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Look for a copy room or janitor's closet. You want to find the amount of air that is regularly exhausted from those rooms. If air is also supplied into the space you need to subtract that number to find the total exhaust.

An ASHRAE equation for Pressure (∆P) in PA is:
Q=165.5*A*√∆P
Here is an example scenario:
3 foot wide closed door with a 0.5" undercut
A=0.125 ft
∆P=5 (required for LEED)
Solve for Q (Air Flow in CFM)
Q=165.5*0.125*√5 = 46.3 CFM
In order to ensure this room has a negative 5 PA differential pressure to an adjacent space you need to have roughly 50 CFM total exhaust (Exhaust CFM - Supply CFM). Area of the room is irrelevant for this equation but for LEED and local building codes you may also need to ensure that you have at least 0.5 CFM/SF exhaust.
The door also needs to be self-closing to achieve this credit.

Hi Dylan,

from this ASHRAE equation: Q=165.5*A*√∆P

What is A? and is it a variable?

Q=VA; flow = velocity * area; A is area of door undercut. 3feet * 0.5"/12inch/ft) = 0.125 feet.

Hello Dylan & Michelle,

Thank you both for the answer provided.

1. Can i have the exact source where you got the equation?

2. If i do not have undercut door then how can i calculate the CFM requirement for delta P=5 pa? Also, if i do not have undercut door and no way to supply air to that room but only exhausting air, isn't it a violation of code (ASHRAE 62.1)?

3. How do i maintain minimum 1 pa pressure differential in the same room while average 5 pa pressure differential condition still in place?

Waiting for your response.

Thanks
SM Rahman

1. I believe the source is the ASHRAE fundamentals book. I don't have the book on me to check where exactly it is. Why?
2. You are correct - if you don't have a way for the make up air to get in - you are not providing fresh air - you are creating a vacuum in the room.
3. You can have a variable exhaust system so you can go between a minimum of 1 pa and average 5 pa or just do a constant exhaust at 5 pa.