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Using hot air in conservatory

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Posted by David on August 29, 2003, 11:56 am
Has anyone tried pumping the hot air form the top of a conservatory
into the floor ?
I am planning to put in a thick concrete floor above insulation
(polystyrene ?) and am thinking about laying 4 or 6 inch drainage hose
in the concrete. Then with a couple of fans, piping the hot air from
the roof apex down under the floor.
The idea is to use the concrete pad as thermal storage, keeping the
conservatory warmer on marginal evenings.
It may still be necessary to vent some or most of the hot air on hot
days, but the system could be useful for some of the year.
The fans could be thermostatically controlled so that if the air temp
rose above say 22C they start automatically and store the excess heat.
Anyone have any experience with this ?

Posted by Nick Pine on August 30, 2003, 12:15 pm

Rich Komp pumps air from the ceiling under a Bath-like "hypocaust"
floor in his Maine house. The floor is hollow concrete blocks with
the holes lined up, under a layer of concrete.  

Lots of buildings in Soldier's Grove, WI work this way too, with low-
thermal-mass solar attics and blowers and motorized dampers. Drew Gillett
and I have written a story about this solar town which should appear in
the Nov/Dec 2003 issue of the ASES magazine Solar Today.

How big would the fans be? How much electrical power? How much heat can
they move, compared to the overnight greenhouse heat loss? C cfm of airflow
with a dT temp diff can move about CdT Btu/h, or more, if moist air condenses
below the slab. What controls the heatflow out of the slab?

You can store about 8/12x25 = 17 Btu/F-ft^2 in 8" of concrete or 3" of water.
On a 20 F night, a 40 F greenhouse with 1 ft^2 of R1 glazing per square foot
of slab would need 20 Btu/h-ft^2 of heat. An 8" R1.6 slab with an R0.7 airfilm
resistance might supply this if it were 40+20xR2.3 = 86 F.

As an alternative, you might lay a greenhouse poly film duct flat on
a welded-wire fence shelf below an insulated ceiling and use a small pump
to circulate water through the duct from a larger store on the ground.
A 4'x100' roll of 2"x4" welded-wire fence costs about $0. A 100' roll of
30" poly film duct (about 48" laid flat) costs about $0. A trough on the
ground or a poly film shelf under the first might collect condensation.

A 4' wide-duct with a slow ceiling fan might keep an 8' deep greenhouse
40 F on a 20 F night with water at 40+20R0.7 = 54 F.  

Most greenhouses need dehumidification in wintertime. Plants can evaporate
about 1 lb of water per day per square foot of floorspace...


Posted by andy on August 31, 2003, 3:21 am
Don Rosce of Halifax NS (DHW solar thermal centre of Canada) builds
homes in exactly this way. He has been at it 20 years or so and has
many examples to show for it - and many happy homeowners. The air is
constantally drawn into a centre duct from a high point in the open
concept building envelope. It then circulates down and slowly out in a
star fashion through the thick concrete slab via 4" ducts. The air
then re-enters the building through floor registers near the outer
walls. There is an electric heater in the main duct for use when extra
heat is needed. The fan circulates air constantally and quietly.

There is a 1988 article on the technique in Harrowsmith magazine. The
June issue perhaps. He also never builds a square room........


Posted by Michael Dewolf on September 3, 2003, 6:32 pm
 What about the threat of Legionaire's disease?  The flat tubes could
get some condensation in the summer time, and puddles could form.

Michael D

andy.swingler@swingsys.com (andy) wrote in message

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