How to Estimate Greenhouse Heating Requirement
Before buying heating equipment for the greenhouse it is essential that you know roughly how much heat is necessary to maintain the temperature at the desired level during cold weather. To this end, you must first calculate the rate of heat loss.
Heating greenhouses is still a very misunderstood subject and much of the equipment available is far from perfect. It is not unusual for beginners to buy heaters that are quite unable to cope during severe cold spells. What may seem a financial saving on equipment and fuel can lead to a great deal of waste.
Once you have calculated how much heat the plants in your greenhouse will need during the coldest spells, you can decide whether or not a heater is necessary, and – if it is – you can work out an approximate figure for fuel consumption. However, even the most expensive fuels need not be a serious burden in the average small greenhouse.
When assessing heating requirements, there is an important basic point that should be borne in mind: the higher the temperature in the greenhouse compared with that outside, the faster heat will be lost – and the faster your heater will have to develop heat to maintain the temperature you want.
Clearly, you must aim for the lowest necessary temperature for fuel economy, and take all precautions to avoid waste. Heating equipment should distribute heat evenly, so that no part of the greenhouse structure is raised to an unnecessarily high temperature. Many beginners tend to have excessive levels in winter, whereas nearly all the common popular greenhouse plants are perfectly happy with about 4-7°C (40-45°F).
Assessing heat loss
To find out approximately how much heat your heater will need to produce, you must first get an idea of how fast your greenhouse will lose heat when the temperature outside is at its lowest, and that inside at the desirable minimum. Armed with this knowledge, you can choose equipment with the output to balance the loss; only if the outside temperature continues to drop (as in a very severe winter) may some form of extra heating be needed.
Construction materials like glass, wood and brick have widely different thermal conductivities and rates of heat loss. The area of these materials, and the heat loss through the floor, are additional factors to take into account.
Summing up, to calculate how much heat your greenhouse will lose at any desired interior minimum temperature, when the temperature outside is at its lowest, you need to know the following points:
1 The desired interior minimum temperature.
2 The expected exterior minimum.
3 The area of construction materials.
4 The area of the ground covered.
Let us assume that No 1 is 7°C (45°F) and that No 2 is -7°C (20°F) which is a good few degrees of frost. Various methods of calculation can be used to estimate heat loss, using factors to take into account the thermal conductivity of the different construction materials. However, they are often lengthy and complicated. Here, we give a method involving simple graphs that are based on figures already worked out for indoor/outdoor temperature differences of 14°C (25°F), ie. 7°C inside and — 7°C outside; 19°C (35°F); and 25°C (45°F).
The graphs give an estimate of the heat lost from glass, and brick or wood (that are more or less the same in this respect), for any section of the greenhouse. There is no need for a graph for heat lost through the floor, since this is near enough 100 x its area in sq m (10 x the area in sq ft), the figure obtained being kilojoules, or British thermal units, per hour (kJ/h or Btu/h). All good-quality heating equipment should be rated in terms of these units — don’t buy any unless this is specified by the manufacturer. The area of glazing bars can be ignored, being quite small in comparison with the main areas of construction materials.
An estimate to within about 950 kJ/h (900 Btu/h) is good enough — equal to about 250 watts in electrical terms. Any figure of kJ/h can be converted to watts by dividing by 3.6 (3.4 in case of Btu/h).
Take the example of an average-sized greenhouse of 1.8 x 3m (6 x 10 ft), with a wooden base wall, glass sides and roof, to explain the calculation.
By referring to the graphs below, we find the following:
For glass, 15.75 sq m (175 sq ft) equals about 6100 kJ/h (5800 Btu/h).
For brick or wood, 5.76 sq m (64 sq ft) equals about 1100 kJ/h (1050 Btu/h).
For the floor, 5.4 sq m x 120 (60 sq ft x 10) equals 650 kJ/h (600 Btu/h).
This gives a final total of 7850 kJ/h (7450 Btu/h).
From this it is seen that if there is about 7°C (12°F) of frost outside ( — 7°C or 20°F), to keep the temperature at 7°C (45°F) inside, a heater giving about 7800 kJ/h (7500 Btu/h) must be installed. If this figure is divided by 3.6 (3.4), we get about 2.2 Kw for an electric heater: a 2 Kw or, to be safe, 2-1/2 Kw electric heater should therefore be installed.
Although these graphs assume the popular 7°C (45°F) minimum, and the average 7°C (12°F) of frost, it should be realized that most plants will be quite safe should the temperature in the greenhouse fall for short spells to merely frost-free.
Provided the heating equipment has some form of thermostatic control, no heat should be wasted if the equipment is overrated. In fact, it is wise to err on the side of equipment that when turned on full, gives a higher heat output than necessary, rather than risk an underestimate.
The graphs above enable you to discover the maximum heat loss rate that your greenhouse is likely to experience during the winter – and thereby choose heating equipment of the appropriate output. As well as the more likely temperature difference of 14°C, graphs are also included for 19°C and 25°C (25°F, 35°F and 45°F).