To study in depth the full implications of heat is well beyond the scope of this section on this site. It will suffice to translate the three methods of heat distribution, ie conduction, convection and radiation, into practical terms in relation to greenhouse heating. Briefly, conduction is the transmission of heat through a substance from molecule to molecule, convection the transference of heat through a liquid or gas by circulation of the heated portion, and radiation the transference of heat energy by electromagnetic waves from heat source to absorbing object, and between objects.
The short wave radiation from the sun passes through the glass of the greenhouse, warming everything it contacts, floor,, bench, plants, pots, and so on. These in turn re-radiate the heat back into the greenhouse on long waves which cannot pass through the glass. The heat trapped within the greenhouse sets up convection currents, establishing in effect a convection cycle of warmed air.
Artificial heating systems
Where an artificial heating system is introduced into the greenhouse it will operate independently of solar radiation, and more generally and preferably when solar radiation falls below an effective level. There are several forms of artificial heating available. Hot water pipes are a good method of heating: the metal is a good conductor of heat and in turn transmits radiation heat; and both pipes and objects warmed by radiation set up convection currents. Hot pipes also transmit heat by conduction when they are in actual contact with the soil. Electric tubular heaters are another frequently used method of greenhouse heating and operate in the same way as water heated pipes. Direct or indirect warm air heaters, free discharge or fan assisted, are becoming increasingly popular for greenhouse heating. The air on discharge sets up convection currents. Fans push the heated air out more quickly, though convection currents still subsequently develop.
Soil warming cables, used mainly for propagation benches, become warm by conduction, then transmit this heat by conduction and radiation to the soil or sand. Mineral insulated cables can also be used for warming the air.
Some variable factors in greenhouse heating
One important point which should be stressed is that these heating processes can readily be upset or disturbed. For example, one side of a greenhouse may be much colder than the other, despite a well-designed perimeter system of warm pipes, because a very cold wind blowing along that side is causing very rapid heat loss through the glass. Similarly, a fan heater may fail to direct warm air to all parts of the greenhouse, because a cold wind outside has produced an internal curtain of cold air.
Lack of uniformity in temperature throughout a greenhouse can be checked by the use of integrating jars or, less accurately, with a number of ordinary thermometers. These integrating jars, which are silver-foil-covered, water-filled bottles containing thermometers, are left in different areas of the greenhouse long enough for the water content in the jar to achieve uniform temperature (there may be difficulty in this respect during short summer nights). On inspection at 7-8 am the temperatures recorded may be found to vary considerably.
The particular problems of heating greenhouses, centre around the unavoidable rapid loss of heat through the glass by conduction, and inevitably also through leaks in the structure. There is therefore great need of a constant high output and uniform source of heat sufficient to overcome this rapid heat loss. A central source of heat, such as an oil heater or an electric fan or convector heater, will not warm a greenhouse as uniformly as it would the room of a solid building. Ideally a greenhouse should have a complete ring of warm pipes to give off radiation heat and initiate convection currents, and in the centre there should be further radiation heat from pipes, which would also set up smaller convection currents.
Where there are benches there should be allowance for convection currents between the bench and the outside of the greenhouse (frequently this is prevented) in addition to radiation heat to plants on the bench by individual pipes. The diagram above shows the greenhouse longitudinally, but the ends of the greenhouse should develop similar convection currents, it being good practice to complete the circuit of pipes under the door by containing them in a grille.
Although a centrally sited unit heater is unable to achieve uniform heat distribution, this matters less as the season progresses and outside temperatures rise. Warm air fan heaters are slightly better than natural convector heaters (electrical, oil or gas), as they do move the air positively which assists with the mixing of warm and cold air, though there are inevitable cold spots. Warm air heaters, however, can be as efficient as warm pipe systems if the air is distributed in polythene ducts.
Calculating heat losses
It is essential to make exact measurements of the greenhouse so that the heat loss through all external surface areas can be calculated accurately. The areas of glass (including glazing bars) and the areas of base walls of brick or wood or other material should be measured separately since each will lose heat at different rates. Heat loss is calculated by the rate of thermal transmission (loss of heat by conduction) or µ value; different materials have different u values. These are, despite metrication, usually still quoted in British thermal units, one unit being the amount of heat needed to raise the temperature of 1lb of water 1°F. The metric term is w/m2°C. The following are the accepted µ values for various materials and, in keeping with modern practice, they are quoted in slightly higher figures to allow for inadvertent heat loss. Note also that heat losses will be higher in exposed situations, lower in sheltered ones.
Average thermal transmission coefficients for different materials in w/m2/°C (Btu/ft2/hour/°F).
Btu/FT2/hour/°F = 5.678 w/m2/°C
w/m2/°C = 0.176 Btu/ft2/hour/°F
|Glass including glazing bars||7.94||1.4|
|Brickwork 11.2cm (4-1/2in)||3.63||0.64|
|Brickwork 22.5cm (9in)||2.66||0.47|
|Concrete 10cm (4in)||9.9||1.75|
|Concrete 15cm (6in)||3.46||0.61|
|Wood 2.5cm (1in)||2.83||0.5|
|Fibreglass and rigid PVC||
Broadly similar to glass with the exception of PVC materials such as Biolex 2000 sheeting, which has a heat loss 30-40% less than glass. Polythene of various grades, usually 150 mu or 180 mu, for practical purposes taken as:
|PVC and new thermic films||Usually slightly less than basic polythene, between 10-20% Bi- or triple-walled acrylics and polycarbonates 30-50% less than glass. The same is true for twin-skinned polythene structures with a fan system to keep the skins apart.|
Technically there is also a heat loss through concrete or earth floors, but this is so small it can be ignored. In actual fact, the floor can often serve as a considerable store of heat for re-radiation, but any heat loss is allowed for in the higher figures quoted above. It is now a relatively simple matter to calculate the total surface area of the greenhouse.
|Sides 2 x 2.4m (8ft) x 1.8m (6ft)||= 8.64 m2 (96 sq ft)|
|Ends 2 x 1.8m (6ft) x 1.8m (6ft) x 1.8m (6ft)||= 6.48 m2 (72 sq ft)|
|Roof 2 x 2.4m (8ft) x 1.05m (3ft 6in)||= 5.04 m2 (56 sq ft)|
|Gable ends 2 x 90cm x 45cm (3ft x 1ft 6in)||= 0.81 m2 ( 9 sq ft)|
20.97 m2 (233 sq ft)
As this is an all-glass greenhouse the total heat loss is 20.97 x 7.94 = 166w/m2/°C (233 x 1.4 = 326Btu) This figure is the difference per degree C or degree F between inside and outside temperatures and it is then necessary to calculate for the heat lift required; this is done by multiplying the 166 (326) by the selected heat lift: an 11°C (20°F) lift is adequate for frost protection and normal temperatures in spring; a 22°C (40°F) lift will ensure a reasonable level of temperature throughout the year, even when it is 6.7°C (20°F) out of doors; the 16°C (30°F) lift is a compromise between the two. Thus the figures to be aimed at
166 w/m2/°C x 11 (20) = 1826 watts (6520 Btu) = approx 1.8 kW
166 w/m2/°C x 16 (30) = 2656 watts (9780 Btu) = approx 2.6 kW
166 w/m2/°C x 21 (40) = 3486 watts (13040 Btu) = approx 3.4 kW
give the respective heat losses for the various lifts. These figures represent the actual heat requirements from the heating system, no matter what type or design this may be. Note that exact conversion from metric to imperial terms is not practical.
It is usual to have a slightly larger heat production unit than necessary to allow quick response and to cope with periods of excessively low temperatures. Where a hot pipe system has the boiler sited away from the greenhouse (perhaps in a shed or garage), there will be some loss of heat en route to the greenhouse, a quarter to a third being the usual allowance for this loss. Allowance may also have to be made for an exposed situation.
Designing a heating system around the calculated heat requirement will depend on the type of crop to be raised and practical matters such as the availability of electricity, oil or gas, and the possibility of linking the system to an existing one in the home.