System Diagrams: Key Features of my Air System Diagrams

So, at this point, you may be wondering what the connection is between the envelope discussion in the previous post and Air System Diagrams, which is the theme for the current string of posts. To illustrate that, I am going to use the sample system diagram from the post where I brought up the air system diagram concept, and then highlight certain features. The colored boxes are areas I will blow up to illustrate various things.

Sample system diagram v2 r1a details_thumb[1]

As a reminder, if you want a larger copy if the system diagram, I have included links to a .pdf version, a .dwg version and a .dxf version in the previous post.

Using the Physical Arrangement of the Building as a Background for the System Diagram

One feature you might notice in the example is that the system diagram is laid out against a background that reflects the actual physical arrangement of the building.

  • The air handling system is in a basement mechanical room, so in the system diagram, it shows up below the occupied areas, except for the warehouse office area, which is also in the basement.
  • There is a shaft that runs up through the building that provides a path for return air, and houses the supply and outdoor air ducts. The outdoor air intake louver is on the exterior wall of this shaft.
  • The mechanical room that the air handling unit is in acts as a relief plenum and there are louvers on the wall that relieve air to the exterior of the building.

Buildings are very complex structures and generally, it is not possible to accurately depict every nuance of their physical arrangement in a two dimensional schematic  like a system diagram where there are other factors (like “untangling” the system) driving how you lay out the drawing. Having said that some aspects of the building arrangement and the juxtaposition of the system in that arrangement may be desirable to capture if possible.

For instance, in the diagram above, the arrangement of the drawing documents schematically that the relief location is separated from the intake location.  Its shown on a different level and different face of the building, reflecting the physical reality. This is an important detail to document in terms of the ability of the economizer to deliver its design intent. If the intake and relief louver where physically adjacent to each other (introducing the potential to recirculate relief air instead of bringing in fresh outdoor air), I might have tried to structure my drawing to illustrate that since it is a potential operational issue.

How far you go to capture the physical arrangement of the building in your diagram is a judgment call. Typically for me, it depends on what I am trying to do with the drawing in terms of documentation and diagnostics. Usually the concepts of untangling things and getting the order of connection right will win out over reflecting the details of the building’s physical arrangement accurately. However, in a subsequent post, I plan to discuss an example of an air system diagram where the details of the building’s arrangement were critical to diagnosing a problem, so stay tuned if you want to know more about this aspect of drawing development.

Include an Example of Each Zone Type

Air handling systems can have tens or even hundreds of zones. Depicting each and every one of them on the system diagram may not be worth the time it would take or necessary for the drawing to be useful. However, including each type of zone is important because it allows you to document the various zone control strategies, pressure relationships, and other important details.

For the system in our example, there are not that many zones, but there are a number of zone types so an example of each is included in the diagram. For zones that are typical of many, the diagram labels them as such. You could even include a note that indicated how many zones each typical  type there were, the total flow the represented, the room numbers associated with them, etc. and provide a bit more information that way.

Each of the zone types in my diagram has some feature (or features) that make it slightly different from the other zones in terms of how it operates. The diagram attempts to illustrate these features.

Warehouse Office Zones

The warehouse office zones are pressurized slightly to minimize the infiltration into the area if the doors on the loading dock are open.

Sample system diagram v2 r1a Office_thumb[1]

The diagram highlights this feature and even calls out the design exfiltration flow rate. Sometimes this parameter is indicated directly on the contract documents. But sometimes, it is only implied by the difference between the supply and return flow. And sometimes, this desirable feature may not be included in the design. But bottom line, the amount of air, if any, that is provided for pressurization is an important parameter for the operating team to know about.

Museum Zones

The example building also included a small museum area housing historic artifacts from the early days of the University it is associated with.

Sample system diagram v2 r1a Museum_thumb[3]

The environmental control system in the museum area includes some enhancements that are targeted at preserving these artifacts, including constant air flow, higher levels of filtration, and humidification. It is also designed be pressurized to minimize the impact of adjacent zones and the outdoor environment on the higher quality environment that is targeted for the space. All of these features are documented on the system diagram.

Snack Kitchen Zone

The primary feature that distinguishes the snack kitchen from the other zones is the exhaust hood it contains.

Sample system diagram v2 r1a Kitchen_thumb[1]

Documenting this and identifying that some of the make-up air for the hood will be drawn from adjacent areas highlights the designers intent to manage cooking odors by pulling air into the space.

Student Union Zones

The VAV reheat arrangement serving the student union area is not particularly unique.

Sample system diagram v2 r1a Union_thumb[1]

But there are a number of zones that are like this in the facility and documenting them is important in addition to documenting the zones that have unusual or unique features.

Student Lounge and Lobby Zones

The distinguishing feature of the lounge area is the large amount of air provided for exfiltration in an effort to keep the lobby and adjacent lounge comfortable despite the anticipated heavy traffic rate through the lobby entry.

Sample system diagram v2 r1a Lounge_thumb[1]

It also is the location from which air is transferred through the restrooms to provide the code mandated exhaust flow that is required.

In this particular facility, the potentially high student volume in the union and lounge area dictated a high ventilation rate, which translates into a high minimum outdoor air flow rate along with a commensurate exhaust flow rate to satisfy conservation of mass and energy (the goes inta’s gotta equal the goes outa’s as my mentor Al Black would always say). The designer leveraged this requirement to ventilate and condition the restrooms with out having to provide a separate zone of control for them.

Restrooms typically have low internal gains but high exhaust flow requirements that are usually related to the number of fixtures they contain. By transferring a large volume of air from the adjacent spaces into the rest rooms and then out through the toilet exhaust system, the exhaust requirements are more than satisfied, the ventilation requirements for the high occupancy areas are met, and the restroom temperatures are maintained at a value that is only a bit above the areas from which the air was transferred.

This nuance of the design is captured by the information presented in the system diagram, documenting it for the operating team.

Illustrate Impacts from Other Systems

If you look closely at the museum zone, you will notice another feature I like to include in my system diagrams.

Sample system diagram v2 r1a Museum Bubble _thumb[1]

Specifically, the architect incorporated a lot of exterior glass into the museum to create the ambiance they were seeking. In a Midwestern environment, large expanses of glass can feel very uncomfortable on a cold winter day. To combat this, the designer included finned tube radiation along the perimeter below the glass.  This will bath the glass with a layer of warm air and as a result, people in the space will feel warmer than if the glass was not treated this way. The bottom line is that the heat from this system will complement the heat from the air handling system reheat coils and help keep the space comfortable.

But, if the finned tube elements are not properly controlled, they could also “fight” with the cooling provide by the air handling system when the weather is warmer. Documenting the finned tube system on the air handling system diagram serves as a reminder of the design intent (comfort during extreme weather) and the potential for unnecessary simultaneous heating and cooling.

Document Conservation of Mass and Energy

In studying the diagram and reading through the discussion up to this point, you probably have noticed another key feature that I like to include in my air system diagrams. Specifically, they illustrate conservation of mass and energy.

Slide Show 9122012 91959 AM.bmp_thumb[1]

The laws of physics will govern how our system operates, so depicting how that will successfully be accomplished is a desirable feature for our air system diagram. And, if you are developing an air system diagram and can’t depict conservation of mass and energy, then you probably have discovered a problem that is either puzzling everyone else or will become a puzzle when the building comes on line.

Either way, you area in a position to be a bit of a hero because you will be able to unravel the riddle, thanks to your system diagram.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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