Two types of warm-air gas-fired heaters have proved satisfactory for the hobby greenhouse; one requires a chimney, the other does not.
A compact unit space-heater, AGA (American Gas Association) approved, is adequate for a small to medium-sized greenhouse, say up to 10 to 18 feet long. It comes equipped with a safety pilot and, ideally, with electric or non-electric thermostatic control. A metal smoke pipe or masonry chimney will have to be provided. Install the heater at the end of the greenhouse walk against the gable and away from the door, so the chimney or smoke pipe can be carried up and out without reducing your growing area. Larger heaters can be equipped with an electric blower-fan kit to assure circulation of warm air through the greenhouse.
The other, a non-vented heater, requires no chimney; the combustion chamber is sealed off outside the greenhouse. Only outside air is used; no noxious fumes are combusted into the greenhouse and plants and gardeners are entirely safe. The heater is usually installed in the wall-gable end or side, and extends only 8 1/2 inches into the greenhouse. The largest size needs only a 17- by 20-inch wall opening. My second greenhouse was heated satisfactorily with such a heater, but we positioned it on the sill in place of one light of glass in the center gable panel and closed the opening with a filler panel of aluminum.
A special fan-control switch prevents circulation of cold outside air before the heater has warmed it sufficiently to assure draft-free distribution of heat. The heater is AGA approved and comes with a safety pilot and high-limit gas shut-off switch. Cabinet is of welded construction with a heat-exchanger guaranteed against burnout for five years.