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How to buy the best greenhouse

Greenhouse heaters

Article 6 of 7

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Greenhouse heaters

To get the most from your greenhouse, it's worth keeping it frost-free by using a greenhouse heater. Lining the glass with bubble insulation will help to retain the heat and keep your greenhouse snug in the depths of winter.

What's more if you're using a heated propagator in your greenhouse, most of them need a background temperature of at least 5C – preferably 10C – to maintain the compost at the 18-21C favour by most seeds. You'll also need a minimum air temperature of 10C to grow on tender seedlings and young plants. 

Which greenhouse heater should I choose?

Electric greenhouse heater

If you greenhouse has an outdoor socket with an RCD nearby, a thermostatically controlled fan heater is ideal. They're cheap to run and the fan not only helps distribute the heat but also encourages air movement which in turn reduces the risk of fungal diseases which favour still, damp air.

Bottle gas greenhouse heater

If you don't have an outdoor socket, a bottle gas heater with a thermostat is a good option. Bear in mind that they're heavier and bulkier than electric heaters. They're also trickier to switch on and have no fan so there may be cold spots in the greenhouse, particularly at floor level. Gas heaters produce water vapour so you'll need to leave vents open for it to escape or you could run into problems with fungal diseases. Good ventilation also ensures that the heater doesn't run short of oxygen and start giving off toxic gases such as carbon monoxide.

Paraffin greenhouse heater

These need to be filled up very frequently, and safety-wise there's not much to recommend them as they tend to be easy to knock over and are they're a fire risk.

Other factors to consider when choosing a greenhouse heater

How powerful a greenhouse heater should I buy?

As a general rule, winter temperatures fall no lower than -4C in the mildest parts of the UK, such as the coast of southwest England, and -12C in the coldest areas, such as parts of the Scotland and the Pennines. You'll need an electric fan heater rated at least 2kW to guarantee keeping a standard 6ft x 8ft greenhouse frost free in most parts of the UK. If you opt for a gas or paraffin heater it'll need to be about 20 per cent more powerful to compensate for the heat lost via the greenhouse vents left open to allow poisonous gases and water vapour out and oxygen in.

The importance of a decent thermostat

A good thermostat will switch the heater on and off at a frequency that will maintain the required temperature, rather than let it fluctuate between being too cold and too hot. Paraffin heaters and some gas models don't have a thermostat at all. As far as electric heaters are concerned, it's better if the thermostat only switches off the heating element, leaving the fan permanently on to circulate heat and create good air circulation. It's a bonus if a fan heater can be used just as a fan during summer to help temperatures down and discourage fungal diseases.