Which type of greenhouse heater should I get?

Before choosing a heater, decide what you are heating for. Keeping a greenhouse just above freezing to overwinter pelargoniums, fuchsias and a citrus costs a fraction of what keeping it at ten degrees for winter salads costs. Insulate first, partition second, and only then size the heat.

Safety

Step by step

1. Decide the temperature you actually need.Frost-free, just above freezing, covers most overwintering. A cool house at around five degrees suits tender perennials and citrus. Ten degrees and above for winter growing is a different order of cost entirely.
2. Insulate before you heat.Bubble lining, sealing the worst draughts, and partitioning the house all reduce the heat needed. Money spent here saves money every day the heater runs; money spent on a bigger heater does not.
3. Heat a partitioned zone, not the whole house.A bubble-wrap curtain across a third of the length, with the heater and the vulnerable plants inside it, is the standard economy. Most people only need to protect a modest number of plants.
4. Check what power supply you have.If there is no electricity at the greenhouse, that decides the choice for you unless you install a supply. Getting a proper outdoor circuit run is a job for a qualified electrician and is notifiable work in most of the UK.
5. Choose a horticultural model with a thermostat.A frost-setting thermostat is what keeps the running cost down — the heater runs only when it needs to. Household fan heaters are not built for a damp environment and are the wrong tool, whatever the wattage.
6. Ventilate whatever you burn.Paraffin and gas both produce water vapour and combustion gases. A permanently open vent or louvre is required, not optional, and the humidity they add makes disease control harder all winter.
7. Fit a max-min thermometer and check it.It is the only way to know whether the heater is doing what you think, and whether the setting is costing you more than it needs to.

Tips

• The running cost of an electric heater depends on the wattage, the thermostat setting, how well insulated the house is and the current unit rate. Work it out for your own case rather than trusting a general figure — the difference between a lined and unlined house is very large.
• A heater on the floor at one end with a fan blowing along the length heats far more evenly than the same heater in the middle with no air movement.
• Fleece thrown over vulnerable plants on the coldest nights buys several degrees for nothing, and is often enough on its own for occasional frosts.

Common mistakes

• Running a domestic fan heater off an extension lead from the house — Household heaters are not rated for a damp environment, and a long extension lead carrying a high-current appliance down a wet garden is a serious fire and shock risk. This needs a proper installed outdoor circuit.
• Using a paraffin heater in a sealed, fully lined greenhouse — Combustion needs air, and without ventilation you get carbon monoxide, condensation running down every surface, and disease. Lining and burning fuel must go together with a permanently open vent.
• Buying a heater before insulating — You end up paying to heat air that leaks straight out through single glass. Lining first typically halves the heat needed.

Alternatives

• Electric fan heater with frost thermostat: The standard choice. Accurate thermostatic control, no water vapour or combustion products, and the air movement it produces actively suppresses botrytis. Needs a properly installed outdoor supply. Buy a horticultural model rated for damp conditions, not a household one.
• Electric tubular heater: Low-output tubes fixed under staging, usually run through a separate thermostat. Silent, cheap to buy, and excellent for keeping a small partitioned area or a propagation bench frost-free. No air movement, so ventilation still matters.
• Paraffin heater: No power needed, cheap to buy, and genuinely effective for frost protection. Produces a large amount of water vapour and combustion gases, so it needs permanent ventilation, and it needs refuelling and wick trimming. Fuel cost varies considerably.
• Bottled gas heater: More controllable than paraffin and available with a thermostat. Same combustion issues — water vapour, carbon dioxide and the need for permanent ventilation — plus the handling and storage of LPG cylinders. Only ever a model designed for greenhouse use.
• Electric soil-warming cable or heat mat: Not a space heater. Heats a propagation bench or a small area of border from below, at a tiny fraction of the running cost of heating the air. Often the right answer when what you actually want is germination, not a warm greenhouse.

Questions people ask