Positive Building pressure

[Badbird2000]

Hey guys, sorry if this has been covered before. It comes up from time to time on jobs i work on. I have an elementary school addition. We are positive by 2,300 cfm after all the exhaust and o.a. as required by the student loads. I swear i remember doing a calculation years ago for building pressurazation but can't find anything. How do you calculate how much positive pressure you should be for a building? I know it depends on how many doors you have, windows, the volume of the building, etc.

Am i just making this up in my head? I worked in Florida for 16 years, and we always made sure we were positive by quite a bit. Keep out humidity.

Thanks guys!

Wayne

 

[MTES93]

I am not sure you can accurately determine what it will be... normally you can control it reasonably well to 0.05 to 0.08 inches I suppose w/ your mechanical system (depending on what it is). As you mentioned you will have doors, windows and the general tightness of the building that causes variable leakage rates. Too much positive air pressure can cause door closure issues...

Not sure if using air pressure to control humidity works great as the vapor pressure difference between humid outside air at temperature X and relatively dry indoor air at temperature Y is typically much much greater than you can achieve w/ air pressure alone.

 

[ChasBean1]

Volume doesn't matter, it's just the net open area for leakage - but usually with a higher volume there is more net open area at the envelope. The estimation I've brought up in the past (from ASHRAE Applications) is:

Q = 2610 x A x (DP)1/2

• Q is the difference in supply and exhaust flow rate in cfm,
• 2610 is a conversion factor,
• A is the net open crack area of the room (in square feet, generally not measurable), and
• DP is the differential pressure in inches water column.

There is no building pressure rule of thumb nor should pressure be a design criterion for a typical building. A good rule of thumb is about 5% positive ventilation – 5% more total air in than out. Resulting pressure depends on tightness of the building. It could be +0.003” or +0.03” or neutral if there are some open windows.