Heating, Cooling and Ventilating Your Greenhouse

First time greenhouse owners might be a little confused by the need to heat a greenhouse. After all, isn’t the main point that they make use of the sun’s rays to heat up the inside, to grow plants earlier in the season?

Importance of Heating and Cooling

That’s true, but in the winter and early spring the sun’s rays are too low to have any great effect and of course the temperature plummets at night time. So heating is needed to protect the plants, particularly young seedlings not yet frost hardened and not ready to be planted out.

In the height of the summer the opposite happens and the greenhouse becomes a sauna, so cooling is the order of the day. Ventilation aids off cooling but is also important in its own right, fighting off condensation, even in cold weather. Wooden greenhouses retain heat more than aluminium framed ones so temperature fluctuations are not as extreme.

Electric Heating

The options for heating a greenhouse depend on what power sources are available. If you have an electrical supply in the greenhouse then you can plug in a fan heater or an oil-filled radiator. Radiant heaters aren’t such a good idea as plant material could drift in the night and start a fire.

You can use overhead infra-red radiant heaters, just be careful where you site them or you could get a very hot head yourself! But one advantage of having electric power is that you can have heated mats or cables under the soil to provide localised heat.

Oil-filled radiators are quite expensive to run but very safe and can have thermostats and timers so they are very controllable. High-end ones also have a frost control setting so they can be set to come on only if there is a threat of frost.

Non-Electric Heating Options

Without electricity you are restricted to heaters that have their own fuel source like coal, wood, gas or paraffin. Obviously the risk of fire is high with some of these heaters, solid fuel and wood in particular. Stoves of this type are really only viable in larger greenhouses with a proper solid floor to put them on and the room to have plenty of space around them.

Many small paraffin fuelled heaters are enough to heat a small greenhouse but have small reservoirs, so in winter you can be filling them up every day. So larger ones are easier to use, but make sure they are vented so that fumes are taken outside.

Ventilation is Important

This is another point where ventilation becomes important. All but the cheapest greenhouses should come with an opening window or vent of some kind but often it’s not enough or it’s in the wrong place. Louvre windows are common but they are very bad at letting in drafts in the winter so are best avoided.

There should be at least one vent high up in the roof. Hot air rises so that’s where you need the escape points. An automatically opening vent, tripped by a temperature sensor, is a bit of a luxury but worth its weight in gold if the greenhouse is left unattended for a while, when you’re on your summer holidays, for example.

Cooling is usually down to introducing blinds to cut down the effects of the sun in the height of summer. Blinds can be used in roof sections and it’s worth finding out if blinds are an option for the greenhouse you buy, even if you don’t buy them straight away. Fitting blinds to a greenhouse that doesn’t have places for the fittings can be very frustrating.

Consider a Fan

Finally, regardless of the heating and ventilation methods you go for, consider installing a fan. Whatever sources of heat you have the fan will distribute heat and moisture equally around the greenhouses, ensuing it passes ventilation points too.

While not adding anything in itself the fan helps to make all the other systems just that little bit more effective.