A variety of gas unit heaters are available to heat hobby greenhouses. Some are designed to hang from the structure of the greenhouse; others sit on the floor. Costs for gas unit heaters range from $300 to $550 for 20,000 to 60,000 BTU units, respectively.
Unit heaters burn gas in a firebox. Heated air rises through the inside of a thin-walled heat exchanger on the way to the exhaust chimney. A fan draws air in from the greenhouse, across the outside of the heat exchanger, and out into the greenhouse. Thus, most of the heat is removed from the exhaust before it exits the structure. The exhaust chimney must be sufficiently tall to maintain an upward draft and extend above the greenhouse roof. An 8- to 12-foot chimney is usually sufficient. All open-flame heaters must be vented to the outside and given a fresh air supply for complete combustion. Fresh air should be provided by an unobstructed opening to avoid carbon dioxide buildup.
For larger greenhouses, a plastic tube system may be needed to distribute the heat evenly within the house. The system consists of a perforated polyethylene tube suspended overhead in the ridge and extending the length of the greenhouse. A fan connected to the tube blows warm air from the heater into the tube for greenhouse distribution. This system can also be used for circulating internal air when heating or cooling are not required.
A variety of electric resistance heaters can also be used. Costs range from $100 to $350 for 5,000 to 17,000 BTU units, respectively. Those available in department stores and home centers are usually only adequate in the smallest greenhouses for starting seedlings in the spring. Larger units can be purchased, but operating them can be costly.
