A greenhouse heater is essential if you want to work well into the autumn in your greenhouse, or overwinter plants there. Here's a guide to sorting through the various kinds of greenhouse heaters, so you can choose and install the type that will best fit your needs, and be most beneficial for your plants.
Step 1 - Choose the Type of Heater
Propane, gas, and paraffin heaters will all need ventilation, since they burn fuel and release moisture into the air. If you go with any of these, you'll need to install some kind of vents or windows, and you'll generally want to avoid running these heaters when you're not in the greenhouse.
Electric heaters will require a power source, but they can be connected to a thermostat and run when the greenhouse is unoccupied.
Step 2 - Get the Right Power for Your Greenhouse's Size
Most gas and electric heaters have a BTU rating (British Thermal Units). Figure out how high you need the BTU to be by multiplying the square footage of your greenhouse interior (length times width) by 100. For a gas heater, divide the BTUs by 100 to get the gas flow in cubic feet per minute (CFM) it will take to operate the heater.
Step 3 - Choose the Location for Your Heater
Most electrical and gas, propane, and paraffin heaters come in stand-alone floor models. They'll need a well-aired spot, away from roofing, walls, and furniture. Some thin-strip electric heaters need to be installed on a wall near the ceiling, along with a protective cover to prevent contact with the strips. Some of these strips can reach a temperature of 175 degrees Fahrenheit, so for safety, you should install any such heating elements well out of the reach of small children. Fan-based electric heaters also need to be installed near the ceiling, so they can circulate well and reduce mold buildup.
Step 4 - Install
Gas heaters will either run on individual tanks, or connect to the gas main line. Paraffin is readily available at camping goods dealers and other hardware centers. Electric heaters will require an outlet, which should be secured to a wall or ceiling in its mounting brackets.
Step 5 - Test
Turn the heater on and test its performance. If it causes the greenhouse windows to steam up excessively, lower the heat setting. Keep the indoor temperature well above 32 degrees Fahrenheit (freezing temperature) during the winter months to protect your plants, fruit and vegetables from frost damage.