Heat Calculations for greenhouses

There is, in many ways, the same basic similarity between greenhouse and domestic heating. One hears so much about the efficiency of the different methods that confusion is inevitable. Yet the artificial heating of any building is a precise science, and the cost and efficiency of every method can be readily calculated. Perhaps the real truth is that although accurate information is available on heating systems from many sources, many people do not take the time to assimilate it properly, or are too readily swayed by lavish National advertising.

SOME VARIABLE FACTORS

There are, nevertheless, quite a few variable factors to be considered. All materials have a conductivity factor which allows the passage of heat in both directions. Glass is a good transmitter of heat and this indeed is why it is so effective in allowing the passage of solar heat. Glass, therefore, is a poor insulator and while it can, unlike many plastics, effectively trap the reflected long heat waves, it cannot store heat for long, which means that when the outside temperature drops below the temperature of the greenhouse, heat will readily flow back through the glass. Reduce the area of glass and heat loss is reduced accordingly, as also is solar heat transmission, a matter which has already been referred to. Increase the wind speed on the outside surface of the glass, and loss of heat is still more rapid. Allow air spaces below or around doors, badly fitting ventilators, or glass, and the heat loss is still further increased, this time by the physical loss of heated air, and the entry of cooler air. Double glazing is effective in a dwelling house because two layers of glass, hermetically sealed, trap an insulating blanket of dead air between them. Double glazed greenhouses are also available or can be constructed, but these are costly. A crude form of double glazing is to line the inside of a greenhouse with polythene, leaving the ventilators free of course, and while this can be effective to a degree, it gives rise to excessive humidity unless ventilation is particularly effective, as it can be with extractor fans.

CALCULATION OF HEAT LOSS

Heating engineers always start their appraisal of any heating project by calculating the heat loss of the building concerned.

They do this by assuming an accepted figure for the thermal conductivity of various materials. These are approximately:

 

Per sq. ft. per hour,
per degree
Glass (including its framework) 1.1 BTU per hour

(British Thermal
Unit)
4| in. brick wall or composition .5 BTU per hour

block

Double brick wall, 9 in. .4 BTU per hour

Wood 1 in. thick .5 BTU per hour

Asbestos (sheet or corrugated) 1.1 BTU per hour

(approx)
Concrete 4 in. thick .75 BTU per hour

Double glazed glass (properly sealed) 5 BTU per hour

Note: There is also technically a heat loss through the floor or ground of the greenhouse, but in practice this is frequently ignored, as the ground area of a greenhouse is often a very effective storage medium for heat, throwing this back into the greenhouse.

To make use of these figures for the purpose of calculation, it is necessary to measure up the total areas in square feet, and this is quite a simple matter. It is better to draw a plan and append the necessary measurements to this. The brick base walls (if any), the total area of glass, and the ends are all measured up and the areas multiplied by the appropriate figure.

ARRIVING AT THE NECESSARY HEAT INPUT

The figure of 259 BTUs is the heat loss of a perfectly tight greenhouse, which is an unusual occurrence. Normally there are leaks and one must also take into account the effect of exposure. Generally one third is added to allow for these additional losses, although this figure could be much greater in a very exposed situation, and less in a very sheltered position with a very tight greenhouse. Assuming an addition of one third, the figure of 259 becomes 343 BTUs. The decision must now be made as to what level of heating is required. This will depend on region, and of course on the type of cultivation intended. A greenhouse to be maintained at 65°F. In all weathers would require a45-50°F. ‘lift’ over the outside temperature, assuming that it could be as low as 20° F. out of doors. Therefore to calculate the heat input the figure of 343 is multiplied by 45 or 50. Generally speaking for a temperate or intermediate house a 35°F. Lift is allowed, and for the cool house a 20°-25°F. Lift. Assuming an intermediate house temperature is the aim, the figure of 343 is multiplied by 35 and therefore becomes 12,005, which is the heal requirement figure in BTUs no matter what type of heat-ing system is involved. For W/metre square DegreeC X 5.6.

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