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re: erv selection criteria
20 may 2002
>a 32x32x16' house contains 16k ft^3 of air... 30 cfm is equivalent to
>0.11 air changes per hour... a house with 0.11 ach on a cold day is
>heroic, a minor miracle... extraordinarily difficult.

and at 0.11 ach, the air leaks provide all the ventilation.
making an erv useful requires something more like 0.01 ach.

>how would we quantify this? say every 10'x10' envelope section contains
>a 4x10'x6"x1/32" perimeter crack, and we want 30 cfm to flow through
>4,096 ft^2 of envelope...

each 10'x10' wall section has a duct with 40/32/12 = 0.104 ft^2 of cross-
sectional area, so the entire envelope has 4096/100x0.104 = 4.27 ft^2,
and the average air velocity is 2x30/4.27 = 14 fpm. each wall section
has 40x0.5x2 = 40 ft^2 of heat exchange surface, so cmin = 30 and ntu
= au/cmin = 4096/100x40x1.5/30 = 82 and e = 1-exp(-82). not bad :-) 

>...a foot of concrete (about 25 btu/f) inside 12" of styrofoam (r60)
>makes rc = 25btu/fx60ft^2-h-f/btu = 1500 hours (63 days)...

whoops. shoulda wrote:

rc = 25btu/fx60h-f/btu = 1500 hours (63 days)...

nick




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