Guide to Heating Greenhouses
Artificial heat is needed to guide tender shrubs, young seedlings and exotic house plants through the average British winter.
Greenhouses provide some protection for overwintered plants even if they are not heated, shielding them from rain, snow and freezing winds. However, glass does little to protect plants from the effects of low winter temperatures — at best, an unheated greenhouse of all-glass or glass and timber construction is two or three degrees centigrade higher inside than outside. If the outside air temperature falls dramatically below freezing point, so too will the temperature inside the greenhouse.
With a polythene-clad greenhouse or tunnel, the protection against low temperatures is even less.
An unheated greenhouse will serve you adequately for growing most plants from spring to late autumn. Many gardeners shut down their greenhouses for the winter although they are useful for overwintering more delicate plants or for enjoying blooms unspoilt by cold weather — ericas, camellias, winter pansies, polyanthus and primroses. Certain lettuce varieties can also be grown in an unheated greenhouse.
However, heating a greenhouse, even just to keep it frost-free, widens the range of plants that can be grown enormously. Seeds can be sown early to produce sturdy seedlings and bedding plants by spring, and many tender shrubs, climbers and house plants will flourish in the controlled environment of a heated greenhouse or.
Unfortunately, both glass and plastic allow heat to escape very easily, so heating a greenhouse, however small, is a wasteful and costly business. Consider whether it is worth heating the entire greenhouse or just part of it — it may be possible to divide the structure into two or more compartments, heating just one small area in the middle.
Makeshift double-glazing may be installed by securing sheets of clear polythene across the inside edges of the glazing bars during the winter. This will reduce heat loss, but remember that it will also cut down the light transmission greatly and often create problems with condensation and ventilation. Repair any broken panes of glass and fit draught-excluders to doors and ventilators if necessary in autumn.
In an open position, the winter sun can provide free warmth. A lean-to greenhouse may be against a wall that will store outside warmth or warmth from the house, and maintain frost-free conditions overnight in all but the very coldest spells. Cold winds carry away heat, so in an exposed garden a windbreak of trees or tall shrubs may cut down the fuel bills.
The cost of most greenhouse heaters is quite high so evaluate their usefulness before buying them. There is no point in choosing the smallest, cheapest model in the hope that it will suffice — unless it is able to raise the temperature constantly above freezing, plants will die just as surely as if they hadn’t been given any heat at all. On the other hand, an excessively large and elaborate heater will be expensive to buy and run.
Most manufacturers of greenhouse heaters give recommendations with their products regarding the size of the greenhouse for which each model is suited — usually quoted according to the floor area or total surface area of glass — and on the fuel consumption rate in watts or kilowatts (electric) or thermal units (oil and). Your local board or oil/gas supplier will be able to give you quotes for the cost of these fuel units and may be able to give you an off-peak tariff to reduce night-time running costs.
For most amateur greenhouse gardeners, a minimum air temperature of about 7°C (45°F) should be adequate. In these conditions, a wide range of tender garden plants can be overwintered successfully and many house plants will also survive, provided you do not over-water them. Higher temperatures can be created in small compartments for propagation purposes.
To calculate the heater output requirements to maintain a winter temperature of 7°C (45°F), first measure the total surface area of the greenhouse — all sides plus the roof. The multiplication factor then required to calculate the heater output in British Thermal units (BTUs) per hour of oil or gas, or watts (W) of electricity, depends on whether you measure the surface area in sq m or sq ft:
★ (area in sq m) x 355 = BTUs
★ (area in sq m) x 108 = Watts
★ (area in sq ft) x 33 = BTUs
★ (area in sq ft) x 10 = Watts
For example, a greenhouse with a total surface area of 20 sq m will need 20 x 355 = 7100 BTUs per hour, or 20 x 108 = 2160 watts (2.16kW) of heat output.
By maintaining a minimum temperature of 13°C (55°F) you will be able to grow a much wider range of plants during the winter, but the fuel costs will be very high. Again, calculate the required BTUs per hour or watts as above, but use a further multiplication factor of 1.4. Hence, a greenhouse with a total surface area of 20 sq m will require 20 x 355 x 1.4 = 9940 BTUs per hour, or 20 x 108 x 1.4 = 3024 watts (3.024kW) to maintain a temperature of 13°C (55°F).
These calculations give only very approximate heat requirements — individual greenhouses vary enormously according to site, construction and climate. Buy a heater which will give slightly more heat than required, rather than less.
Commercial growers often maintain temperatures of 18°C (64°F) or more throughout winter, but this is beyond the scope of the amateur gardener.
Types of heaters
In practice there is little difference between the costs of the various fuels, though prices do vary from year to year and from region to region. Some fuels are also more efficient than others.
It is often more economical to use two fuels rather than one for heating. The cheaper, such as paraffin, can be used for background warmth, and the dearer, such as electricity, can be used to attain maximum heat with thermostatic control.
Electricity is most likely to be trouble free, gives excellent automatic control, produces no harmful fumes and does not raise humidity in winter. There is no need to transport or store fuel, and electrical equipment is usually compact.
For a small greenhouse, an electric fan heater is ideal. These are cheap to install — they plug into a 13 amp waterproof socket — and are portable. Fan heaters usually incorporate a thermostat which controls the fan and the heat output simultaneously. The air circulation is good for the plants and minimizes fungal diseases. A disadvantage is that if the fan breaks down, heat loss is total.
Tubular electric heaters — aluminium tubes sealed at both ends and containing a heating element — are also highly efficient. They are screwed to the uprights around the sides of the greenhouse about 15cm (6in) clear of the floor.
If they are mounted in banks, there should be 2.5cm (lin) space between each tube. Tubular heaters are often sold in banks of three or four, but the heat distribution is usually better if they are fitted singly all round the greenhouse. They respond well to thermostatic control, and if one fails, the others should keep out the frost.
Convector heaters, which produce a current of warm air without a fan, are fairly inexpensive and easy to install, but heat distribution is not as good as with fan or tubular heaters. If you are using convector heaters, have one at each end of the greenhouse.
Electric storage heaters, using off-peak electricity, are not really suitable because the heat output cannot be efficiently controlled — it is not possible to link them to a thermostat.
Cuttings root slowly if the soil is cold, and seeds are slow to germinate in cold composts. To heat the soil or compost in a propagator or frame by air warmth would be slow and costly. And a high air temperature in winter or spring would produce unwanted top growth in plants being propagated. The simple and economic answer is to use electric soil-warming cables. These are inexpensive and easy to install.
Cables are made in lengths to fit any area and give the correct amount of heat — follow the manufacturer’s instructions carefully. They are available with a thermostat fitted. The wire is fully insulated and usually has a braided metal earthed sleeve for safety. The heating section is usually a distinctive colour and must never be cut.
Soil-warming cables can be connected direct to the mains supply, or special low-voltage cables can be operated via a transformer —safer if there is a risk of damage to cables from garden tools.
Paraffin heaters are somewhat cheaper to buy than electrical heaters, but the running costs are about the same. The best kind to use are those that burn with a blue flame. They produce a certain amount of carbon dioxide and water vapour, which increases the humidity and can be beneficial to plants if ventilation is adequate.
It is important to use good-quality paraffin oil because low-grade oil, when burnt, can give off sulphur fumes which may damage plants. However, with larger and more sophisticated paraffin heaters there is a flue which takes the fumes outside the greenhouse. Condensation is reduced by the heated flue, creating a drier atmosphere in winter.
Paraffin heaters are probably most useful as emergency heating during cold spells to supplement electrical heating, or during power cuts.
Bottled gas heaters can give a high heat output and are controlled by a sensitive manual valve. The gas bottle is connected to the heater unit by a neoprene hose. This is an economical heating system, but the gas bottles, which are cumbersome and heavy to handle, need refilling or replacing regularly. Gas heaters also produce water vapours and require free ventilation.
Hot-water pipes heated by an oil or gas boiler can be used for heating a greenhouse, but they are costly to install, if the greenhouse is some way from the house.
However, hot-water pipes can be installed in a lean-to greenhouse or conservatory, where they form an extension to the main house. Thermostatic control is possible, but the pipes retain the heat and do not respond quickly to temperature changes.
Thermostats for use with a greenhouse heating system should be easily adjustable, graduated in degrees and be able to regulate the temperature to one or two degrees.
Set the thermostat according to readings taken from a minimum/ maximum thermometer in the greenhouse. This records the highest and lowest temperatures since the previous day. Adjust the thermostat so that the minimum temperature does not drop below the desired level.
With an electrically heated greenhouse with thermostatic heat control, it is possible to wire fan ventilators into the system so that humidity and air flow can be continually adjusted automatically to suit the temperature.