Before selecting a heater, check the electric and gas rates in your locality. It may be more economical to use electricity because of low kilowatt-hour rates, or gas may cost less than you suppose. However, don't try to economize on the heater itself—only the best is good enough in the long run. Greenhouse manufacturers or local contractors will tell you the ratings of various heaters. They can also estimate the amount of heat in BTU's necessary to adequately maintain the night temperature you want in a greenhouse of given size. Some greenhouse catalogues include tables for calculating the BTU's (British Thermal Units) required per hour. These are calculated on the basis of an outside night temperature of zero F and a specified inside night temperature, with conversion factors given to enable you to figure correctly for your own conditions, in case your outside temperature does not drop to zero F or you desire an inside night temperature other than that specified in the BTU tables.
Select a heater that will furnish slightly more BTU's than you figure will be required. Extra cost at the time of purchase will be nominal, but you may need the safety margin if your greenhouse is in an extremely windy location. Also, if later you decide to increase the size of your greenhouse, or to grow plants that need a higher night temperature, you will probably not have to change your heater.
What will be the cost of heat to maintain a certain constant temperature throughout the growing year? Quite a bit less than most gardeners imagine. In most sections of the country heat must be furnished during the cold months, starting in October and tapering off in April. Through late fall and early spring when days are quite sunny, you will usually have to furnish heat only from sunset to sunrise, as the sun will provide the heat required through the day. This makes for a considerable saving in heating costs. My second greenhouse, 13 by 15 feet, maintained a 55 F night temperature some eight months for an average of $100 for the entire season—much less than I had anticipated in the Westchester suburbs, north of New York City.
At this point, a special caution is appropriate. If you live where power failures occur frequently, you might like to investigate a heater with a built-in non-electric thermostat. If you prefer a heater without this safety device, then an automatic warning system and some kind of auxiliary heating equipment for use during an emergency will serve you well.
In a greenhouse with automatic ventilation, a gas heater—if it requires installation of a chimney—must also be automatically controlled by a thermostat. When roof vents open, warm air flows up and out of the greenhouse, causing a down-draft of outside air through the chimney. If the heater is operating at the time of the down-draft, noxious fumes and other combustion products are spread through the greenhouse, possibly resulting in damage to plants and danger to anyone working there. If the heater and the ventilators are operated automatically—each controlled by its own thermostat at different settings —the heater cannot function when the ventilators are open. Thus the hazard is eliminated.