Lags, the Two-Thirds Rule, and the Big Bang, Part 2

In the previous post, I describes a significant emotional event I experienced in an early attempt to use a remote duct static pressure sensor to control a large variable air volume system. The remote sensor approach represented an application of the two thirds rule to make the system more efficient.

In this post, I will look at why a remote duct static pressure sensor has the potential to deliver energy savings compared to controlling a VAV system based on a fixed discharge pressure.

Why Worry About the Two-Thirds Rule

At the time of the project behind this blog post, the reason for wanting to apply the two thirds rule was a personal and corporate goal to be energy efficient. But it was not a code driven requirement.

However, work by ASHRAE during the late 1980’s and 1990’s resulted in Standard 90.1. which in so many words, mandated applying the two thirds rule as a code requirement for many systems. But before we look at what current codes would require, let’s explore why the two thirds rule concept saves energy in the first place.

Two-Thirds of What?

The real question about the two thirds rule for many is “two thirds of what?” I am frequently asked this question by operators and technicians who have heard of the concept and are interested in its benefits but are uncertain of how to implement it.

In other words, is the rule saying the sensor should be at a point that is:

  • Two thirds of the horizontal distance from the discharge of the fan to the most remote point in the system on a plan view of the facility? Or,
  • Two thirds of the vertical distance from the fan to the most remote floor? Or,
  • Two thirds of physical length of the longest duct run from the fan? [i]

As we will see, all those interpretations would work. In fact, the rule could have been called:

The “75 to 100 percent out the duct rule” (per the Honeywell Gray Manual)[ii], or

The “15/16ths” rule, or

The “27/32nds” rule.

The bottom line is it was intended as a guideline, not an exact solution, that encouraged moving the sensor out into the distribution system.

Contrasting Discharge Pressure Control with Remote Pressure Control

To illustrate the benefit associated with controlling for a remote duct static pressure, lets contrast what happens for a simple system if it is control for discharge pressure vs. a remote pressure. This example is based on a SketchUp model I use for Existing Building Commissioning (EBCx) training, which has its roots in some of the systems I have seen in existing hotels serving meeting rooms and ball rooms.[iii]

Controlling for Discharge Pressure Near a Fan Location

Consider the system illustrated below (the ceiling of the mechanical room has been removed to reveal the distribution ductwork serving the two zones in the ballroom above).

Ball Room AHU Technically, we could control fan speed based on the pressure near the fan discharge—for instance, after the two elbows and transition (Point A).

Engineering calculations similar to those illustrated subsequently under Controlling for a Remote Duct System Pressure reveal that a static pressure of 1.102 in.w.c. is required under design conditions at Point A to deliver design flow.   Meaning that this metric would become the set point for a control process referencing that location.

As the load in either of the zones served by the system drops and the terminal unit dampers throttle, the discharge pressure will tend to go up. Upon detecting this, a properly designed control process would reduce the fan speed (or, for the MCI Building, close the IGVs) to return the discharge pressure to set point.

Examination of the fan-energy equation …

Fan bhp

… reveals in this scenario, energy would be saved for two reasons. One is that the flow rate dropped, meaning one of the terms in the numerator became smaller, which will make the result smaller even if nothing else changed.

But the pressure drops through the filters, coils, and other components of the air-handling unit that are upstream of the discharge sensor also will drop due to the reduced flow rate. The square law [iv] …

Square Law

… allows us to quantify this for the new flow condition based on the design flow conditions.

As a result, the total system static pressure would be reduced, even if the discharge static pressure were held constant. Thus, a second term in the numerator of Equation 1 became smaller.

Clearly, then, a system designed to reduce flow as load drops will save energy compared with a system with a steady flow rate, even if the design discharge static pressure is held constant for all hours of operation.

If the square law is to be believed (in other words, if you have a modicum of respect for Isaac Newton and Johannes Kepler and those that followed), the pressure required to move air from Point A to Point B also will drop as flow drops. But because the control process is forcing discharge static pressure to the design requirement—even though that amount of static is not required at the reduced load condition—the terminal-unit dampers will need to throttle to dissipate the unnecessary pressure the fan is creating, which can also create a lot of noise.

Therein lie the improvements that can be achieved by applying the two-thirds rule.

Controlling for Remote Duct-System Pressure

Consider what would happen if we located the sensor immediately ahead of the point where the duct splits to serve the two ballroom zones: Point B in the first illustration(which just happens to be about two-thirds of the way to the terminal-equipment location).

A Cautionary Tale

Before going further, there is a point I feel compelled to make about the specific code requirements that would drive a design decision process to use remote duct system pressure to control a VAV system. 

In the first draft of this post, at this point in the discussion, I wrote:

For current design projects, ANSI/ASHRAE/IES 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, prescriptively requires that duct static-pressure-sensor location be such that a set point of no more than one-third of total system static-pressure drop is required. Clearly, then, a sensor cannot be located at the discharge of a fan.[v]

At the time, I didn’t have the most recent copy of the referenced guideline, but I did have the 2019 ASHRAE Applications Handbook, so I referenced that.

One of my colleagues, in their review, pointed out that despite what the handbook says, my statement was not correct, which is why I include this little cautionary tale.

ANSI/ASHRAE/IES Standard 90.1-2019, now says:

Static pressure sensors used to control VAV fans shall be located such that the controller set point is no greater than 1.2 in. of water. If this results in the sensor being located downstream of major duct splits, sensors shall be installed in each major branch to ensure that static pressure can be maintained in each.[vi]

The standard includes an exception that allows facilities with DDC systems to implement a trim-and-respond control strategy like the one recommended in ASHRAE Guideline 36, High Performance Sequences of Operation for HVAC Systems,[vii] to be used to achieve compliance. DDC systems may or may not be required depending on a number of variables as illustrated below, which is a screen shot of ANSI/ASHRAE/IES Standard 90.1-2019 Table – DDC Applications and Qualifications.


I believe the current language in 90.1-2019 is unchanged from what the 2016 version of the standard would require. That implies that the 2019 ASHRAE Applications Handbook reference is to a version prior to 2016.

My point here is that even though the handbook represents ASHRAE’s position on a subject, in the code compliance scenario associated with a design process for a new construction project or a retrofit, the code in force is what will govern. In other words, I should have gone straight to the source and dug out the code and verified what I had read in the handbook before I wrote those lines in the first draft of the blog post.

Having said that, even if you go straight to the source; i.e. the governing code, things may not be as clear as you would hope, especially in existing buildings.

Existing Building Complications

In the mid-1980s – when my significant emotional event in the MCI building happened, neither of the standards and guidelines referenced above existed. In fact, the technology for performing a trim-and-respond strategy did not exist. Thus, our goal was to deliver the benefits of a concept that was being used as a general guideline for improving energy efficiency.

If I was working on the MCI project today (the project associated with the story behind this string of blog posts), either as a new construction project or as a retrofit, I would need to comply with the more specific language of the governing code. But the governing code may or may not be the most current version of a given standard, depending on where the jurisdiction is in terms of updating the codes they enforce. As a result, things can start to get a little “murky”.

And in my experience, in the existing building operations arena, this can get even “murkier”. Most of the time, the facility operators and technicians I get to work with have a passionate desire to improve the performance and efficiency of their systems. Frequently, they are crippled in their efforts by the realities of their operating budgets and equipment. Every year I run into one or two operators who are working with systems that have pneumatic controls and who don’t have the budget to upgrade to DDC. But what they do have is the skill and interest in making what they have work better once they understand how to go about doing it.

That means that for operators in a facility that does not have the technology in place to comply with the “letter of the law” (a trim and respond strategy for controlling duct system static pressure), the approach we used for the MCI building could deliver a significant portion of the savings that can be achieved. 

Returning To our Discussion

For a sensor located at point “B” in Figure 1, engineering calculations would reveal that a pressure 0.975 in.w.c. is required at Point B to maintain flow to the two symmetrically ducted zones served by the system. Thus, if we were to use our control process to maintain this pressure, we would deliver the design flow rate to each zone.

If we used one of the terminal units to do zone-level scheduling by stopping airflow to half of the ballroom if it was not in use when the other half was, the demand for airflow would be cut in half. But, if we maintained 0.975 in. w.c. at Point B when the inactive zone shut down, we would deliver design airflow to the half of the ballroom still in service.

The image below illustrates the pressure drop calculation just to give you a sense of what something like that looks like..

Fan Static Projection v2

The graphics are screen shots from the ASHRAE fitting database, which was used to do the math for the fittings in the analysis.

In addition to looking at the design flow rate, the calculations also  look at what would happen to the pressure drop in that section of duct if the flow were reduced 50 percent using both the square law and the more precise One Point Eight Five to One Point Eight Nine Law. Because the difference between what the square law and what the more refined calculations predict for this short duct run is in the third decimal place, I simply will reference the numbers as predicted by the square law for the purposes of this discussion.

How it Works

Under design conditions, if a sensor at Point B were to meet its targeted set point of 0.975 in. w.c., the fan would be forced to deliver 1.102 in. w.c. at Point A because that is the pressure needed to overcome the resistance due to flow between the two points and deliver 0.975 in. w.c. at Point B. This is the same result as would be achieved by a system that simply controlled for the design static pressure at Point A.

However, at 50-percent flow, a system controlled by a sensor at Point B would force the fan to deliver only 1.007 in. w.c. (the 0.975 in. w.c. required to deliver design flow to either zone from Point B plus the 0.032 in. w.c. required to deliver 50 percent of design flow to Point B). Thus, at part load, the total system static requirement is reduced from what would be achieved in a system controlling for a fixed discharge static pressure.

Good News and Bad News

The Good News

By moving the sensor used to control fan static pressure out into the duct system, we can maximize the energy savings in a variable-flow application. The same is true regarding the location of a sensor controlling the distribution pumps in a variable flow pumping application.  In fact, if you want a detailed look at that, you will find it in a string of blog posts I did a while back about applying the two thirds rule to a pumping system, complete with pump curves and everything.

In any case, selecting the location for the remote sensor is a balancing act, with energy savings pushing the sensor to the most hydraulically remote branch in the system and caution pushing the sensor back toward the fan because the most hydraulically remote branch can be challenging to identify in a large system.  And, it can move around in the system as load conditions change. In fact, for a large system using the remote sensor strategy, it may be desirable to install several sensors and use low-signal-selection logic to dynamically choose the appropriate sensor.

The Bad News

The bad news is that moving a sensor out into a distribution system introduces a lag into the control process. For the system in the model, an air molecule leaving the fan discharge will take only about 1.5 seconds to reach the remote-sensor location, so the lag is likely not much of an issue. But for a large high-rise, the implications can be much more significant.

For example, for one of the systems in a 475 foot tall high rise that I did work in, on a time-rate-distance basis, an air molecule that left the AHU on the top level would take 10 to 12 seconds to reach the terminal unit it served on the lower level.   This slide from a presentation I do about the project, which includes a scale drawing of the duct system will give you a sense of what I mean.


For the MCI Building, the distance to the remote sensor was in the range of 300 ft and the time-rate-distance lag that was introduced probably approached 8 to 10 seconds.

Because of the dynamics of large systems, the lag we are discussing is much more complex than a simple time-rate-distance assessment would lead you to believe. I will discuss why this is in a subsequent blog post. But for now, the take-away is that lags can make control-process tuning challenging and generally are the enemy of tight control. This was the issue I failed to recognize with my initial fan static pressure control system design for the MCI Building and is the reason I blew up the duct.

In the next post, we will take a closer look at exactly what lags are in the general case. Once we establish that, I will do a post that looks at the lags I was dealing with in the MCI building, with a focus on what turns out to be a very complex transportation lag.

Finally, I will wind up the series by looking at how we solved the problem in the MCI building, a solution which is also applicable in the general case if you are dealing with a large, complex system.


PowerPoint-Generated-White_thumb2_thDavid Sellers
Senior Engineer – Facility Dynamics Engineering     Visit Our Commissioning Resources Website at

[i] This is the generally accepted meaning. Interestingly enough, nobody really seems to know where the “two thirds” part came from. Chuck Dorgan did some research about that at one point and concluded that it evolved from a recommendation made in a technical guide developed by one of the major control system vendors in the late 1970s that targeted providing support for their field technicians who were running into the requirement at the time. Personal discussion with Chuck Dorgan, approximately September 20, 2010.

[ii] The Honeywell Gray Manual is an industry classic and was the text book Honeywell used to train new engineering recruits after hiring them. Originally published in 1934, it went through 21 editions with the latest I know of being 1997. While, it is not current with regard to the control system technology in our buildings these days the fundamental principles it discusses like psychrometrics and different applications still apply and are explained in layman’s terms and I frequently recommend it to folks coming into the industry, especially if they do not have a technical background. You can download a copy at

[iii] Incidentally, the duct configuration on the discharge of the fan in the model and related system effect is abysmal; in class I also use this system as an example of how not configure the fan discharge and also to discuss what you can do about it if you find it as an existing condition. For a longer discussion of system effect that uses an earlier version of this model, visit this blog post.

[iv] The square law has its roots in the Darcey-Weisbach equation, which assumes fully developed turbulent flow. ASHRAE research has demonstrated that for most applications, the Square Law is really the One Point Eight Five to One Point Eight Nine Law because there are places in our systems where we do not have fully developed turbulent flow. But for field work, preliminary estimates and developing a general understanding of how things work, it is reasonable to use an exponent of 2 instead of 1.85 – 1.89. Plus, it’s easier to do the math on a slide rule that way (I still carry one around).

[v] 2019 ASHRAE Applications Handbook, Chapter 48, page 48.8.

[vi] ANSI/ASHRAE/IES Standard 90.1-2019, paragraph VAV Static Pressure Sensor Location, page 235.

[vii] ASHRAE Guideline 36-2018, paragraph 5.1.14 Trim & Respond Set-Point Reset Logic.

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Lags, the Two-Thirds Rule, and the Big Bang, Part 1

MCI Building 02This string of blog posts started out as an ASHRAE Engineers Notebook Column.  But they got to long for that format, so I decided to post them here.  The story is an example of how I was mentored by a building and it’s systems and learned a number of lessons that I use to this day. 

My mentor in the story is a building that was, at the time, known as the MCI building, on the riverfront in St. Louis Missouri, where I lived and worked during that period of my life.  I believe it is now called the Deloitte Building.  In any case, it is the teal colored building in the picture to the left.  I will call it the MCI building as I write this because that is how it was known to me at the time.

The Situation

From 1984 – 1986, I had the privilege of working for Murphy Company under Tom Lillie in their Design/Build department as a combination field engineer, start-up engineer, and control system designer.  I can’t remember what it actually said I was doing on my card, but basically, that is what I was doing.

One of the projects I worked on was the MCI Building.  Although the industry was moving from pneumatic control to Direct Digital Control (DDC), the owner wanted to stick with pneumatics, primarily because of budget constraints, but also because of uncertainty about how well their operations staff would be able to deal with the new technology.  Tom placed his trust in my abilities as a control-system designer and startup technician for the large (two, nominal 90,000 cfm units in parallel) variable-air-volume (VAV) air-handling system that would serve the facility. My work on that system brought about a “significant emotional event”— a phrase coined by Jay Santos, PE, co-founder of Facility Dynamics Engineering.

The Story

Significant Emotional Events

A significant emotional event is an attention-grabbing, eye-opening incident that changes the way you think about and approach something in a very profound, fundamental way. This event clarified one of the principles that David St. Clair wrote about in “Controller Tuning and Control Loop Performance, a Primer” —namely, “IT ALL DEPENDS ON THE LAGS!”[i]

Setting Up the Significant Emotional Event

With Tom’s blessing, I applied the two-thirds rule to the duct static-pressure-control process for the Inlet Guide Vane (IGV) equipped MCI Building supply fans, using a high-quality pneumatic control system featuring two-pipe transmitters and Proportional plus Integral plus Derivative (PID) capable receiver controllers. At the time, the two-thirds rule was an emerging energy efficiency recommendation that advocated moving the duct static pressure control sensor from the fan discharge to a point out into the distribution system. But it was not yet a code or efficiency standard requirement.

I will discuss the rule in more detail in the next post , in this series, and you can find an illustration of it applied to pumping systems in a previous string of posts.  But suffice it to say that our engineering calculations indicated that we should install the sensor that would control the supply fan static for the system at the supply main on the second floor using a set point of 0.75 in. w.c. to accrue the two-thirds rule benefits. In terms of linear feet of duct, this turned out to be about two-thirds of the distance from the 12th floor penthouse location of the air-handling unit; just saying.

When the time came to bring the system on line for the first time, I stationed myself at the remote sensor so I could watch what was going on there.  Ray Baltimore, a very gifted control system pipe fitter that I was working with was up in the penthouse 12 floors above me coordinating things there and monitoring the process from that perspective.

Upon initial startup, the discharge-static-pressure safety switch (3.5-in.-w.c. set point, 4.0-in.-w.c. duct-pressure class) tripped, even though the pressure at the sensor location that I was monitoring never reached the targeted set point of 0.75 in. w.c.

Believing we were dealing with a control response problem; we narrowed the throttling range of the controller and restarted the system. After the discharge-static-pressure safety switch tripped again, and after consulting the specifications to verify the duct-pressure class, the safety switch setting was increased to 3.75 in. w.c., the throttling range of the controller narrowed further, and the system restarted.

Not Quite Connecting the Dots

Following yet another safety trip, restrictors were added to the pneumatic tubing serving the IGV’s to slow them down and allow downstream pressure to build without exceeding the discharge safety set point.

When we restarted the system, discharge-static-pressure safety switch did not trip. But after 10 minutes, the actuators had not moved far enough to get the system to set point because of the large actuator volume and the reduction in flow imposed by the restrictors.

After experimenting with several restrictors, we concluded that we had simply traded a safety trip problem for an unresponsive system problem. So, we removed the restrictors, and increased the safety setting to 4.0 in. w.c. Upon restart of the system, the discharge-static-pressure safety switch tripped once again.

The Big Bang;  A Significant Emotional Event

Assuming there was a tolerance on the duct static-pressure class rating, we increased the discharge-static-pressure-safety-switch set point to 4.25 in. w.c.

That’s when it happened: the big bang. Ray, always the humorist and trying to put a positive spin on things radioed …

Well, at least we know the duct pressure class is right.

Sadly, I had just performed an (unintentional) destructive test verifying the duct system pressure class. While destructive testing may have its place for verifying that things like airbags in a car will work in an actual crash, it is not the approach recommended by SMACNA for verifying duct pressure class.

As the fan spun down, David St. Clair’s words hit home. And it was also apparent why “It’s all about the lags” was in all capital letters, with an exclamation point, in an extra-large font, in a highlighted box at the end of the lags chapter in his book.

Up until then, I had not appreciated what he was saying at all. Now, I fully appreciated it and had added my very own exclamation point.

Solving the Immediate Problem

I desperately wanted to capture the savings associated with using remote duct pressure instead of fan discharge pressure to control the supply fans. But to maintain schedule, I concluded that I would need to move the transmitter to the fan discharge and control the system based on that for the time being.

Ray and I made plans to gather the necessary hardware and make the change.  The tinner was already putting the blown duct joint back together and figured they would have the system ready to go again before they went home for the weekend. But between the changes that Ray and I would need to make to the control piping and the fact that some of the parts we were having air-freighted in would not arrive until Saturday, it looked like we would be working the weekend.

Another Mentoring Story

When I called Tom to tell him the bad news and what our plans were, he kind of chuckled and said something like …

Well Dave, we aren’t the first people to do this two thirds rule thing, so there must be a way to make it work and I bet you guys will figure it out. And I’m sure your temporary plan will work until then.

But you and Ray have been working hard and you have that new little baby sitting at home.  God put us on this earth to do certain things and it wasn’t to constantly be messing around with buildings.   So go home and take a break.  I’ll meet you on site Monday to brainstorm a solution and help get the temporary control plan working. 

Pretty cool;  like I said, I can’t remember the exact quote, but I won’t ever forget the intent and message about paying attention what is important in life.

The Temporary Fix

After moving the sensor to the fan discharge, we were able to tune the control loop to allow the system to start and achieve stable operation at the targeted 3.00 in.w.c. set point without a safety trip.

One obvious solution to our problem was simply to let go of the concept of controlling the system based on pressure at a remote point. But if we did that, we would not be delivering the efficiency we promised our client.

And in the bigger picture having both been mentored by Bill Coad,[ii] Tom and I wanted to make our system as efficient as possible. Thus, my quest to understand the reason I could not get the system to work using a remote sensor continued.

Not Every System Will React This Way

(Thank Goodness)

I want to emphasize that I am not saying this problem will occur in every VAV system out there and that controlling static pressure based solely on a remote sensor in the duct system won’t work. Obviously, it works in many situations.

But in this particular case, due to the dynamics of the MCI Building system, the duct-pressure-class limit was exceeded at the fan discharge before the desired operating pressure was reached at the remote-transmitter location; i.e., our problem was related to a lag. This caused me to realize that the dynamics of some systems may require a different approach for achieving the benefits of duct static pressure control based on a remote pressure in the system.

Coming up On Lags, the Two-Thirds Rule, and the Big Bang

In the next post, I will look at exactly why using a remote static pressure sensor to control a VAV system will save energy compared to simply controlling for discharge static pressure.

In a third installment, I will take a closer look at exactly what lags are in the general case.

In the fourth installment of the series, I will look at the lags I was dealing with in the MCI building, with a focus on what turns out to be a very complex transportation lag. I believe there are also reasons aside from the system lag dynamic that result in this problem occurring on some but not all projects, occurring on all projects and I will highlight them in this installment.

Finally, in Part 5 of this series, I will look at how we solved the problem in the MCI building, a solution which is also applicable in the general case if you are dealing with a large, complex system.

In closing, I wanted to thank the Engineering Notebook team for their initial feedback on the article, which helped me focus it and address some technical questions it brings up.  And I also want to thank Michael Ivanovich and Scott Arnold of AMCA, who jumped in and helped organize the original article into the more manageable string of five articles that have evolved to this string of blog posts.


PowerPoint-Generated-White_thumb2_thDavid Sellers
Senior Engineer – Facility Dynamics Engineering     Visit Our Commissioning Resources Website at

[i]     This is still available for purchase, at and is well worth the money if you are trying to understand PID control loops in practical terms.

[ii]     Bill Coad was the vice President of McClure Engineering when I interviewed there in 1976. In the 1980’s Bill wrote an article for the ASHRAE Journal titled Energy Conservation is an Ethic. (ASHRAE Journal, vol. 42, no. 7, July 2000) But he was thinking that way long before he wrote the article. That philosophy, conveyed to me during my interview in 1976, was one of the things that caused me to want to get into this field. You could say it changed my life. You can find a copy of it on the ASHRAE website at

In addition, we have a page on our website with a lot of the other things Bill wrote, which are still applicable today since he dealt in fundamental physics.

Posted in Air Handling Systems, Controls, Mentoring and Teaching, Pneumatic Controls | Leave a comment

New Resources

Its been a while since I have posted and my (potentially annoying) holiday post has been sitting there for over a month.   All I can say is that I have been working on FDE’s commissioning resources website to add some new content and it simply took some time.

The good news is that there is some (potentially useful) new information available there, and this is a quick post to give you a “heads up” about a place where you can find:

Energy Design Resources Design Briefs

A bit over a month ago, I discovered that the Energy Design Resources website that is linked to multiple references in the Resource List and the Existing Building Commissioning Skills Guidebook no longer exists.  So there are a bunch of links in both of those documents that will not longer work.  I am working to fix that and get copies of both resources with repaired links up on the website.

Meanwhile, I made a page on our commissioning resources website where I am posting the EDR resources that are referenced by both of those documents so the will be available to you.  On the one hand, you could consider them to be “dated” since some of them were written over 10 years ago. 

But on the other hand, since they are based on physical principles that Isaac Newton identified over 300 years ago, which still seem to be reasonable explanations for the phenomenon going going on in building systems, you could decide that they are potentially useful after all (in case you did not guess it, this would be my position).

And since they were created with public benefit funds, I figure that posting them for folks to access does not violate any copyrights.   I want to emphasize that I am only doing this to make what I believe to be useful information available to folks who might need it.  If you have issue with this, please let me know and I will either take the information off the website or work with you to come up with a mutually agreeable way to make the information available.

Bureaucratic Affairs Building HHW System Logic Exercise Answers

For quite a while now, we have been using SketchUp models as a way to provide training experiences and there are a number of model based exercises that you can work  your way through on the Cx Resources website.  One of these exercises involves modifying the logic for a heat exchanger in the mythical Bureaucratic Affairs Building in Golden Girl, Missouri to add a reset schedule to the control process. 

The answers to the exercise have available in the form of logic diagrams, a narrative sequence and a points list for a while now.  But I have been working on a series of informal videos that walk you through the answers and I completed the first two of them today and posted them on the website.

These videos give you an overview of the problem and then illustrate one possible answer for adding a reset schedule to the existing heat exchanger control process, which uses a fixed set point.  Adding the reset schedule will save energy and improve comfort.  The details of why are discussed a bit in the videos and in more detail in the information provided to support the exercise.

My point here is to let you know they are available.  I also plan to do a couple of videos that look at some of the other logic associated with the HHW system in the Bureaucratic Affairs building including the occupied/unoccupied cycle logic and the pump lead/lag sequencing.  This logic is included in the hhw_logic_diagram_vweb_-_full.xlsm file that is provided on the answers page.  The videos will simply walk you through how it works.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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Flexible Flyer; a Holiday Story

clip_image002Like it or not, you have probably noticed that I endeavor to do a “seasonal” blog post of some sort around the holidays.  Past topics have ranged from cute pictures of my granddaughters to snow crystals to George Hinke’s depiction of life at the North Pole.

I even managed a semi-technical post about how while most of us are having a magical, often romantic Christmas Eve and day, there are folks out there doing their job to keep the lights on and keep us safe in our homes and hospitals and doing other things we take for granted, like keeping the internet running. So take a minute to say thanks for that and remember them if you happen to be reading this.

At this point, I will pause for a moment and say that if you find my occasional forays into the non-technical to be annoying, you should stop right here and come back in a week or so because I plan to have a new technical post up by then.  Or, wait for the February ASHRAE journal because I have been given the honor of being invited to be on the team that writes the Engineering Notebook column and my debut will be in the February edition.  Alternatively,  you can roll your eyes and continue anyway because you are kind of intrigued.

Having done due diligence and forewarned you,  I will continue with what will become a (what I believe to be romantic) holiday story in the spirit of previous holiday posts.

In writing those posts, I realized the skill set needed is somewhat different from the technical writing skill set that I have been honing for the past 20 or so years of my career.  And, I realized there is apart of me that is interested in learning how to do non-technical writing.

Initially, this was triggered by finding my Dad’s letters home from the war, including ones written right before and after he went into Omaha beach as a Sea Bee, which I found gave me a lot to think about, especially when I “connected some dots” from conversations I had with him while growing up.  But in starting that process, I also realized that I had a lot of wonderful gifts from my ancestors in the form of memories and that I wanted to honor them somehow.

So, I am in the process of writing a string of stories that I will occasionally self publish as a series of books.  I am even working with a professional writer, Rachel Starnes, who is patiently mentoring me in this regard.  She has been a joy to work with and given me a lot of insight and guidance.  So, if you are exploring writing and looking for someone to work with, you may want to get in touch with her.

love letters It turns out the self publishing thing is actually fairly easy using one of the self publishing applications like Blurb.  I used that resource to make Kathy a (well received) picture book that was the history of our relationship as a gift on the occasion of our 10th anniversary.  It looks like a for-real book like you would buy at a bookstore,with a glossy cover jacket and everything.  But there is only one copy; the one she has, which is kind of cool.

In any case, in terms of recording (my version) of the family history, the idea is self publish a series of story books that I will give as gifts to our children and grandchildren.  This will likely cause them to sigh, roll their eyes, and place the book on a shelf or in a box someplace.  There it will remain until, perhaps, somewhere in their 40’s – give or take a decade – they remember it, go looking for it (perhaps a bit frantically), rediscover it, and maybe find it to be interesting after-all.  And I am pretty sure that doing this will be good for me if nothing else.

My point in bringing this up is that I was “drawing a blank” on what to post for the holiday’s this year. But then, for some reason – probably because I had been through my shop  retrieving a Christmas decoration from the attic over it – I realized I might have a pretty good seasonal story to share. So here we go.

Specifically, in pulling down the folding stairs to the attic over the shop, my eyes came to rest the Flexible Flyer you see in the photo at the top of the post. That was my Dad’s sled when he grew up on a farm near Bellefonte or more specifically, Waddle, in Central Pennsylvania. I spent a lot of time riding it in my youth.

The story I was reminded of happened before I was around, maybe even before I was a “twinkle in my Father’s eye” as they say. But the event I am about to relate may just have triggered the twinkle one snowy Christmas Eve, at least that is what I like to think.

Central Pennsylvania has a lot of rolling hills and the land that the farm was on was no exception, sitting in the Nittany Valley with gently rising hills on either side of it. Some of the hills were woodlands, but some had been cleared for crops and pastures over the years. The picture below will give you a sense of it.

clip_image004These cleared areas made for great sled rides and my brother and I spent many hours on the Flexible Flyer racing downhill, across a pasture past the oak tree in the bottom left corner of the picture, making a hard left turn at the end of the run to slide to a stop before coming to a sharp 3-4 foot drop down to the road.

If you misjudged the turn, which we did on occasion, you ended up taking the plunge. But with all the layers of clothes we had on, bruises were minor if any and the danger was minimal given it was a country lane with very little traffic.

In any case, as far as I know, on the Christmas Eve I am referencing, Mom and Dad were engaged, and Mom – more of a a city girl than not – had already been to the farm once or twice but never when there was snow until this occasion. So, Dad thought it would be fun to introduce her to the joys of sled riding and talked her into making a trip down the biggest open pasture on the farm with the Flexible Flyer in the opening photo being their means of transport.

I can imagine this would have been a romantic sounding notion to my Mom. Perhaps, in her mind’s eye, she was thinking they would need to snuggle up together to make the trip; certainly my Dad was not thinking of sending her hurling down a hill on a sled for the first time ever all by herself.

This being the late 1940’s/early 1950’s, I would also imagine that in the normal course of events, proper, attractive, young, unmarried women were not supposed to spend much time snuggled up to a man, even if it was the man they were engaged to. In fact, I suspect they were not even supposed to be considering it.

Holding hands, or maybe even arm in arm might have been O.K. once you were engaged. But certainly not sitting front to back with arms and legs wrapped around each other to ride a sled or worse yet, lying front to back on top of each other. But at this point in the conversation, I suspect Mom had not thought in practical terms about how they would both fit on the sled.

Dad, on the other hand, might have.

But my guess is that Mom probably just thought that it would be all snuggly and romantic and would probably involve a hand-in hand walk to the top of the hill together. Plus, she had never really been sled riding.

So, she said yes, and in my mind’s eye, I see the two of them heading out towards the barn together to retrieve the sled, hand in hand as she had hoped, Dad in his work clothes and the red leather hat he always wore when home working with his Father and brother on the farm. And Mom in a taupe, knee length coat, a burgundy scarf covering her head, and black ankle high boots with fur rimming the top of them on her feet.

From there, they probably walked up the valley a bit along the line separating the woodland from the cleared land for a little over a third of a mile to get to that perfect sledding field, all of this in the gray mid-afternoon twilight of the near winter solstice with a quiet, gentle snow falling around them, the kind where the snow crystals are spectacularly huge, and intricate, and delicate, and make you feel as if the sky is snuggling you.

I imagine them quietly talking as Dad recounted growing up on the farm, something he was fond of doing because of his love for the place. He had spent many of the hours of his life prior to the war, plowing, and otherwise working the field they would have been walking along.

Or maybe he gently teased Mom about being a city girl who, until he brought her home to the farm with him for that first crucial visit, had always had the benefit of indoor plumbing, something that would not arrive at the farm for several more years. And now, here she was, all adapted and comfortable with occasional periods of rural life and about to go for her first real sled ride.

And I imagine the occasional quick kiss along the way, since young love is inspired to that sort of thing in settings like these.

In any case, eventually they would have arrived at the upper left corner of the field in the picture below.


That probably does not look like much of a hill, but the angle and the corn in the foreground make it a bit deceiving. If you look at it on a topographical map, the drop is in the range of 100 – 110 feet from the start of the run to the bottom of the valley in about 500 feet of horizontal distance.


The red dot on the map is about where Mom and Dad were standing when they were at the starting point of the sled run. The black outlined square next to the orange dot is the barn, where they retrieved the sled, and the black dot next to the blue dot is the farmhouse. The light blue line above the word “RAILROAD” is a little creek that marked the very bottom of the valley.

The creek itself was only 6-12 inches deep, even during the rainy season, but it was several feet wide and in some places, there was a 2-3 foot drop from the elevation of the field to its banks; not that different from the drop to the road if you miss-judged your turn at the end of the run in the field where my brother and I sledded in our youth. More on why that matters in a minute.

To give you a sense of the elevation change, when you are standing at the top of a typical flight of stairs and looking down, you are looking at an elevation change of about 12 inches vertically for every 18 inches of horizontal distance covered. In that context, imagine riding a sled down your basement stairs (I am sure there are kids how have tried it) (if there is a kid reading this DO NOT DO THAT).

Now envision riding down a slope about 1/3 as steep and you have a sense of what my Mom saw when they arrived at the starting point. That is still pretty steep, and hills always look steeper from the top, at least they do for me. I suspect the same might have been true for Mom.

So, I would not be surprised if, having arrived at the starting point, Mom might have expressed second thoughts about taking the ride. My Dad probably had anticipated this and had a plan. Truth is, even with a steep hill, with fresh snow on top of the stubble of a hay field, it takes several runs down the hill to sort of “break things in”, at least in my experience.

The first run will be spectacularly slow and short; you may even need push yourself along a bit at the end of it to feel a bit of satisfaction about the distance gained. The second run builds on the first by an order of magnitude or more for both speed and distance. And at some point, usually a couple of runs at the most after the first two, you “break the snow barrier” and find yourself flying down the hill at breakneck speed.

My guess is that when Mom expressed some second thoughts, my Dad offered to make a run or two on his own to show her how safe it was. These runs, of course, would under-state what would eventually happen if you persisted at things a bit. But after watching my Dad make one or two slow, relatively short runs down the hill, Mom agreed to giving it a try.

At this point, I suspect the practical considerations regarding proper conduct of an engaged but unmarried young woman needing to somehow fit on one sled with her man emerged. This was likely met with mixed emotion since it may have been my Dad’s plan all along and may also have been the subconscious reason the snuggly sled ride appealed to my Mom.

I don’t really know what happened at this point; it was never discussed. Perhaps they tried Mom in front with Dad behind her, his legs on either side of her and his arms around her grasping the rope that would steer the sled. And I suspect Mom may have found this to be quite satisfactory other than for the fact that she was staring point-blank down a hill that looked like a near vertical drop with nothing in front of her to protect her.

At that point, maybe they decided to try switching positions. But about the time Mom stepped astride the sled behind Dad, it probably occurred to her that perhaps this was not the most proper of positions for a young woman wearing a skirt to assume.

As it turned out, they settled on Dad lying on the sled, gripping the steering arm with his hands (which in my experience works really well) with Mom lying on top of him, arms wrapped around him and gripping the sides of the sled, probably for dear life.

At that point, Dad would have pushed off and they would have started down the path carved out by may Dad on during his demonstration runs. From having done it, I suspect that two or three demonstration runs would have gotten things pretty close to the point where you would “break the snow barrier”, but not quite.

So maybe their first run got half way down the hill with enough speed to make it pretty exciting, but with enough distance between where they slid to a stop and the bottom of the hill to make things seem very manageable and safe to my Mom. However, the course was now set for “breaking the snow barrier”.

What with getting to lie on top of her man with her arms wrapped around him listening to my Dad’s infectious laughter as they slid down the hill, Mom probably thought another run would be fun. Dad, of course agreed, so the climbed back up the hill, positioned themselves on the sled, perhaps after a quick, glowy-eyed kiss, and headed down the hill.

This time, they “broke the snow barrier”.

Your Father took me sled riding for the first time down a huge hill where it turned out that you had to jump a creek to finish the ride!

Generally speaking, that is how this event was related to my brother and I (or others when we managed to figure out a way to get Dad to start telling the story with Mom in the room).

Full disclosure, a lot of this is speculation on my part; the details never were divulged in the telling of the story. And were you to write down what was actually related during a telling of the tale and compare it to what I have written, it would be much shorter. But in my mind, and I guess in my heart, this could easily be an accurate appraisal of the details.

In any case, I suspect that right about the time they “broke the snow barrier” and started to accelerate instead of decelerate half way down the hill, Mom noticed the creek.

Remember the creek; the light blue line on the topographical map?

Dad, of course knew it was there.

Part of the fun of the ride was jumping it, which worked …

…most of the time.

But if it didn’t, you made the 2 or so foot drop to the banks of the creek and maybe into the creek. Not a big deal really if you are young and a farm boy. But probably not what you would like to subject your future bride too.

On the other hand, if you pulled it off, as you had countless times before, then you would be your future bride’s knight in shining armor.

At the time, Dad may not have realized he already was Mom’s knight in shining armor.

Or maybe he did but thought it would be good to reinforce that sort of thing. But either way, for Dad, jumping the creek was the culminating event in the over-all experience of farm-land sled riding.

For my Mom, it had the potential of a nightmare; what if they missed and ended up in the creek? It would could ruin her coat, and her hair, and she would be freezing, and it was not a short walk back to the farm house, which did not have central heating, just fireplaces.

Phil, please don’t do this!

Dad, realizing that perhaps he had pushed a bit too hard, but also realizing that there was a point where physics was governing all of this, decides to try to circumvent the creek jump by making a hard-left turn, dissipating speed by sliding sideways, just like my brother and I would do to avoid ending up in the road.

If it worked, it would look like the plan all along. But if it didn’t; well, not good.

In the end, the balance of forces almost, but not quite, worked out. In other words, Mom and Dad and the sled at a relatively slow speed, dropped the two or so feet to the banks of the creek and then started to roll towards the ice covered water.

All of this happened in fractions of a second and there was no thought involved, just instinct; Dad grabbed Mom and stopped their roll at the water’s edge with his coat just touching the ice and Mom safely on dry land.

There they sat for a second …

… or a minute …

… or an hour …

… or a year, or an eternity, wrapped around each other gazing into each other’s eyes, adrenalin pumping, anger and excitement pounding against each-other in their veins.

And then they kissed, and not just a peck on the cheek.

And somewhere in there, I think I might have become a twinkle in their eyes.

At that point, I suspect reason and proper conduct prevailed and they jumped up, surveyed the damage, realized it was minimal, and headed back to the farm-house.

My guess is that on the walk back, Dad apologized for not mentioning the creek jump as being a part of the over-all experience to my Mom.

And Mom, while acting appropriately upset about it, tells him that it was O.K. and that other than nearly ending up in the creek, it was quite exciting. This followed by a quick kiss that was not quite as quick as the ones on the way out.

So, there is a Christmas Eve story for you, enhanced, of course, by my imagination filling in the gaps between what I was told and what I saw in my parent’s eyes and heard in their voices when they told it.

May you all have a happy (and romantic) holiday season.


PowerPoint-Generated-White_thumb2_thDavid Sellers
Senior Engineer – Facility Dynamics Engineering                                 Visit Our Commissioning Resources Website at

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Electric League of the Pacific North West Training Opportunity

This is a quick post to let you know that I will be helping to support a two day technical class for the Electric League of the Pacific Northwest.   

Interstitial Space 03The class will be a two day long technical class that will use the existing building commissioning process as a way to work with the technical skills that commissioning providers will use in virtually any commissioning process.  The class is designed to be interactive and provide some hands-on exposure to the various skills via exercises with SketchUp models and exercises in the Fred Hutchison Cancer Research Center mechanical spaces.

I have added the class to a calendar on the the Training Opportunities page of our Cx resources website and you can find additional information there.  Or you can use this link to go directly to the Electric League’s web page for details and a registration link.


PowerPoint-Generated-White_thumb2_thDavid Sellers
Senior Engineer – Facility Dynamics Engineering     Visit Our Commissioning Resources Website at

Posted in Uncategorized | 2 Comments

Build Your Own Psych Chart–The Spreadsheet

A while back, I started a string of posts in memory of Bill Coad about how to build your own psych chart in Excel based on the basic psychrometric principles and equations.   This is what the completed product looks like.


So far, I have put up the following posts.

  1. Build Your Own Psych Chart – In Memory of Bill Coad
  2. Build Your Own Psych Chart – A Few Fundamental Principles 
  3. Build Your Own Psych Chart – Creating the Data for the Saturation Curve and a Bit About VLOOKUP

I still intend to finish the string and provide guidance on how to complete the chart. 

But in the meantime, folks occasionally ask me for the spreadsheet so they can reverse engineer it on their own.  So, I decided to post it on a support web page that I have on our Commissioning Resources web site at this link so you can download it to work with if you want.

FYI, the page is not a public page so you need to use the link in the preceding sentence to get to it because you will not be able to navigate to it directly on the site.


PowerPoint-Generated-White_thumb2_thDavid Sellers
Senior Engineer – Facility Dynamics Engineering     Visit Our Commissioning Resources Website at

Posted in Excel Techniques, HVAC Calculations, HVAC Fundamentals, Psychrometrics | Leave a comment

The Functional Testing Guide

When I was working at Portland Energy Conservation Incorporated (a.k.a. PECI), one of the things that I helped to develop was the Functional Testing Guide. 

FTGuide Picture

At its heart, the guide was a compilation of functional testing strategies that were donated to the cause by experienced providers.  What we realized after we collected the tests was that they are great descriptions of how to go about performing a functional test on a specific system.

But what was lacking was information regarding why a particular test sequence mattered.  And given that the number of possible HVAC system configurations can be quite large, as can be seen from the following relationship …

Number of Systems v1

… the guide developers realized that it would be desirable to supplement the donated test sequences with information that provided guidance about the systems and components targeted by the test strategies and as well as general information on what constituted a good test strategy.  That way, people using the guide could adapt the test strategies it contained to their unique needs and understand exactly what they were doing.

The result was a tool that, in addition to the test strategies, contained:

  1. System and component specific guidance about how various building systems and their components functioned.
  2. An index of all of the tests and test guidance documents included in the guide with links to each one.
  3. A checklist tool that could be used to help a provider develop a test strategy, including links to the applicable guidance information as well as desirable preparatory steps and precautions to be taken when executing the test.
  4. A control system design guide that provided information regarding the control system design process, the selection and application of sensors, point lists for common HVAC system types, and a point list tool.

All of the tests were in the form of Word or Excel files, which allowed the users of the guide to edit them to their hearts content and make them their own.   And, even though the development of the guide occurred about 12-15 years ago, since the physics of the building systems it targeted for testing has not changed much, the information it contains is somewhat timeless.

Recently, I discovered that the websites that provided access to the guide have been taken offline and retired. And it turns out that I was not the only one, because a number of people contacted me to see if I knew of a way to gain access to it again.  As a member of the development team, it was gratifying to realize that others had found the document useful, but not particularly surprising since, as I mentioned above, the content is somewhat timeless.

It turns out that the answer to that question is “yes”.  More specifically, if you look closely at the image at the beginning of this post, you will discover that the FT Guide website had a download button.   If you clicked that link, a zip file would be downloaded to your computer that contained all of the content in the guide.

If you unzipped the file, it would create a directory on your hard drive that deployed the content in manner that let you run it locally, just as if you were working with it on line.  That was possible because the guide had been deployed using html (Hypertext Markup Language).

Since I have a copy of the download file, and since the guide was developed with public benefit money, I have concluded that I can, with out causing some sort of copyright infringement thing, make it available on FDE’s commissioning resources website. I guess it is possible that someone could tell me to cease and desist with this, but I figure I can cross that bridge when and if I come to it.

So, the primary purpose of this post is to make you aware of that.  In other words, if you go to FDE’s commissioning resources website and pick the Functional Testing Guide topic under the Resources drop down menu, you will find the zip file for download along with some information about how to go about installing the guide on your personal computer and working with it.

But I also thought I would include some content here that highlighted some of the features of the guide for those who are not familiar with it.  The links below will jump you to the indicated topics.  At the end of each topic, a “Back to Contents” link will bring you back here.


Getting Started

When you launch the guide, it should open in Internet Explorer (assuming you either associated it with that application or used “Open with” to select it) and look like like the image at the top of this post.

To use any of the three elements, you need to select the “View Online” button, even though you actually are not working with it on line.

(Back to Contents)

Exploring the Functional Testing Guide

The primary element of the tool is the Functional Testing Guide.  When you click on that “”View Online” button, you should get a screen that looks like this.


In my system, I get the little warning message that you see in the box with the gold header.   For me, simply ignoring it seems to work and if I select “Allow blocked content” the HTML actually does not work as well as if I just ignore it.

(Back to Contents)

Understanding the FT Guide

The first time you open the guide, you may want to review the section titled Understanding the FT Guide.  The figure below is extracted from that section and is a pretty good summary of the structure of the guide, which is related to its evolution and funding levels.


The original edition was published in May of 2013 and included the Functional Testing Basics module, the Air Handling Systems module and the Air Handling Systems Reference module, all packaged as one element.

When the guide was expanded:

  • The functional testing basics portion of  the original guide was extracted and made a stand-alone module because the concepts it contained generally applied to any testing process, irrespective of the system the test was focused on.
  • System modules were created for Air Handlers, Chillers, Condensers, Boilers, and Pumping as a methodology for organizing the applicable tests and related, system specific functional testing tips and benefits.
  • An Integrated Operation and Control Module was created to expand and emphasize the integration focus associated with commissioning and operations.
  • The remaining elements of the original guide (the Why We Test part) were retained as a reference to supplement the air handling system tests in the Air Handling system module (the How to Test part).

There was insufficient funding available to develop reference guides for the other system modules but at the time, the vision was that this might materialize and allow further development.  This never happened.

(Back to Contents)

Functional Testing Basics

As a first time user of the guide and/or someone new to functional testing, you may want to review the Functional testing basics section of the guide because it covers quite a few important functional testing concepts as can be seen from the table of contents listed below.

  • Introduction
  • Understanding the Fundamentals
  • The Commissioning Process
  • The System Concept
    • The System Diagram
    • Detailed Sequence of Operations
  • Testing Hierarchy
  • Documentation
    • As Built Submittals and Shop Drawings
    • Installation Inspection Report Forms
    • Performance Sheets
    • Installation and O&M Manuals
    • Balance Information
    • Operating Sequence
    • As Found and As Left Conditions
  • Training
    • When to Start
    • Supplemental Information
    • Control System Training
    • Factory Training
  • Verification Checks
    • Verification Checks Development
    • Verification Checks Checklist
    • Factory Inspections and Tests
    • Factory Supervision, Assembly, and Start-up
    • System Readiness
  • Warranties
  • Temporary Operation
  • Elements of a Functional Test
  • Basic Tools, Instrumentation, and Equipment
  • General Precautions and Preparations
  • Observing Tests
  • Returning To Normal
  • Getting To Team-based Solutions
  • Trend Analysis as a Functional Testing Tool

(Back to Contents)

The System Modules

The system modules provide the following information for each system type.

  • Key Commissioning Test Requirements, which focuses on basic items that should be considered in the context of testing a particular system type including safeties, sensor checks, actuator sequencing checks, etc.
  • Key Preparations and Cautions, which pretty much what it sounds like.
  • Time Required to Test, which also kind of what it sounds like.  But it does not give specific information in the form of it will take 30 minutes to test the freezestat.  Rather it provides information in the form of general guidelines that take system complexity and size into consideration.
  • Testing Guidance and Sample Test Forms;  the screen shot below will give you a sense of what the information in this section is like.


The links will take you to the actual test forms, which are in Excel or Word format.  That means you can download them and edit them to your heart’s content, add your company logo and print it out in your company’s colors.

(Back to Contents)

The Reference Guide

As I mentioned above, the reference guide only exists for air handling systems.   As you can see from the screen shot below, there is a chapter in it for each component in an air handling unit.


If you open up a chapter, you will find a lot of detailed information about the component, as can be seen from the screen shot below, which is the table of contents for the economizer section.


(Back to Contents)

The Integrated Operation and Control Module

This module was added in the second addition of the functional test guide.  It is a case study of the commissioning of a hypothetical building that goes through a renovation process.  The intention was to tie all of the functional testing concepts together in a realistic representation of what the process might look like out in the field.  So basically, it is a nerd novel about commissioning.

The module includes all of  the basic information about the building such as equipment schedules, system diagrams, etc. in an appendix.   The problem is that the link to the appendix was never added to the table of contents.  So it is really hard to find it.  But fear not, there is a workaround, which is described on the FT Guide web page on our Commissioning Resources website.

(Back to Contents)

The Appendices

The FT Guide contains a number of appendices including

  • Appendix A – On Overview of the Commissioning Test Protocol Library (CTPL)
  • Appendix B – Functional Testing Guide Resources (links to other resources like ASHRAE, AMCA, etc.)
  • Appendix C – Calculations
  • Appendix D – Description of Tests
  • Appendix E – Test Sources

While all of the appendices have useful information, I wanted to highlight Appendix C and D in particular.  Appendix C provides quite a bit of information regarding how to do energy calculations for air handling systems, as can be seen from the screen shot of its table of contents below.


And, while the calculations discussed are in the context of air handling systems, the concepts can be applied to other system types.  For instance the formulas used to calculate fan and pump bhp are very similar.

Fan bhp

Pump bhp

As a result, the techniques outlined in FT Guide Appendix C for calculating savings due to a reduction in static losses or assessing a load profile for a fan system also apply for assessing savings due to a reduction in pump head or developing a load profile for a pumping system.

(Back to Contents)

Resolving a Viewing Problem

If you look closely at the screen shots, you will notice that there is a little tool bar at the bottom of them that gives you a tip for searching the page and also sets up a printable view of the page.   The problem is that for some of the pages, the tool bar prevents you from reading the last line or two as illustrated below.


I have not been able to figure out how to fix this bug, but I did find a work around.   Specifically, if you select “Print Preview” from the tools menu …

Print Preview

… and then pick “Only the selected frame” …

Only Selected Frame… and then page to the end, you should be able to read the last line or two.

Last page

(Back to Contents)

Exploring the Checklist Tool and Test Directory

The second edition of the FT Guide added a checklist tool and test directory that was intended to make it easier to identify the test targets and find a test.  When you launch that portion of the tool by clicking on the “View Online” button, you will find two other options, as illustrated below.


If you select the View Checklist Tool option by clicking on it, you will be take to a page that has links to a number of checklists.

imageIf you follow one of the links, it will take you to a list of items you should be considering as you develop a functional test for that element of the system.  Here is what that looks like for the economizer and mixed air section.


The little icons link you up with pertinent information in other parts of the tool to help you understand why a particular item is on the list.  A list of precautions to be considered when executing a test on a given system element is also included.

If you select the “Test Directory” button from the Checklist Tool and Test Directory home page, you are taken to a page that provides access via links to all of the tests contained in the tool.


If you over over a link and right click it and pick the “Save target as” option …

Save Target

… a window will open up and allow you to save the selected checklist to your hard drive as a Word or Excel file.   From there, you can open it up and edit it as needed to match your specific system requirements and reflect your company’s logo and other standards.  The screen shot below illustrates what you get if you open the Cooling Tower Prefunctional Checklist (test ID #78) from the table in the previous image, which is in the form of a word document.


(Back to Contents)

Exploring the Control System Design Guide

The control system design guide evolved with the functional testing guide because having a working, well designed control system is essential for delivering a functional building system.  In addition, testing the control system is always going to be the first step in testing the controlled system for any functional testing process .

When you click on the “View online” button for the Control System Design Guide, the following page will open up in your browser which provides access to the content.


If you open the How to Use the Design Guide module, it will give you a good overview of what the different chapters are about.  So rather than repeat that, I will just highlight a some of the features that are incorporated in the guide that I think are really useful.

(Back to Contents)

Valve and Damper Schedule Spreadsheets

If you explore the Control System Design Process module, in addition to learning about some of the challenges associated with designing control systems, you will be presented with ways to address those challenges, including some tools like spreadsheets for sizing control valves and control dampers and developing control valve and control damper schedules as you can see in the screen shot below.


If you hover over the Link to Valve Schedule Spreadsheet button, right click, and then pick the “Save target as …” option, you can save a copy of the spreadsheet on your hard drive.

Once it is there, you can open it up and edit it to your heart’s content to match your personal likes and company standards.  Nothing is locked and everything is totally editable;  why you could even make it pink with purple polka dots if you wanted to.


If you “play” with the spreadsheet a bit, you will discover that the valve sizing equations are built into it.  Meaning that if you fill in things like your targeted flow rate in gpm and your targeted pressure drop in psi, then the spreadsheet calculates the targeted Cv for the valve.  A similar tool is provided for damper schedules.

(Back to Contents)

Sensor Selection and Installation Guidance

As you can see from the screenshot below, Chapter 3 contains a lot of information to help you make decisions regarding the sensors that will feed data into your control system.


The information is presented in both a narrative and tabular format providing guidance regarding the pro’s and con’s of different technologies, and the accuracy to target for different applications along with application specific installation and calibration recommendations.


(Back to Contents)

System Configurations and Recommended Point Lists

The 4th chapter of the CSDG focuses on 15 common air handling system configurations and the recommended points for each one.


To me, the point list tool that is included in this section (and also linked up in Chapter 3) is one of the most valuable tools in the guide.  In my opinion, if you were to provide a point list with the features of included in this point list tool along with an outline narrative control sequence for your projects, you will have taken a huge step forward in terms of improving the quality of your control system design.

Slides 41-47 of a presentation I did for the Oregon ASHRAE Control Trade Show will give you some insight into why I say that.  And you may also find that presentation to be useful as a general introduction to the concept of control system design and commissioning.

In any case, if you save the spreadsheet to your hard drive and open it up, you will find something like this, which is an open, fully editable tool, just like the valve and damper schedule tools I mentioned previously.


As you can see, the spreadsheet has a tab with a recommended point list for each of the system types described in the guide.   The yellow column is something you would not publish with the point list if you used it for a project.  But it provides guidance for you as the designer and specifier regarding if a point is absolutely essential or could be optional if budget was an issue.  This is accomplished by referring you to notes in a supplemental html file that is also included via a link in the guide.


(Back to Contents)

So there you have it, an overview to the Functional Testing and Design Guide along with a link for accessing it from FDEs commissioning resources website.


David SellersPowerPoint-Generated-White_thumb2_th
Senior Engineer – Facility Dynamics Engineering     Visit Our Commissioning Resources Website at

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