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Maintaining Positive Pressure inside an Office Area

Maintaining Positive Pressure inside an Office Area

Dear Friends,

I am designing an Office Space air-conditioning system with the following estimates:

Cooling Load : 25 Ton
Conditioned air flow into Office space: 10000 CFM
Fresh air supply: 1000 CFM (ambient air from outside for maintaining IAQ - added in the mixing plenum of the return air from office)
Toilet Extract: 1000 CFM (transfer via Toilet door louvers)

But the specification requires the office space to be maintained at 5 Pa pressure difference with respect to the ambient.

Would anyone please suggest me some guidelines to achieve the intent?

Thanks in advance for the support.

RE: Maintaining Positive Pressure inside an Office Area

It depends strongly on how much leakage the room has through windows, doors and etc. You are already positive in the space.

Measure the pressure inside and out side the space with all doors and windows in the space closed. This is where to start.

RE: Maintaining Positive Pressure inside an Office Area

You have 1000 cfm exhaust so you should have about 1200 cfm makeup.

There are no guidelines but meeting the 5 Pa intent could be adjusted by changing your makeup vs. exhaust ratio.

RE: Maintaining Positive Pressure inside an Office Area

Provide space pressure sensor that would modulate RA/OA dampers to achieve space pressurization set point.

RE: Maintaining Positive Pressure inside an Office Area

Thanks @willard3, @ChasBean1, and @Cry24

RE: Maintaining Positive Pressure inside an Office Area

ASHRAE Handbook, Fundamentals, Chapter 16 "Ventilation and Infiltration" has a lot of information on this.

With 1000 cfm ventilation and 1000 cfm exhaust, your building is essentially neutral, although you'll still get infiltration due to effects like wind pressure. Also if you don't have dedicated equipment or fans providing the outdoor air, or reheat, your building pressure will swing negative when supply air fans shut off or slow down due to the reduced fresh air intake.

RE: Maintaining Positive Pressure inside an Office Area

If you read ASHRAE and codes you will find that the given values for outside air and exhaust air are minimums. It never hurts to exceed these values. I recommend exceeding them anyway by about 10% to allow for real world conditions in which you will never get exactly 1,000 CFM of OA or EA.
Also to maintain positive pressure, I typically design to provide 10-20% more OA than EA. Will this result in the 5 Pa difference? I don't know, it depends an the variables listed by Willard. I don't usually have the time to analyze things to that degree.

If you will have intermittent exhaust or you want to be super critical of maintaining that pressure difference, then using indoor/outdoor pressure differential sensor with variable dampers/VFDs is the way to go.

RE: Maintaining Positive Pressure inside an Office Area

Thanks @FL Engineer; @dbill74

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