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Ventilation Question

Ventilation Question

(OP)
In regards to IMC 403.2.2:

Say you have a building with two spaces; an office and a single bathroom. The bathroom has a single fixture and needs 50 CFM of exhaust; the office space has 5 occupants and is 416 SQ. FT. needing 50 CFM of outside air (5*5+416*0.06 = 50). Does the unit that serves this space bring in 50 CFM of OA or 100 CFM of OA (with 50 EA from the bathroom and 50 EA from the office space)? I used to assume 50 CFM of OA but the IMC with commentary has me questioning myself. In particular:

Ventilation air cannot consist entirely of reused air, meaning that the ventilation air that is transferred into a space cannot consist entirely of the ventilation air that was required to be delivered to an adjoining space. Transfer air will consist of some reused ventilation air and some unused ventilation air, because the space from which the transfer air is taken is, in effect, over ventilated. For example, if the required ventilation air for a kitchen area is not introduced directly into the kitchen, the air would have to be introduced into an adjacent area from which the air is transferred into the kitchen. Consider a large toilet room that needs 700 cubic feet per minute (cfm) of exhaust and is accessed from a lobby. If the required outdoor air is not introduced directly into the toilet room, 700 cfm would have to be introduced into the adjoining lobby in addition to the required ventilation air for the lobby, then air from the lobby could be transferred to the toilet room. For kitchens, some transfer air is typically taken from dining rooms. The makeup air requirements for the kitchen exhaust hood system will often exceed the required ventilation rate for the kitchen; therefore, the makeup air with the required minimum outdoor air component can serve as the ventilation air for the kitchen. In the case where transfer air is taken from a dining room, the air is actually outdoor air that has been supplied to the dining room by the dining room ventilation system. Any of the required outdoor air for the kitchen that is to be supplied by transfer air from the dining room will have to be added to the required outdoor air rate for the dining room.

I am having a brain fart here; could I get some clarity?

RE: Ventilation Question

The requirement for adequate OA separate from the ventilation air is quite clear to me from the above.

RE: Ventilation Question

Willard3: OP has office space attached toilet. Office need 50 cfm fresh air and toilet required 50 cfm exhaust. Toilet need to be negative pressure and office need positive pressure.so as to me, 100 cfm fresh air is correct, additional 50 cfm will be extracted.

RE: Ventilation Question

RandomUserName (great name!):

Good question and post but I’m not fully in tune with your dilemma: Typical office cooling will need about 1 cfm/ft2. Your exhaust is 50 cfm so you’d need an approx. 400 cfm service with 12.5% minimum outdoor air to match the exhaust. I’d do 15% to keep a slight positive…

Sometimes these things can get over-complicated.

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