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re: recovery of solar heat from attic
5 jan 1996
george weinert  wrote:
 
>a few years ago, i read about a company that was going to build heat 
>exchangers to go into your attic space to recover heat during daylight 
>hours, and bring some portion of that heat into the living space of 
>your home...

perhaps this was ed palmer of solar attic, inc. at 15548 95th circle, elk
river, mn 55330-7728, (612) 441-3440/7174 fax/email solarattic@aol.com. their
system may be covered by us patent no. 5,014,770, issued may 14, 1991,
which may expire in 2008, or perhaps sooner.

>i also seem to recall having read about a study done on a home in 
>minnesota(?) which blew hot (warm?) attic air into the house any time 
>the attic rose above 68f(?).  i seem to recall that the home had 
>significantly reduced utility bills, and that the payback time on the 
>equipment was rather short.

i seem to recall a documented 25% savings, with a conventional roof. 

>i am interested in perhaps building and installing such a system in my home,
>if it's actually cheap enough, reliable enough, and effective.

it seems to me that a system like this can be cheap, reliable and effective,
especially if your next steep-sloping south roof is made of clear thin single-
layer dynaglas or replex corrugated polycarbonate plastic (not fiberglas),
like mine. this material costs about $1/ft^2 and it is commonly used in
commercial greenhouse roofs, and it has a 10 year guarantee against yellowing,
and an expected mechanical lifetime of at least 25 years. its lifetime can be
extended and the attic made cooler in summer by covering it on the outside
with a large sheet of 15 cent/ft^2, 80% greenhouse shadecloth.

>any construction details or pointers would also be greatly appreciated.

well, the plastic comes in standard lengths of 12' and a width slightly more
than 4', so it can be overlapped 1 corrugation for support on 4' centers. and
you want to keep warm house air out of the attic at night, to avoid heat loss
and condensation. which to me means a passive plastic film backdraft damper
near the bottom of the supply duct, near the attic floor. the duct itself might
be a polyethylene film tube, say 24" in diameter, bought from a commercial
greenhouse supplier for about 30 cents per linear foot, with a large slow fan
at the top pushing warm air down from near the roof peak into the house. the
return duct might be a 2' x 2' piece of 1-2" foil-faced foam in the floor at
the other end of the attic, with a hinge on one edge and a $50 grainger 4z451
reversible 115vac gearmotor and some sort of spool attached to the shaft to
wind up a 1/16" nylon string attached to a screw eye in a rafter, with a couple
of limit switches. you might control this with one or two cooling thermostats
in the attic and a heating thermostat in the house, in series with the fan,
eg 2 or 3 grainger 2e158 thermostats ($14.05 each.) 

>btw, i live in the mild climate of the sf bay area, so i beleive my attic
>actually does warm up significantly even on 'cold' days (mid 40's, brrr ;-)

seems like this should work pretty well there, especially in the spring and 
fall, even with a conventional roof. some of my phila area neighbors tell me
their attic fans with thermostats turn on in the middle of the winter...
 
a transparent roof should be able to collect at least the heat equivalent
of about 1 gallon fo oil per year per square foot of vertical south-facing
projection of the roof glazing. 

or maybe you'd like a small nuclear engine in your attic :-)

good luck. 

nick



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