Greenhouses are by nature poorly heated structures. The thin membrane of plastic or glass that covers the greenhouse allows heat to enter the greenhouse during daylight hours, but also allows heat to escape at night. One option for heating your greenhouse is a portable propane heater. You can purchase portable propane heaters at hardware or hobby stores. Heating a greenhouse with a portable propane heater is one of the most cost-effective means of heating your greenhouse, according to the Alabama Cooperative Extension Service website.
Contact your local propane company to inquire about renting or purchasing a propane tank for your greenhouse. Your propane dealer will be able to help you determine what size propane tank will meet your needs. Smaller tanks will need to be refilled more often during winter months when the propane heater is in continual use.
Place the tank near the greenhouse.
Dig a trench from the wall of your greenhouse to the propane tank using a trenching machine. You can rent a trenching machine from an equipment supply store. Connect gas pipes to your propane tank and run them through the trench to the greenhouse wall.
Drill a hole through the side of your greenhouse wall using a spade bit or masonry bit. If your greenhouse is a plastic structure, you can simply slip the gas pipes underneath the plastic.
Connect your propane box heater to the gas pipes. Place the box heater in the centre of the room for maximum radiation of heat.
Purchase exhaust fans for your greenhouse that can be mounted to the greenhouse frame. Cut a hole in your greenhouse plastic that is slightly smaller than the exhaust fan and is located in front of the frame where you plan to mount your fan. An exhaust fan will help you vent hot air and carbon dioxide out of your greenhouse before they can harm your plants. Fan capacity for your greenhouse should be large enough to vent the room completely at least once per minute.
Mount your exhaust fan to the frame using wood screws.
Tape the greenhouse plastic to the frame of the exhaust fan using plastic greenhouse tape.
Place a thermometer inside the greenhouse to monitor the temperatures inside the structure. Turn on the ventilation fans any time the temperatures inside the greenhouse becomes too warm for the plants.