I have a small tenant space in a large high rise commercial building. 100% outdoor air is supplied down a shaft, each floor has a main heat pump which mixes that outdoor air with return air from the floor (40% OA, 60% RA). That mixed air is conditioned by the main floor heat pump and supplied to our space (the sensor for the main heat pump is located within our open office space). The interior of this space (mostly open office) has diffusers serving it directly from that supply duct, while the perimeter spaces (2 conference rooms and some perimeter offices are fed from smaller heat pumps located above the ceiling with the 40/60 main heat pump supplying air into the back of the individual heat pumps. My question is this: the space as a whole is supplied 2,730 cfm from main floor heat pump, of that amount 40% is equal to 1,100 cfm of outdoor air. This office employs 35 people max, but when you add up the conference rooms and consider all of the offices occupied, the occupancy goes as high as 55 people (taking zero diversity). 55 times 10 cfm per person is 550 cfm, exactly 1/2 of what we are supplying. I noticed that one conference room only has 120 cfm going to the back of it's local heat pump which is NOT enough outdoor air per ASHRAE 62 for a room with 8 people. Will USGBC allow me to look at the space as a whole, or do I need to prove that every space within it meets the 10 cfm threshold even though we are supplying twice the required amount of outdoor air into the entire space?
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Ilona Johnson, PE, CEM, LEED AP
AssociateLilker EMO Energy Solutions
8 thumbs up
July 16, 2014 - 9:01 pm
I started to respond based on Case A, but then I realized you are mentioning the 10 cfm because you are pursuing Case B. But I'm curious about the context. If you are providing two times the required 10 cfm per person, then you are providing 20 cfm per person. What is the area of your space? With 20 cfm per person, I'm curious if Case B is appropriate. You may be close to meeting ASHRAE 62.1 requirements in other spaces excluding this one conference room.
I once tried submitting Case B on a project, but then the reviewer came back and said we should make minor modifications to meet Case A. This might mean rebalancing air supply in certain spaces such as your conference room that receive insufficient ventilation...