Greenhouse Gardening: Forms of Heating

Natural gas heater

Although a great deal of interest can be obtained from an unheated greenhouse. It is impossible to exclude frost from it at all times of year. It cannot, therefore, be considered as a permanent home for tender plants.

The range of plants is greatly increased if frost can be kept out even in the coldest weather and is still further increased if a minimum temperature of 7°C. (45° F.) can be maintained. There are many tropical plants which require even higher temperatures but the cost of heating a house can increase disproportionately as these much higher figures are reached.

There are five main ways in which a greenhouse can be heated: by solid fuel. Electricity, gas- or oil-fired boilers or by paraffin. The most convenient of all is undoubtedly electricity but, of course, all the different methods have their advantages and disadvantages. I will now discuss these in more detail.

Solid Fuel Boilers

Solid fuel boilers heating water and circulating this through a system of pipes are a good form of heating. They are probably the cheapest to operate, burning anthracite, coke or special fuels, but they require the most attention. In small greenhouses the boiler is normally installed in the end wall, often alongside the Hue. Unfortunately in such a position they are exposed to changes of wind which may affect the rate of burning. This effect can be reduced by erecting a shelter around the boiler and building a tool or potting shed onto the end of the greenhouse to contain the boiler. Water from the boiler is usually circulated by thermosyphon action through 4-in. diameter metal pipes which must have a steady rise of about 1 in. in 10 ft. to their furthest point from the boiler and an equally steady fall back to it.

Oil-fired Boilers

Oil-burning adaptors for solid fuel boilers are available and may be thermostatically controlled, thus reducing the amount of attention they require to the minimum. For larger installations special oil-fired boilers are manufactured and have a very high degree of efficiency and automation.

Gas-fired Boilers

These are as easy to operate as those burning oil and are equally adaptable for full automatic control by a thermostat. Some care should be taken to site the boiler and its flue where there is no danger of gas fumes being carried into the greenhouse for they are. If anything, even more damaging to plants than fumes from an ill-adjusted or dirty oil burner. This kind of heating can, how-ever, be almost as expensive as electricity.

‘Natural Gas’ Heater

A new heater making use of natural gas was introduced in the summer of 1971 which promises to alter many ideas about greenhouse heating. Thermostatically controlled and completely automatic it is claimed to be cheaper to run than paraffin and to cost only half as much as electricity. In addition, the greenhouse atmosphere is enriched with carbon dioxide, so encouraging plant growth. Thus, the established commercial glasshouse practice of atmospheric enrichment has been brought within the amateur gardener’s sphere. The heater has a safety valve which prevents the main supply from being turned on unless the pilot flame is alight.

Paraffin Heaters

No form of heating is simpler or cheaper to install than a portable paraffin heater. It is advisable to choose one specially designed for greenhouse heating rather than to use a stove made for household use as paraffin fumes can be deadly to some plants. The greenhouse heaters are designed to reduce to a mimimum the risk of fumes and are often lilted with tubes or other devices to distribute their heat as evenly as possible.

It is sometimes thought that paraffin heaters dry the atmosphere but precisely the opposite is actually the case, for as paraffin burns, water vapour is produced and the atmosphere remains humid. Paraffin heaters should be kept spotlessly clean and should never be turned too high nor stood in draughts.

Electric Heating

Electric heating may prove more costly than solid fuel, oil, paraffin or gas heating but it has the compensating advantage of ease of control, cleanliness and safety from the point of view of plant health. It must be installed by a competent electrician for exposed or ill-made electrical connections can be lethal in the damp atmosphere of a greenhouse.

Many different types of electrical heating apparatus have been developed for greenhouses. Tubular heaters, positioned along the side of the house, have much the same capacity for even distribution of heat as hot water pipes. They may be installed in single lines or banks according to the degree of heat required. The normal loading is 60 watts per ft. of tube.

A more compact heater is the fan-assisted type. Such heaters are readily moved about, and will push hot air to all parts of the greenhouse. A quite typical small unit would have a loading of 3.000 watts, ie. equivalent to 50 ft. of tubular heating. When heat is not needed, such healers can be used to circulate cool air.

During recent years, under-floor heating has been tried with some success, two methods being adopted: warming cables below a concrete floor and night-storage heaters. The trouble is that when morning comes the heat has built up in the storage unit or the concrete whether it is wanted or not and there is no means of getting rid of it except through the house. So although it can be economical because off-peak electricity is used it can also be very wasteful.


All forms of electrical heating may be controlled thermostatically and special thermostats are available for greenhouse use. The most sensitive is the rod-type thermostat and it should be mounted as near as possible to the centre of the house where it will register the mean temperature. It is an advantage to shield the rod from direct sunlight by silver foil as this will enable the thermostat to give a more accurate reading.

Soil Warming by Electricity

Warming the soil from below by means of electricity offers numerous advantages to the gardener. Like growing earlier-than-normal crops of tomatoes and lettuces, and easier rooting of cuttings and germination of seeds.

There are two main methods of soil warming by electricity: low voltage current, reduced from the mains voltage by a transformer, which is passed through bare wires, or the full mains voltage passed through insulated soil-warming cable. In either case wire or cable should be buried 4 to 6 in. in the soil.

When laying wire the soil is first excavated to the required depth and a good layer of sand scattered over the bottom. This is then raked level. Next, the required length of cable, to be ascertained from the maker’s recommendations, is spread evenly over the whole surface in lines as nearly parallel and equidistant as possible. Small, hairpin-like pegs of bent galvanised wire will be found useful to hold the cable in position.

With low voltage wire there is no need to take any special precautions to protect the wire, but mains cable can be dangerous if it is accidentally severed by a spade, trowel or other tool. To guard against this it is an excellent plan to lay a length of galvanised wire netting right over the cable. More sand is thrown over this and then the soil is replaced. The cable should be plugged into a waterproof switch socket provided for it in a convenient place, preferably well above soil level where it is unlikely to be accidentally splashed with water.

It is possible to obtain soil-warming cables complete with built-in thermostats so that the current is switched off automatically when the soil temperature rises above a certain point. This is useful and economical but not essential as the loading of these cables is never very high usually enough to provide a temperature of about 16° C. (60° F.).

01. March 2012 by admin
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