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  Heaters, vented & otherwise
From: "Weaver, Bill" bill.weaver at hp.com> on 2006.11.21 at 16:11:47(14820)
The major problem with unvented heaters is the buildup of ethylene gas
is present in some natural gas supplies. Ethylene is the gas released by
ripening fruit like apples that accelerates flower maturity/death. I
been using an unvented heater in one orchid greenhouse for about ten
and I haven't had any problems.
I should note that a properly installed "unvented" heater is generally
not really unvented. At the very least an air vent is required to
combustion air otherwise the flame would eventually go out. (Soon after
setting off your carbon monoxide detector) Plus, some sort of "outlet"
vent is generally called for as well. This causes some of the heated air

to exhaust out the outlet vent and draw combustion air in the other
Cuts back a bit on the effiency, but prevents ethylene buildup.

(BTW unvented heaters are no longer legal in California)

Bill Weaver

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