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Maureen,
If we use a figure of 875 square feet of glazing for the entire greenhouse
(glass may look nice, but it's one of the worst from an energy efficiency
standpoint), use the zone 7a average minimum of zero °F, and assume you want the
interior of the greenhouse to never go below 50°, your greenhouse will
require about a 15 kW electric heater or one with an output of about 50,000
BTU's. (Most heaters are specified by their energy input, so check
on the efficiency when deciding on one.)
I don't know what the electric power rates are like in Atlanta these days,
but here in southeast Pennsylvania, having propane tanks installed and using
that as the fuel is a LOT cheaper.
The hot water radiant heater has the advantage of applying the heat over a
large area, making it more uniform, but as you'll want lots of air movement for
the health of the plants, that's less of an issue.
When I replaced my heater, I got one with separated combustion - the
combustion air is drawn in from outside of the greenhouse through a duct running
inside of the heater exhaust, rather than from within the greenhouse. In
addition to simply being better for the heater controls to not be exposed
to the humidity, the preheating of the combustion air makes it more efficient
and it doesn't draw all of the humidity out of the greenhouse. My fuel
usage versus the standard design was reduced by over 40%.
Ray Barkalow - First Rays Orchids - www.firstrays.com Plants, Supplies, Artwork, Books and Lots of Free Info!
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