System Diagrams: The Envelope as Part of the Air Handling System

In my previous post, I mentioned that in my perspective, the building envelope is a part of the air handling system.  That concept is one of the drivers behind how I draw air handling system diagrams.  My bottom line there is that the envelope is what the outdoors from the indoors;  the uncontrolled environment from the controlled environment.

Having said that, bringing in air from the outdoors, treating it, and distributing it through-out our buildings is an important part of how we keep them safe and confortable.  A well thought out, well implemented, and well maintained envelope can make managing that process much easier than it is when the envelope has issues.

So, to provide a foundation for my next post, which goes into some of the features I incorporate into my air system diagrams, I wanted to spend a bit of time discussing envelope issues and the challenges they can represent for building operations.  I will also discuss how we generally use outdoor air in buildings.  I believe such an understanding will help clarify why I do some of the things I do when I develop an air system diagram.

The picture below, taken during the early phases of the construction cycle for a high rise project up in Seattle, illustrates some of the envelope issues that can potentially impact an air handling system.

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By their nature, envelopes are full of thermal bridges (red arrows) and gaps (blue arrows) that need to be properly treated to keep the environment inside separate from the environment outside. If they are not properly treated initially or properly maintained over the course of time, then operating challenges can present themselves.

Envelope Leakage and its Impact on Air Handling System Operation

To illustrate the impact of envelope issues on air handling system performance, lets look at a relatively new high rise I worked on during my tenure at PECI, along with Hannah Friedman and Larry Luskay.  The building had major breaches in the envelope that manifested themselves as occupant discomfort, frozen pipes and the inability to pressurize the lobby among other things. These problems were significant in the relatively mild Portland, Oregon environment. So you can imagine what they might have been like in a more extreme environment.

To try to quantify the problem, we used the building’s air handling systems to perform a sort of “blower door” test.  The test we ran was based on the ASTM standard E-779-99 test procedure, but was streamlined to allow us to run it in a fairly limited time frame.

When we performed the test, we discovered that the envelope leaked in the range of 250,000 – 300,000 cfm. And, even at that leakage rate, with no relief dampers open, no doors, or windows or loading dock doors open, etc., we were not able to force the neutral plane in the building down below the 2nd floor.

That test led us to an insight and an energy savings opportunity. Specifically, we reasoned that up to a point, we could use the leaks in the envelope as a relief air system during the economizer cycle. By doing this:

  • We saved fan energy because we could shut down the return fans part of the time.
  • We saved perimeter heating energy.

To understand these opportunities, we need to understand the ways we use outdoor air in buildings.   Typically, we introduce outdoor air into a building for three purposes.

  1. “Free cooling” via the economizer process.
  2. Ventilation to control contaminants.
  3. Air to create pressure relationships, either between adjacent spaces or between the interior and exterior of the building.

Items 2 and 3 will need to be provided any time the facility is in normal operation to keep it safe and comfortable (the bottom line goal for our HVAC processes).  The outdoor air brought in for an economizer process is above and beyond the outdoor air introduced for ventilation and pressurization.

Air for Economizing

As most of the people reading this blog know, large buildings require cooling for the core spaces year round because things like lighting, office equipment and machinery are constantly generating heat.   And, unlike the perimeter spaces, there is no loss of energy to the outdoor environment during the winter.  Economizers attempt to minimize the need for mechanical cooling by moving outdoor air through the core of the building to offset these gains when possible. 

We call this “free cooling” and it is “free” in the context of what it would cost to provide the cooling using machinery.  But, as most building operators will tell you, the process is not with out a few costs of its own, including complexity and the related maintenance costs, and possibly, higher filtration costs.  But that is a different topic.

My point here is that the amount of outdoor air introduce by an economizer cycle varies with the cooling load. When you implement they cycle, it’s important to realize that by bringing in this extra air to perform a temperature control function (cooling), you create the potential for a building pressure control problem; i.e. if you don’t provide a way for it to exit the building, you will over-pressurize the building, which can cause a number of problems.

  • At pressures above about 0.10 to 0.15 inches water column (in.w.c.) for the cross-sectional area represented by most doors, you will exceed the allowable door opening force associated with ADA (Americans with Disabilities Act) requirements. The typical value I run into is 15 pounds for exterior doors, but I believe that this is ultimately set by the local code authority rather than the actual ADA standards.
  • If you exceed the door opening force and the door opens outward, you will blow the doors open, which can create a security problem.
  • If you exceed the door opening force and the door opens inward, you create the potential for people not being able to open the door.
  • You may not achieve the required flow rate because the flow through the system is restricted by the lack of a relief path.
  • The pressures variations created by an economizer equipped system can interact with other systems serving the facility and cause operating problems in those areas.

The traditional approach to an economizer design assumes that the building is air tight, meaning that the design of the system needs to include a relief path. Typically, that translates to a relief louver or relief hood with some sort of damper arrangement that is coordinated to work with the economizer, either directly by following the same signal as the economizer dampers, or indirectly by following some sort of proxy or indicator of the activity of the economizer process, like building pressure.

There may or may not be fans involved;

  • For a small facility with short airflow paths, the supply fan may be the only fan necessary.
  • For a larger building with a short, un-constricted return airflow path but a long and/or constricted relief air flow path, a relief fan may be required.
  • For a larger building with a long or constricted return air flow path, a return fan may be required and the relief system is usually on the discharge side of the return fan.

For the building I have been discussing (and for many buildings) the assumption that the structure is air tight is not a particularly good one.   But for Larry, Hannah and I, that shortcoming lead us to an opportunity to shut down the return fans some of the time, which I will discuss in more detail in a minute. 

    Air for Ventilation and Pressurization

    In contrast with the economizer process I have been discussing, bringing air in to ventilate or control pressure relationships in a building is not an option if you are going to keep the building safe and comfortable.   Typically, the ventilation air is removed  via some sort of forced exhaust process, like the toilet exhaust system or a hood exhaust system.

    In some facilities, additional air may be brought into the building to ensure certain pressure relationships are maintained. For instance, a designer may set up a system serving a lobby so that it brings in extra outdoor air to minimize infiltration through the lobby doors.

    Ultimately, you need to design and operate the system so the ventilation and pressurization air is introduced under all operating conditions. As a result, you will need to treat this air, cooling and dehumidifying when it is hot and humid outside and heating and perhaps humidifying it, when it is cold and dry outside and filtering it no matter what.

    All of these processes represent an resource consumption requirement for the facility on a number of fronts.

    • Electrical and thermal energy will be consumed to heat, cool, humidify, and dehumidify the air.
    • For desiccant dehumidifiers, thermal energy is typically required to renew the desiccant as a part of the ongoing process.
    • Water will be required to humidify the air or if an evaporative cooling process is used.
    • For some humidifier and evaporative cooler designs, there will be other consumables like electrodes and evaporative media that must be replaced occasionally.
    • Humidifier and evaporative coolers may also have some water treatment costs in the form of chemicals or UV lighting associated with operating them.
    • Desiccant based processes often need to have the desiccant replaced after some period of time.
    • Generally speaking, the more outdoor air you handle, the more you will need to spend on filters.  This is a topic all to itself and if you want to know more, you can tune into a series of three video clips on YouTube where I discuss operating filters on a life cycle cost basis vs. time in service.  (The preceeing link takes you to the first video;  the second is at this link and the third is here.)
    • Labor is required to operate and maintain all of this equipment
    • A waste stream is generated by the depleted resources like dirty filters or the blow-down water from evaporative cooling processes.

    The bottom line is that handling outdoor air represents a significant operating cost in terms of energy and other resources.  That’s one reason why documenting how a system is using outdoor air on a system diagram is important.

    Leveraging the Air Handling System Operating Sequence to Address an Envelope Leakage Problem and Save Fan Energy

    In the example building I am using in this post, the air handling systems and their associated return fans are located on the 8th floor and the 16th floor. Air is returned up a shaft and then is either recirculated or ejected from the building at relief louvers on the mechanical levels.

    As a result, even though the leaks in the building that we documented would have been able  to get rid of a lot of the relief air associated with an economizer cycle, they were not “allowed” to do it because the return fans and relief system operated to bring all of the air back to the equipment room and relief louvers to get it out of the building.

    Once we realized how leaky the building was, we realized that we might be able to leverage that feature to save some  energy.  In general terms, the sequence we ended up with was something like this. 

    • We didn’t start to open the relief dampers until we saw a positive building pressure.  To do this, we were looking at data points in the lobby, about half way up the building and at the top of the building because there were doors to the exterior at all of those locations and we hoped to be able to maintain positive pressures at all of them. 
    • In addition, we didn’t start operating the return fans until at least one of the building pressure data points  we were monitoring reached a level beyond which where we were going to have problems with the ADA requirements.   We were also concerned about the whistling sound  that can be generated by air going out through the cracks around the door. 

    The noise issue showed up on the upper floor during some seasons before we reached a pressure that was a code problem.  It seemed to be  some combination of the physics of the door and its frame, stack effect, and tolerance of the people sitting in the immediate area.

    Making these changes allowed the relief air to leak out the cracks in the envelope rather than forcing it back to the equipment levels to go out the relief louvers and help keep things more comfortable.

    Leveraging the Air Handling System Operating Sequence to Address an Envelope Leakage Problem and Save Heating Energy

    The perimeter heating energy savings mechanism associated with the improvement I have been discussing is a little less obvious  than the fan energy savings until you realized that because of the holes in the envelope, before we made our changes, the neutral plane in the building tended to hover between the 6th and 8th floor.

    When we started to use the cracks in the envelope as the relief system, we forced the neutral plane down to between the lobby level and the 2nd floor most of the time. As a result, floors that used to experience infiltration at the perimeter (the floors below the neutral plane during the winter months) were pressurized.

    When the floors where negative, air that leaked in at the perimeter represented a heating load during the winter months that generally was picked up by the zone control equipment serving the area where the infiltration occurred.  Pressurizing those zones changed things a bit.

    Specifically, with the zones pressurized, warm air from the building leaked out instead of cold air from the outside leaking in.  But, the trick was that the warm air that leaked out was the relief air from the economizer cycle. And that air was actually air from the outside that was introduced into the building by the economizer process to provide “free cooling” for the core of the building.

    That meant that the internal gains in the core had warmed the air up rather than some sort of active heating system.  Since the air was warmed up by the internal gains, the perimeter system did not have to provide the energy, thus the savings. 

    In other words, in addition to providing free cooling in the core, the economizer was providing a form of heat recovery to handle the perimeter infiltration loads.  As long as we brought no additional air into the building to perform this function beyond what the economizer process was bringing in, we saved energy.

    More on Envelopes and Related Commissioning Issues

    For those who are interested in more information on envelope issues from a field perspective, I was a co-author of an ACEEE paper on the topic, which I have placed in my Google Drive folder so you can access it.  The paper looks at a number of commissioning issues that I encountered over the course of my career which were envelope related. It also includes additional references on the topic.

    In my next post, I will return my focus to air handling system diagram development, illustrating some of the key features I incorporate into the diagrams I make.

    David Sellers, Senior Engineer – Facility Dynamics Engineering


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