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July issue of The Irish Garden








Greenhouse Growing : Heating and Ventilation

Heat conservation |  Heating systems | Vents

Heat conservation 

A ‘cold’ greenhouse means that there is no artificial heat provided. A ‘cool’ greenhouse has a heating system that will provide some artificial heat – usually just enough to protect against frost. A ‘warm’ greenhouse has a heating system capable of providing an air temperature of at least 10°Celsius.

Greenhouse flowers can be very rewarding

Greenhouse flowers can be very rewarding

Heating systems

If basic frost protection is all that is required, there are two options – paraffin or electricity. A simple paraffin heater is relatively cheap to buy, and to run, but needs to be lit and refuelled, and can give off damaging fumes if not set up correctly.

An electric fan heater is more expensive to buy, but very easy to operate, usually featuring a built-in thermostat. Seek professional advice when installing an electric heater and be sure to set it up so that it does not get wet.

Running costs for a fan-heater are low, if it is used only for frost protection. A two kilowatt fan-heater will keep a 3.6 metres by 2.4 metres greenhouse free of frost on a night when it is minus 8° Celsius outside.

Heating a warm greenhouse to about 10° Celsius is a more difficult proposition. An extra radiator can be taken off the domestic heating system if a conservatory or greenhouse is attached to, or very close, to the house. Most central heating systems are on a time switch that shuts down the system at night – just when the greenhouse needs it most!

However, usually enough heat will have built up earlier to protect plants adequately. A thermostat over-riding the time switch can be installed to prevent very low temperatures.

Electric storage heaters can be used in a conservatory too. These use night-rate electricity. There must be no danger of them getting wet, for safety reasons. They tend to be bulky too. Free-standing greenhouses are difficult to heat economically beyond basic frost protection. A separate hot-water boiler is ideal, but few people would consider this necessary or affordable.

Vents

On a sunny day in summer, greenhouse temperatures could rise above 40° Celsius. Plants dry out quickly and can be scorched or killed at these temperatures. It will be necessary to allow the hot, dry air to escape.

Vents in the roof and sides should be provided. Opening the door helps too. Roof vents are important not only because hot air rises, but also, because it may not be possible to leave side vents or doors open in a conservatory for security reasons.

Polythene tunnels are difficult to ventilate properly, usually relying on leaving the ends open. This is adequate, once the tunnel is not too long.

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