Positive Building pressure
Positive Building pressure
(OP)
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
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





RE: Positive Building pressure
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.
RE: Positive Building pressure
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.