Greenhouse Heating Systems
Warm Air Heating Systems for the Greenhouse
There are a great many types of heaters suitable for greenhouse use. The advantages of warm air heating can be weighed against the disadvantages. Compared with pipe systems, installation costs are low; heaters are easily moved and so do not obstruct or sterilization; they provide positive air movement, especially if fan-assisted; there can be a quicker response to changing temperatures than with a pipe system, especially where the latter has a high thermal inertia.
Some disadvantages of warm air heaters are as follows:
- temperature variations can be much greater over the greenhouse area than with a well-designed pipe system, especially when heaters are of the free-discharge type; there is not the same ability to warm by radiation as with a well-designed pipe system (although recent experimental work has shown that a well-designed ducted system can give almost comparable results to a pipe system and also give positive air movement);
- where a number of warm air heating units are necessary, this adds considerably to the cost, but total cost may still be considerably less than a boiler and pipe system;
- running costs can be higher than with a well-designed pipe system, taking into account that warm air heaters are either on or off and there is no residual heat such as that contained in the water of a pipe system. Technically speaking, however, the lack of residual heat could be considered desirable in respect of achieving even temperatures.
Solid fuel units
Solid fuel warm air units take the form of a boiler with external flue and a heat exchange unit, the air being pulled in at the base of the unit and discharged through the top, either freely into the atmosphere or through ducts. Smaller types of these are hand-fired and are reasonably efficient but suffer from the disadvantage of inaccurate temperature control. While the combustion of the fuel can be controlled to a certain extent by the damper, it is obvious that this will leave much to be desired in the interests of accuracy.
Steam/air heat exchangers
Live steam passes through a heat exchanger, and warm air is discharged by a fan. These have little application in smaller units.
Many oil-fired warm air units arc available, fired either by natural draught or pressure jet burners and of various sizes. The smallest economic size of pressure jet burner is in the 14.65 — 17.5kW (50,000 — 60, 000Btu) range-but they go up to 30kW (100,000 Btu) or more, and this is capable of heating a fairly large greenhouse of 73 — 83m2 (800-900sq ft). They lend themselves admirably to automatic control.
Small oil or gas fired warm air units
The smallest type of natural draught oil burner is a simple oil stove of which there arc many types, which is in effect a warm air heater. It should be noted that the products of combustion are discharged into the atmosphere, and while the CO2 (carbon dioxide) content of combustion could be useful as a means of CO2 enrichment, the sulphur content of oil could be damaging to crops. These small paraffin burning stoves are suitable for smalland are in the range of 2.1 — 6kW (7,500 — 20,000Btu) output per hour and must of course have their burning rate manually adjusted to meet needs. Recent designs of oil heating units with very accurate heat control are available. They are generally placed centrally in the greenhouse, which has disadvantages for providing an even temperature throughout the greenhouse although some have ‘arms’ and other devices for more effective heat dispersal. In recent years gas-fired warm air units have become available, using propane. (Town gas, with its damaging effects on plant growth, is now no longer available in the UK)
Electric warm air heaters
These are becoming increasingly popular for the heating of small greenhouses. They are fairly cheap to purchase, operate on a thermostat, and are reasonably efficient in use. They are available in different sizes, popular models being the 1,250 watt, 2,500 watt, and 3,000 watt (3kW). As 1 kW is equal to 3, 412Btu, the respective Btu values are:
1,250W model 4,262Btu
2,500W model 8,530Btu
3,000W model 10,236Btu
A range of much larger models is also available.
The main disadvantage of unducted fan heater units is the fact that they must be sited at some central point, and this inevitably results in temperature gradients over the greenhouse. A further disadvantage is that the direct blowing of warm air on to plants immediately it is discharged by the heater should be avoided, as plants could be damaged. Nevertheless, the high efficiency of the small fan-assisted heaters and the advantages of built-in thermostats with which most of them are fitted can result in very efficient heating at low initial cost. Fan unit heaters are extremely useful for the cool greenhouse where frost protection is the aim. Large types of fan heater are available which have polythene ducts for distributing the warmed air, which largely overcomes the problem of even air distribution. With the increasing use of warm air heating units in the home, it could be possible to arrange for a duct to serve the lean-to greenhouse or conservatory built on to the house. This should be taken up with a qualified heating engineer, a basic problem being one of control (as with small or mini bore heating systems) as the home heating system may be designed to shut off at night when there would be the biggest demand in the greenhouse or conservatory.
Electric convector heaters
These are generally fitted with thermostats and operate on the lines of taking air in at the base where it passes over a hot element and is discharged as warm air at the top of the heater. The warm air sets up convection currents and displaces the colder air, but it does this over a narrower zone than warm pipes distributed round the greenhouse. As with fan heaters, therefore, convectors have their limitations in respect of warming the greenhouse air uniformly and cold zones may frequently exist. Whether their shortcomings are acceptable depends largely on the range of crops being grown and whether these are particularly sensitive to temperature differences. The biggest drawback of convector heaters is the gentle movement of air compared to the positive distribution achieved by fan heaters, which encourages movement of both cold and warm air and minimizes ‘dead’ pockets.
Storage heaters operating on off-peak power can be fitted with fans which operate on a thermostat and push out the warm air. So far there has been little use of storage heaters for greenhouse heating, as they offer temperature control problems; research work has been carried out on the feasibility of heating concrete paths, but it is likely that they would also offer temperature control problems. They would seem to have a useful role in conservatories.
Electric tubular heaters
Mains voltage waterproof tubular heaters are a very useful means of heating greenhouses, as they can readily be distributed round the perimeter. Of approximately 5cm (2in) diameter they can be installed standing 15cm (6in) from soil-level, either singly or in banks of two or three. Special fixing brackets are generally supplied with them. For metal houses they are best put on the base wall, if any, or on a wooden support. Electricity loading is usually 60W per 30cm (12in) which involves calculation of the exact kW requirement, remembering that 3,412Btu-1kW (1,000W), which means just under 5.1m (17ft) for 1k W (3,412Btu). For calculating the length needed, the heat loss of the greenhouse is worked out and the length of tubular heaters readily follows.
Tubular heaters give off a fair amount of radiant heat which could scorch plants in the near vicinity, and this should be taken into account. Their operation should preferably be controlled through a thermostat. Mineral insulated (MI) cables have a useful application where thermal requirement is relatively low. They consist of a heating wire encased in an inert mineral material and protected by a copper sheath. Of only pencil thickness, the cables are secured to the sides of the greenhouse (as for tubular heaters) in porcelain holders, and are available in packaged units of 2, 3.3 and 5.2kW. MI cables cost less than tubular heaters of similar loading, but they cost more to install as this is invariably a task for an experienced electrician. Plastic-covered mains voltage heating cables are also available. Infra-red heaters are becoming more widely used in greenhouse heating. They transmit heat of a wavelength absorbed by plant leaves.
Systems linked to domestic heating
There are many instances where it is possible to combine home and greenhouse or conservatory heating, especially when the home system is of the small or mini bore type. A normal pipe system may be designed in the greenhouse or conservatory served by the domestic system, although home-type radiators may be used. Provision must be made for night operation when domestic systems are frequently shut off and this is a matter best taken up with the heating contractor.
Siting the Boiler
In most instances where larger type solid fuel or oil-fired boilers are being used for a pipe heating system, space does not often permit the siting of the boiler in the actual greenhouse, although this is a highly practical course of action because of the radiation heat obtained from the boiler which will, as stated above, be lost if the boiler is sited separately. With smaller installations it is ideal if the boiler can be sited in the actual greenhouse. When sited elsewhere it should be protected from the elements and from wind which will interfere with the combustion rate. Care should always be taken to lag any exposed pipes running between the boiler and the greenhouse to avoid wasteful heat loss. Boilers and their housings should not shade the greenhouse; keep them to the north side if possible. As flue gases are carried away by prevailing winds, north siting is doubly preferable.
All smaller self-contained heating units, including fan heaters of larger size, should be sited in the actual greenhouse, the flue pipes, if any, being taken through the roof by the fitting of a special waterproof gland. Free-discharge heaters are best sited centrally, but when polythene ducting is to be used it is better to keep the heater to the north end, if possible, and run the ducting out from this point.