Types of Greenhouse Heating Systems
Having explained how to estimate the heating requirements of your greenhouse, here we look at various ways of producing that heat.
The wrong choice of greenhouse heating equipment can mean high fuel bills, so make sure you pick the right methods for your particular needs.
Paraffin wick heaters
The most widely-used form of heater seems to be the paraffin wick type. This is convenient and portable but the necessity for manual control can lead to fuel being wasted. In recent years attempts have been made to automate combustion, but the designs are prone to fume and still need further improvement. Another disadvantage of this thermostatic type of heater is that it continues to give off considerable heat and consume much paraffin after if has been turned off.
Many very small heaters are sold, but often they are quite incapable of providing enough warmth, so always check the kilowatt (or Btu) output, and also the paraffin capacity. If a heater is to provide enough warmth it will have to burn adequate paraffin, for you cannot get heat from nothing.
It is important to choose a heater that is designed for the greenhouse, as some domestic types give off fumes that can harm plant life. Generally, the ‘blue flame’ type of heater can be recommended where a good growing temperature needs to be maintained. With this type, extra air can get to the wick promoting more efficient combustion and there is little or no fuming.
A paraffin heater should preferably have hot-air tubes to distribute the warmth more efficiently, otherwise a current of warm air will rise directly upwards and be cooled by contact with the greenhouse roof. This means that a considerable amount of heat is lost before the warm air gets a chance to circulate properly. This rule, incidentally, applies to other forms of greenhouse heater as well, so always try to distribute the heat evenly around the house.
Other constructional features to look for include a stainless steel lamp chimney, a copper tank (that will not rust and leak), and a reservoir of reasonable size so that filling is not a chore. Some designs are fitted with a humidity water trough, but this is both unnecessary and undesirable.
Combustion of paraffin produces too much water vapour as it is, without adding to humidity by filling a water trough. If a trough is fitted, leave it empty.
An especially useful feature is that some designs can be kept topped-up automatically with paraffin from a large drum, thus enabling them to burn for long periods without attention.
Paraffin wick heaters can be used in greenhouses with a floor area of about 1.8 x 3.7m (6 x 12 ft), but in this case the maximum-output heater is usually necessary, or else two or more heaters, depending on temperature requirements. Where high temperatures are needed it is better to choose some other form of heating equipment.
This has now become a popular fuel, but for the type of heater where combustion products are released into the greenhouse, you should exercise the same care over ventilation as in the case of paraffin lamps — otherwise you will have trouble with condensation and unsatisfactory combustion of the fuel.
With natural gas there is better thermostatic control, hence less waste of fuel. Where it is impossible to run a piped main, a bottled source can be used, though this tends to be expensive. Continuous burning can be achieved by using a pair of bottles with an automatic changeover valve that comes into operation when one bottle is empty.
Hot water pipes
These were one of the first forms of greenhouse heating, but unless high temperatures are required or the greenhouse is very large — exceeding the average amateur size — installation and running can be expensive. Water pipes hold heat for a long time and are difficult to control accurately by thermostat. At high temperatures heat exchange is more rapid and control better.
Modern equipment is designed for easy installation and maintenance. Hot water pipes are an especially practical proposition when the fuel used is the same as that employed for domestic heating, for bulk buying at a cheaper rate is then possible.
Although solid fuel is the cheapest for hot water pipe heating, liquid and gaseous fuels are more easily and accurately controlled by thermostat.
It is important to have as generous a run of pipes as possible, and preferable to convey the heat around three sides of the greenhouse.
Don’t put the pipes too near the glass — you may find it best to run them along each side of a central pathway.
This need not be expensive unless wastefully employed — as in the case of using an immersion heater to warm hot water pipes. Electricity is most economical when heaters with a low heat capacity —and hence immediate response to thermostatic control — are used to distribute the warmth. It has the advantage of not giving off water vapour, that could otherwise cause trouble in winter months.
The most suitable appliances are fan heaters, tubular heaters, and convectors.
Fan heaters – These should have the fan and heating element switched on together by a separate, rod-type thermostat. Moving air will quickly transfer the warmth from the heater to the greenhouse sides. Once these are warmed, it is best to let the air remain relatively still.
A rod thermostat between the power point and the heater will give more sensitive temperature control — provided the heater’s own thermostat is on its highest setting.
Tubular heaters – These are designed to hold little heat and are hollow inside.
They should be spread out around the greenhouse and not all banked in one or two positions.
Convectors – These are best put under staging if possible so that the rising warm air is better distributed.
Never let any one small area of the greenhouse structure become hotter than really necessary. The greater the heat gradient between the inside and the outside, the faster heat will be lost.
Dividing into compartments
A greenhouse can be divided into compartments of different temperature with advantage. A permanent partition with a door, or a temporary polythene one, can be used. An extra warm section should preferably be a middle one — or the end farthest from the door. Lining a greenhouse with polythene can cut heat loss by up to 40 per cent — if done correctly. Allow some 13-25mm (1-1 in) of static air between the polythene and the glass.
Retaining heat through insulation
The value of double glazing has been appreciated for some years. However, the sealed-glass variety used domestically is expensive and impractical for greenhouses. Extra glass panes added to the inside have also proved far from convenient because condensation and dirt get behind them in a very short time, and frequent cleaning is not easy when a greenhouse is full of plants. Polythene sheeting is much easier to manage, and can be put up in autumn and left for the winter months only; it is simple to take down, roll up and store for the summer.
The insulation effect of polythene is quite dramatic. At least 40 per cent can be cut from the normal heat loss figure — and this means a corresponding reduction of fuel bills. Lining is especially suited to greenhouses with electric fan heaters or other ‘dry’ forms of equipment. Where paraffin or gas is burnt — and there is no flue — condensation may be a problem, for considerable water vapour is produced during combustion. In all cases, however, the vents should be separately lined so that they can be opened to permit generous and free ventilation whenever the weather permits.
Remember that the sun is a valuable source of free heat, so any insulation arrangements you may put into operation should not obscure light longer than necessary. Only a little winter sunlight will shoot up greenhouse temperatures even when it’s well below freezing outside.