If you use your garden greenhouse to overwinter plants which can’t stand frost, then you’ll need some method of heating it – usually a greenhouse heater. If the greenhouse is attached to the house you may be able to hook in to the home heating system or simply leave a door open between the house and the greenhouse, but freestanding greenhouses need some kind of heat source if they are to stay above freezing when the outside temperatures drop below. There are ways of using natural heat sources like compost or small livestock, and you can reduce the extra heat needed by using water or rock as a heat sink to store daytime heat, but many people install a greenhouse heater of some kind.
Along with the greenhouse heater should come insulation: there’s no point having all your heat disappear through uninsulated walls and glazing! You’ll also want a min-max thermometer to keep track of temperatures, and possibly a thermometer that has a remote readout in the house so you can really keep a close eye on things. See below for details on these.
Greenhouse Heater Fuel Types
The most common greenhouse heaters run on electricity or propane, but depending on your location you may also be able to get kerosene (paraffin) heaters, oil heaters or natural gas heaters. Electric, oil and natural gas heaters require that you have electricity, oil or gas supplied to the greenhouse, whereas propane or kerosene heaters can be freestanding and portable (though propane heaters can also be permanently installed and piped to a large propane supply tank outside the greenhouse). Occasionally people install wood or pellet-fired stoves in greenhouses: like oil or natural gas these need a flue or chimney for safety, and are most often installed against a solid wall. Check local building regulations to find out whether you have to have venting for combustion-based heaters.
Greenhouse Heating System Features
Whatever heat source you use, there are some things you want your greenhouse heater and associated heating system to have.
- A way of distributing heat around the greenhouse (reflectors, ducts and/or fans)
- Temperature control (thermostat)
- Automatic turn-off feature during a malfunction or emergency
- System for handling fumes or combustion gases, if any
Electric Greenhouse Heaters
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There are different designs but most come with indicator lights, overheat and tip-over shutoffs. If you want to add a thermostat you’d need to wire that separately to control the heater – consult a qualified electrician.
Good points of electric heaters are that they do not give off any fumes and thus can’t damage plants that way, they are very easy to install if you already have power in the greenhouse, they are portable, clean and quiet. You need to watch where the fan is blowing hot air – it’s better if it doesn’t blow directly onto plants – and be very careful that the heater and the wiring can’t get wet. If you have a large area to heat you may be better off with several smaller heaters than one large one, so that the warm air is distributed more evenly around the greenhouse.
Propane Greenhouse Heaters
Propane heaters come in two types – those which can be used in an enclosed area, and those which must be vented. The ‘closed area” type have a low-oxygen shutoff system which means that they will not continue to burn under conditions which might cause production of poisonous carbon monoxide.
Smaller models can run off the small 1lb camping propane cylinders but this is very uneconomical if you need to run the heater for extended periods. An alternative is to use a heater which can attach to a large cylinder like those used to run BBQs, using a hose. Commercial-size models might be better attached to an exterior propane tank such as those used to supply home heating and kitchen appliances.
Small propane heaters are quiet, portable, lightweight and odor-free (unless, like me, you hate the smell of the propane itself!). Some can be wall-mounted as well as floor-standing.
Other Greenhouse Heaters
If you have a large home greenhouse or a commercial-sized one, then you might want a fixed, permanently installed heater which runs on oil, gas, propane, pellets or wood. These need a chimney or flue, and are often installed against a solid end wall of the greenhouse with overhead ducting to carry the warm air all the way along to the other end.
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An upgrade is to get one which has a remote sensor in the greenhouse and an indoor display. These have now come down to a very reasonable price, at least if you don’t need a long transmission range, and I think are worthwhile.
Some even have an alarm which will wake you from your sleep when frost threatens, so you can leap from your bed, run outside and spread blankets over your beloved plants. A delightful prospect, but possibly better than losing a lot of hard work!