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).
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 …
… 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] …
… 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 22.214.171.124.1 – 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..
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.
[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 http://www.av8rdas.com/honeywell-gray-manual.html.
[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 126.96.36.199.2 VAV Static Pressure Sensor Location, page 235.
[vii] ASHRAE Guideline 36-2018, paragraph 5.1.14 Trim & Respond Set-Point Reset Logic.