Calculating the average pressure differential

Yes- I would say they are looking for average over time which factors in different occupancy schedules and system modes. I would think that minimum would occur at times when the system is setback.

I have used an ASHRAE equation for this. See 2007 HVAC Applications handbook, page 52.5, eqn 8. Given airflow (room exhaust) and flow area (door undercut or other inlet), you can calculate differential pressure.

I am also confused on this credit. It still doesnt seem like anyone knows exactly what LEED is looking for pertaining to Average Pressure Differential and Minimum Pressure Differential (with doors closed) and how to calculate the values.

Any thoughts on this?

In one our projects, the M&E team have used the ASHRAE equation Sarah is referring to (i.e. Q= 776CA √2∆P/p).

I am guessing that the equation above gives us the average pressure differential. But how can we get the minimum pressure differential?

Anybody ?

Has anyone used the ASHRAE Applications equation to calculate the average pressure differential and had it approved by USGBC? I think Eqn 9 in the 2007 ASHRAE applications (Q=2610*A*√∆P) is appropriate. I've found that this equation corresponds to Figure 2.2 (Relationship between Airflow Offsets, Differential Pressure, and Effective Leakage Area) of the Health Facility Commissioning Guidelines book.

Also, my systems are either running continuously, or they are off, so I am assuming my minimum pressure differential equals the average pressure differential. Has anyone filled out the form to show that average pressure differential and minimum pressure differential are the same, and had it approved by USGBC?

Other thoughts for how to calcule the minimum pressure differential would be appreciated. Thanks.

The ASHRAE equation above is for Pressure (∆P) in "Inches of water." The equation for pressure in PA would be:

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 this room has a negative 5 PA differential pressure to an adjacent space you need to have roughly 50 CFM 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 minimum pressure differential values are the measured values? If yes, how to document these values at design phase?

Thanks

No the minimum pressure differential is a calculated value base on the design.

Thanks Dylan for your reply. If minimum pressure differential values are calculated using this equation
Q=165.5*A*√∆P by considering door closed.How will be average pressure differental values calculated?
Thanks!