Average Pressure Differential - New LEED Online Template

You are on the right track. Although, depending on the space I might consider the average to be with load in the space if that is what occurs most often.
Another thing to consider with minimum Pa would be: If you are exhausting a Janitor's closet using the same exhaust fan as the bathrooms, then the fan could have night time set back, but it would need to maintain a minimum exhaust rate in the space.
Also, don't forget the door also needs to be self-closing to achieve this credit.

Hi everyone. How could I get the pressure differential in Pa? Right now I only have the air flow in cfm inside and outside the copy room. What formula should be used? Is there a need for a test to get that in Pa?

Bobby,
I answered this on the NC section, so I'm copying the formula over:

Assume the door is closed.

An ASHRAE equation for Differential 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
Solve for Q (Air Flow in CFM)
Q=165.5*0.125*√5 = 46.3 CFM

In order to ensure a 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.

A test is not required, this is a design credit.

Dylan:

What is meant by "Average Differential Pressure"? In my case, i have 4 janitor's closets and a recycling room on the same fan. There is no supply air served to these spaces, all make-up air is brought in from the undercut of the door. The fan will operate continuously so using the equation above for both the minimum and average differential pressures will yield the same result which is under 5 PA but more than 1 PA. is there a different equation to use or different parameters?

Thank you for your time in advance.

Stephen,
Average is sort of your typical case. It sounds like your average and minimum are the same case because your system is constant volume.
It is perfectly fine to use the same fan for the 4 janitors closets and recycling room. Just indicate what each space should be balanced to on your plans based on meeting 5 PA all the time.

Dylan,

Thank you for getting back to me. I just want to make sure my numbers seem right, because to me (a complete novice at LEED) they seem high. I have a 3 foot door with a 0.5" undercut. I am exhausting 150 CFM from the room. my delta P is coming out to be 52.5 Pa. Does that number seem too high to you?

Stephen,
You used the equation correctly. 150 cfm for a janitor's closet is a bit higher than typical, but it depends on the size of the closet. Typically they get about 1 cfm/sf.

Where exactly is the ASHRAE equation found? (Q=165.5*A*√∆P)
Is this equation specified in the LEED references?

John,
The equation comes from ASHRAE Applications in the 2011 book it is on page 53.5 (Fire and Smoke Management).
It's been converted from in. of water to Pa to be in the present form above.

LEED does not specify how you are to calculate differential pressure, but I've never be questioned while using that equation.