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re: workshop on ohm's law for heatflow
21 oct 2003
~^johnny^~  
>wrote:

>>most us houses leak a lot more than 15 cfm/occupant. an old house might leak
>>2 air changes per hour (ach), eg 2x2400x8/60 = 640 cfm for a 2400 ft^2 one-
>>story house.
>
>it's called "infiltration".  

a fancier word...

>in theory,  a-a exchanhers can only recover a maximum of 50% of the
>heat gained/lost through the ventilator

perhaps you have erred.

>(unless i am missing something).

counterflow heat exchangers can be close to 100% efficient. if our 8' cube
has 12x8 = 96 linear feet of 1/32"x6" cracks in the envelope with a reversing
fan in an indoor partition that moves 30 cfm from one side of the partition
to the other 50% of the time, a = 96x2/2 = 96 ft^2 and u = 1.5 btu/h-f-ft^2
and cmin = cmax = 30 make ntu = au/cmin = 4.8 and e = ntu/(ntu+1) = 0.83, for
"83% efficiency." a 32' cube with a = 32x12 = 384 ft^2 of crack surfaces and
a reversing partition fan might exchange air with 95% efficiency.

nick




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