Outside Air Calculation for Office

[sfxf]

In my project, one new Air Handling Unit is to serve several conference rooms and large restrooms nearby. The outside air for the conference rooms is based on 20 cfm per person as per ASHRAE standard 62. My question is: How should I decide the outside air requirement for the restrooms? Can I estimate the occupancy in the restroom (the worst case - assuming the restrooms are full) by counting the fixture unit? I am concerned that if I do that, some people may be counted twice because they may be the same people in the conference room?

Thank you in advance for help!

 

[cme]

First off, if the conference rooms are used only for an hour or so, the ventilation rate can be reduced by 1/2 resulting in 10 cfm per person. I would have to read ASHRAE 62 and local lodes again to see where it says that but I believe it says it somewhere. Check it out.

You will find a very high ventilation load from the conference rooms so I would suggest doing the ASHRAE analysis with the X, Y and Z factors.

For the toilet rooms, I use 75 cfm per water closet or 2 cfm/sq ft exhaust whever is greater. Then I supply 1 cfm/sq ft or use transfer air.

 

[VEEKRISH]

sfxf, i would suggest the following -

Calculate Fresh Air based on ASHRAE Standard 62 @ 20 CFM per person for all conference areas.

Then for the rest-rooms, calculate at the rate of 1.5 CFM per sq ft. Ensure that air is not returned back from this space. provide Exhaust @ 2 CFM per Sq Ft for rest room. This will ensure that this area is maintained under negative pressure.

This way you will not reckon the same occupancy twice.

 

[ChasBean1]

sfxf,

ASHRAE in the U.S. (MA) is not code but defines good engineering guidelines. In my area, toilet exhaust is defined. The Massachusetts State Building Code (780 CMR, Sixth Edition, 1997) referencing the Building Officials and Code Administrators (BOCA)-1993 indicates that each w.c. (each crapper, urinal, etc.) shall have 75 cfm exhaust/make-up. Designers here have leeway in HVAC, but not for toilet exhaust volume.

Restrooms don't implode from suction, so the 75 cfm exhausted is 75 cfm made up from other sources, usually transfer grilles, undercut doors, and in some cases, make-up air. So long as the make-up volume is less than the exhausted volume to keep the room negative, you will be fine. But generally, I would recommend exhausting 75-100 cfm per w.c. and making up that air via transfer (no mechanical supply). -CB