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re: air to air heat exchanger
22 feb 2001
graham parkinson  wrote:

>are their any heat recovery ventilator designs that are suitable
>to use in a superinsulated, low infiltration off grid solar house?

yes, but you may not need one if your house has more than 60 cfm of natural
air leaks, eg more than 0.225 air changes per hour for a 2000 ft^2 1-story
house. then again, you can only rely on natural air infiltration if it's
windy outside or there's an indoor/outdoor temperature difference. 

>i've seen some designs that used poly film baffles to exchange the heat -
>any other good ideas out there for low cost home built designs?

poly film baffles sound good. how about mounting a 4'x8' box on an outdoor
south wall? it might thermosyphon on a cold day, especially with some help
from the sun. condensation of water vapor from outgoing warm air seems
important for energy savings, and that can also reduce the heat transfer
surface required, but it seems to me that recovering some of the latent
heat is enough. we don't need to make porous heat exchange surfaces and
try to evaporate the water back into incoming air. plants and fountains
and people and showers can do that.

for 85% effectiveness with a counterflow heat exchanger, we want ntu
= 0.85/(1-0.85) = 5.67 = au/cmin = 1.5a/60, so a = 227 ft^2, with
slowly-moving air and lots of condensation. we might have about 227/32
= 7 layers of film inside the box. incoming 30 f air would be heated to 
30+0.85(70-30) = 64 f, making the average temp in the incoming "chimney"
47 f, and outgoing air would be cooled to 36 f, making its average temp
53 f. if 60 cfm = 16.6asqrt(8'(53-47)), with an empirical chimney formula,
each airpath direction inside the box needs a total cross-sectional area
a = 0.52 ft^2, ie the box needs (2x0.52')/4' = 0.26' or 3" of depth, so
the layers need to be at least 3/8" apart. we might make them 3/4" apart
and use 1x3's or strips of 3/4" foamboard for spacers in a 6" deep box. 

some smart controls (at least a humidistat) and a couple of fans would be
nice, eg grainger's 12vdc 6.8 w 107 cfm 4wt36 $23.59 versions, solar-
powered of course, for times when thermosyphoning isn't enough, and the
bottom might be slightly sloped towards the outdoors to allow condensation
to drain out neatly, and we'd want to turn off the incoming fan when the
outgoing warm air gets close to 32 f at the outlet. (there's no danger
of frost in the incoming airpath, since heating that air reduces the rh.) 

maybe bill kreamer will build one of these as a special version of his
solar air heating panel :-)

nick




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