# Controlling building pressure in high rises



## HCTXFPMCJC (Mar 30, 2012)

Hi, I'm a newbie here, but not to the trade, after 25 years in HVAC in different places around the world I find myself now managing a high rise in Houston TX. now those with there fingers on the pursestrings, in an attempt to save on power wish to control building pressure using VFD's on the exhaust fans, a target differential of 0.010 "WG positive has been deemed appropriate. I have done this myself many times on single story buildings on oil refinery's and negative buildings such as Laboratories and Hospitals, but this high rise is proving to be different.

When this particular building was built positive pressure was calculated simply as 4 x 35K CFM outside AHU's, minus 4 x 25K CFM exhaust fans, minus 20K CFM total toilet exhaust fans = a positive pressure, and that was about it, but not good enough now.

We have been experimenting with placement of indoor and outdoor sensors to obtain an accurate reading of differential pressure from which we get an anologue input with which we can control the VFD's, the difference in natural outside barometric pressure between ground level and 325ft was the first stumbling block we came across, where to put the outdoor sensor? roof and ground level being the only options, then we found that taking an average of the inside pressure, sampling floors 20, 11 & 3 resulted in a sampling tube so long that pressure drop came into play, causing further inaccuracy.

So my questions are these, how is it done at *your* other house? or is it done at all? are the problems I'm coming up with *the reason* it's not done? or am I just missing something entirely? Thankfully they let me experiment on this before buying the VFD's, thank you.


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