How to Heat Your Greenhouse – Greenhouse Heating
Quite a wide range of plants can be grown in an , but as frost cannot be excluded it cannot be used as a permanent house for tender plants. Provide artificial heat so that frost can be excluded in even the coldest weather and the range of plants which can be grown will be increased tremendously. Go one step further and maintain a minimum temperature of 7°C (45° F) and the scope is still wider. However, to put things in perspective, let me hasten to add that many tropical plants need even higher minimum temperatures and the cost of supplying these extra degrees of heat rises disproportionately.
This is the cleanest, safest (from the plant’s point of view), easiest and most adaptable form of heating for the greenhouse, but it is also likely to be the most costly. It is important that all installation should be by a qualified electrician.
Tubular heaters have much the same capacity for even distribution of heat as hot water pipes; the usual loading is 60 watts per foot of tube and they can be installed singly or in banks. Compact fan-assisted heaters can be used and these have the advantage of being portable and of circulating the hot air throughout the house. Another form of electrical heating employs an immersion heater to warm the water in ordinary 4in. water pipes, as with a solid-fuel boiler system but without the boiler. This system incorporates a thermostat.
Special thermostats are available for greenhouse use and the most sensitive of these is the rod-type which should be mounted as nearly as possible in the middle of the house where it will register the mean temperature. If shaded from the direct rays of the sun by silver foil it will record the air temperature more accurately.
Soil warming is another form of greenhouse heating which is especially valuable. It is used to provide early crops when the plants are grown direct in the greenhouse border, and for providing bottom heat in propagating cases to hasten the rooting of cuttings and seed germination. Two kinds of electricalwarming are available: low voltage current reduced by a transformer can be passed through bare wires, or current at full mains voltage can be passed through insulated soil-warming cable. Both bare wire and insulated cable are buried 4 to 6in. deep. Thermostats can be installed in such units, if desired. The loading is usually enough to provide a temperature of about 16° C (60° F).
Solid Fuel Boilers
Boilers which burn anthracite, coke or other suitable fuels are widely used and probably the cheapest form of heating but, on the other hand, they are time consuming and need quite a bit of attention. However, if time is not a problem this does not matter.
Metal hot water pipes of 4in. diameter are used with such systems, the water being circulated by thermosyphon action, and these must have a steady rise of about 1in. in every 10ft. to their furthest point from the boiler with a corresponding fall on the return journey. As the boilers for smallare usually installed in an end wall with the flue nearby, changes in wind strength and direction can affect the rate of burning. It is a good plan to reduce this risk by building a screen for the boiler.
Oil and Gas-fired Boilers
Solid fuel boilers for the greenhouse can be converted to oil burning quite easily and, with thermostatic control, they then need little attention. Gas-fired boilers for the greenhouse are also fully automatic and for the busy home gardener are competitive alternatives to electrical forms of heating; with gas, though, it is important to place the boiler and the flue in a position where gas fumes will.not be likely to seep into the greenhouse.
For simplicity and cheapness of installation nothing can beat the portable paraffin heater, but as most gardeners are aware fumes can be a problem. This possibility will be reduced if a heater intended for greenhouse use is purchased, for these have been designed to reduce the risk of fumes to the minimum, and they are fitted with devices to distribute the heat as evenly as possible. Such heaters must be kept scrupulously clean, never be turned too high and have their wicks trimmed regularly. They must also be kept out of draughts.
The commonly held view that paraffin heaters dry the atmosphere is the reverse of the truth. As paraffin burns water vapour is produced and the atmosphere, therefore, remains humid.
Controlling the Temperature
The four prongs of temperature regulation are insulation, thermostatic controls for certain heat installations, shading and ventilation — all in their different ways of importance to good plant cultivation. The difference which good insulation makes to the heating bills is very considerable, and this is something every greenhouse owner should be aware of. A simple method of insulation is to fit polythene sheeting inside the roof glass but you must then be prepared for a certain loss of light which may or may not be significant depending on the plants that you are cultivating. Even small cracks allow a great deal of heat to escape and cold air to enter, and wooden houses in particular are in need of extra insulation with glass wool, asbestos packing or other suitable heat-insulating materials.
Electrical heater installations and oil and gas-fired boilers can be thermostatically controlled to the benefit of your pocket and the well-being of the plants. A maximum-minimum thermometer is also a very valuable piece of equipment for monitoring what actually happens in your absence.