Kerosene heating is the simplest and cheapest way to heat a greenhouse. And these relatively simple and sturdy heaters will give you years of service. However, even large kerosene heaters are only suitable for small greenhouses--200 square feet and smaller. And even then, they are unlikely to do more than keep greenhouses in relatively mild winter climates frost free. Gardeners with dreams of growing subtropical plants in Minnesota winters will have to look to other, more powerful, heating sources.
Calculate the BTU's Needed
Calculate the temperature difference your plants require. To do this, take the lowest average outdoor temperature in your area and subtract the temperature you would like to maintain in your greenhouse. For example, if your greenhouse plants need 60-degree temperatures to survive and you expect this winter to fall to -10 degrees, your temperature difference is 70 degrees.
Calculate the square footage of exposed glass or plastic in your green house.
Multiply your greenhouse's square footage by the temperature difference.
Factor in the layers. If your greenhouse has 1 layer of plastic or glass, multiply the number that you got in step 3 by 1.2. If your greenhouse has 2 layers of plastic or glass, multiply that number by 0.8. The resulting number will be the minimum required BTU's per hour of the Kerosene heater you will need to purchase.
Heat Your Greenhouse
Make sure that your greenhouse has or is installed with a vent. This can be as simple as an opening in the roof or as complicated as an exhaust fan. Kerosene heaters release carbon monoxide and sulfur which needs to be released.
Install three or more thermostats at even intervals in your greenhouse to monitor the temperature.
Place a circulating fan in the greenhouse. Without it, the heat will not be uniform. Instead, warm air will rise to the ceiling and the cool air will settle around the floor. Place two fans in opposite corners of your greenhouse. Each fan should have a cubic-foot-per-minute air-moving capacity equal to one-fourth the volume of your greenhouse.
Turn on your kerosene heater. Most units come with their own thermostat control and will turn on and off on their own depending on the external temperature. Continually check the thermostats installed in your greenhouse anyway, to make sure that it is heating evenly.
Refill the kerosene heater periodically. Most fuel reservoirs last between 16 and 24 hours.