GAS HEATING Although in itself coal gas is an efficient high grade fuel which will function automatically, it has not been widely adopted for greenhouse heating. This may have been because it could only be used as a fuel for traditional hot-water systems, but the conversion to natural gas has changed this. There are now available free-standing heaters using natural gas as a fuel, which stand in the greenhouse in the same way as an oil heater, and have a flexible gas connection so that they can be moved about if required. They can be used either with bottled gas or from a mains supply of natural gas, but they have the same drawbacks as paraffin or any other flue less flame heater. The by-products of combustion accumulate in the greenhouse and these include
not only carbon dioxide, which is beneficial to plants, and water vapour which, in excess, can be bad for them, but also a mixture of gases which, if the concentrations are sufficiently high, can be decidedly harmful. If the ventilation recommended by the manufacturers (a top ventilator open 1in [2.5cm] plus 6sq in [38.7sq cm] ventilation at a lower level) is given these heaters can be operated at low settings of about 4° to 7°C (39° to 44°F) without obvious harm to plants. At temperatures much higher than this there is a risk that toxic fumes will build up to a concentration that can cause damage. Extra ventilation does not cure the problem as the heater then burns for longer periods to maintain the desired temperature. Gas and oil-fired units fitted with flues and heat exchangers are now becoming available and these should pose no pollution problems. Although running costs with natural gas heating is at present lower than with electricity the movement of prices. Conservatories and lean-to structures built against a dwelling house can sometimes be heated by linking their heating pipes to a gas or oil-fired domestic central heating boiler. The peak demand from the greenhouse comes during the night when domestic radiators are not in use. Depending on the type of installation, minor problems may be encountered, but many such adaptations are in use and working in a satisfactory manner.
OIL FIRED BOILERS Many commercial greenhouse owners have installed oil-fired boilers, with their attendant advantages of automatic control and attention-free running. Unfortunately there is no cheap, simple, oil-fired boiler which has been designed specifically for heating the smaller size greenhouse. There is, however, a conversion unit available which enables an existing solid fuel boiler to be converted to oil burning. This is supplied with a 50 gallon (225l) fuel tank and burns domestic fuel oil of the 28-second grade (this is the time that oil of a certain grade takes to pass through an orifice of a given size). The same manufacturer produces an oil-fired hot air heater, which is used in conjunction with perforated polythene ducting to circulate the warmed air round the greenhouse. This type of heating is valuable for plants which prefer a warm, dry atmosphere, or are adversely affected by moist, stagnant conditions.