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re: air/air heat exchanger
1 mar 2001
abby normal  wrote:
>ashrae's 15 cfm is more geared towards a commercial setting where
>you have a high occupancy density per square foot. a 2000 sq foot home
>could have 4 to 6 occupants, while 2000 square feet of office space
>could have 20 people or more.

the standard mentions 20 cfm/occupant for offices, and 0.2-0.3 cfm/ft^2
for retail stores...
>therefore residential rates are based on habitable rooms. to use 60 cfm
>for a family of four, the fresh air would have to track the occupants
>around the house.

perhaps it's sufficient to be "well-mixed." seems to me that open doors
between rooms would take care of that. some parts of the standard talk
about rooms, eg "hotel rooms," with "30 cfm per room." "residential living
areas" need "0.35 air change per hour but > 15 cfm per occupant," which
sounds like a lot, if large houses need 0.35 ach: 210 cfm for a 4000ft^2
house with 9' ceilings would waste 24h10000dd(210-60) = 36 million btu
per year for "extra ventilation, over $500 with oil at $1.50 per gallon. 

>typically 10 cfm is allowed per habitable room, with the master bedroom
>getting 20 cfm...

i'm not aware of anything like that in the standard. then again, we might
allow people to open windows as needed, vs attempting to legislate that
and sell extra machinery :-) 


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