Heating Your Greenhouse for Four-Season Growing!

What you can grow in your greenhouse is only limited by how warm your heat is. First, select an energy source. Electric heaters are generally less expensive to purchase and work well if you have electricity wired to your site (no unsafe extension cords, please!) Gas heaters use either natural gas or propane. They are inexpensive to operate and require no electricity.

Next, decide how warm you will need to keep your greenhouse. If your goal is to winter-over plants, you need only keep your greenhouse above freezing. If you plan to get an early start with seedlings, you will need a more consistent and constant heat supply.

Heaters are rated by the amount of BTU's (British Thermal Units) they produce. In order to determine the correct size of heater you'll need, you'll first need to gather some information on the size of your greenhouse, the minimum outside temperature you expect for your area, and the minimum temperature you want to maintain in your greenhouse.

Greenhouse BTU Calculator

Do this	Formula	Example
Determine the total square feet of exposed surface area of your greenhouse.	288	Nine sheets of corrugated plastic (4' x 8' sheet) would be 9 x 32 = 288 s.f.
Determine the maximum inside temperature you want to keep your greenhouse.	60	You're growing seedlings, so you want the inside temperature to stay at 60 degrees F.
Determine the minimum outside temperature you expect for your area.	20	You live in Oregon and you expect temperatures to be down to 20 degrees F this winter.
Determine the heat loss factor for the greenhouse covering material	.8	Corrugated plastic has a heat loss value of .8.
Multiply the square footage by the difference in temperatures, then by the heat loss factor.	288 x 40 x .8 = 9216	The total square footage of the covering surface area is 288. The temperature difference is 40 degrees F. The heat loss factor is .8
9,216 is the minimum BTU rating to keep your greenhouse at the desired temperature during the coldest time of the year. Here are three heater options that will more than satisfy the BTU requirement of 9,216. Our Blue Flame 10,000 BTU Gas Heater will adequately heat the greenhouse in the above example. The larger heaters provide 20,000 or 30,000 BTUs for those larger greenhouses.
Blue Flame 10,000 BTU Gas Heater $324.95     Blue Flame 30,000 BTU Gas Heater $399.00 Non-Vented Greenhouse Heater 20,000 BTU $524.00       Recommdation: Although the Blue Flame 10,000 BTU heater will supply the necessary 9,216 BTUs in the example above, we recommend the Blue Flame 30,000 BTU Gas Heater or the Non-vented 20,000 BTU gas heater. Why? There are always exceptions to the lowest temperature you might expect during the winter. Don't risk your entire plant population by not having enough heat source to protect your plants in case the temperature falls below what's expected. An extra $75.00 will insure you have an adequate heat source!