infiltration as make up air

[russmech]

Has anyone come up with a good way to tie an infiltration calculation (such as the ASHRAE "crack method&quot to the requirements for make up air due to a building's exhaust system? UBC gives an exception for combustion air if the room is large enough, but I've never been able to dream up a good connection between that line of thought to an actual CFM of make up air required for an exhaust system. The code inspectors in my area are real sticklers about this subject, but may listen if I can come up with a mathematically logical line of reason.

 

[fredb]

Hello russmech

The reason infiltration is not considered a dependable source of make-up air is because the flowrate can vary considerably depending on outside wind conditions. There are some cases documented where wind pressure has caused vacuum conditions to occur which not only will not allow air into the building but will actually draw air from the building.

My advice, you can't make a good case for infiltration is a dependable source of air is required.

 

[ChasBean1]

Russmech, I don't think I fully undersand what you are trying to do, but the requirements for the makeup air are typically to supply the exhaust volume or slightly more. If you know the building exhaust and supply volumes and the exhaust exceeds the supply, you can reasonably assume that the infiltration volume is the difference between the two.

Building pressure will vary with tightness by:

Q = 2610 * A * dP^.5

where Q is the flow in cfm
A is the net open crack area in ft2
dP is the differential pressure, in inches of water

Regardless, if the flow is known and stablized by use of VAVs/CAVs with flow stations, the difference in cfm should be the infiltration.

If you don't know these volumes because you don't have a recent TAB report or design drawings, a tracer gas test is the best option - apply sulfur hexafluoride (or other tracer) to the building to achieve a known concentration and monitor the decay concentration at certain intervals to determine building air exchange. This air exchange should coincide with known mechanical outside air. The offset between the ventilation results of the tracer test and the mechanical air intake volume will be the infiltration.

Those are some general notes about the subject, but I'm still unsure about your specific question. If you've recently installed a combustion appliance and are trying to circumnavigate code for make-up air, I recommend (for your own protection) to ventilate as prescribed, especially if you have no idea how tight the building is. These inspector-types are "sticklers" because people have died from messing up combustion make-up air.

Anyway, I hope some of these thoughts help. -Chas

 

[DRWeig]

Hi Russmech!

Pay attention to Chas's comment about liability. While trying to make up combustion air this way can cause one set of problems, doing so with exhausted occupied spaces is equivalent to tampering with indoor air quality. It's way better to mechanically make up slightly more than you exhaust to that you keep a little positive pressure in the occupied areas. This way, your OSA load is handled at the coil and doesn't contribute to controlled space conditions.

That's my two cents' worth anyway.

Best of luck with your project!!

Old Dave