Greenhouse Heating and Electricity in the Greenhouse
SOIL-WARMED GREENHOUSE BORDERS
The low-voltage system is usually used for this purpose, the wires being laid 9 in. deep. The electrical loading can be from 5 to 8 watts per sq. ft., which means that a 100-watt transformer will be capable of warming up to 20 sq. ft. of border. Asretains the heat well, it is usually sufficient to have the current on for only 12 hours out of each 24.
SOIL-WARMED GREENHOUSE BENCHES
These are constructed by placing 2-in. layers of sand on the benches, laying the cables on this and covering with a further 2 in. of sand. Mains voltage cables are ideal for this purpose, the electrical loading usually being from 5 to 8 watts per sq. ft.
To convert an ordinary slatted bench, fix 6-in. deep boards round the sides and cover the slats with a layer of roofing felt. Place the pots and boxes of cuttings and seedlings on the surface of the sand and pack the spaces between the pots with moist peat to retain the heat. The heating wires raise the temperature of the sand and this heat is in turn transmitted to the pots and boxes. Control the temperature of the sand at 60° F. (15° C.) by placing a rod-type thermostat at right angles across the bench with the rod just below the surface of the sand. Alternatively, it can be hand switched when a soil thermometer placed under one of the boxes indicates the need.
To take the warm bench a stage further, a propagating case can be built over it. This can then be improved still further by fitting soil-warming cables round the inside of the case to provide air warming, making, in effect, a heated greenhouse within a greenhouse to provide correct conditions for germinating seeds and rooting cuttings. In a heated house this can effect a considerable saving in fuel as a temperature of 60 to 65° F. (15 to 18° C.) can be maintained quite economically within the case, while the general space temperature is kept down to 45 to 50° F. (7 to 10° C).
For heating a small greenhouse, electricity has a decided advantage over other fuels. It is efficient, reliable, clean, economic and, most importantly, it is automatic and needs no attention. There are numerous methods of heating a greenhouse electrically, and the running costs are about the same in each case.
A heating system should replace the heat lost through the fabric of the greenhouse even in the severest weather; to determine the size of the heater required, it will be necessary to calculate that loss. Heating for a greenhouse should be based on a minimum outside temperature of 20° F. (—5° C). So if the greenhouse is to be maintained at a minimum temperature of 45° F. (7° C), the heating system will have to be capable of raising the temperature by 25° F.
For an accurate calculation it is necessary to know the thermal conductivity figure, or ‘U’ value of the various materials used in the construction of the greenhouse, which are as follows:
Horticultural glass 1-0
Woodwork 1 in. thick 0-5
Woodwork 1-¼ in. thick 0-4
Brickwork 4-½ in. thick 0-6
Brickwork 9 in. thick 0-5
Floor soil or concrete 0-33
The calculation is made in three stages:
1. Measure the surface area of the glass (including the sash bars), brickwork, soil, etc., of the greenhouse. Multiply each area by the appropriate ‘U’ value, then add up the total.
2. Multiply the total by 0.39. (This figure takes into account the factor for fortuitous losses—1.33, and the fact that there are 3.412 B.T.Us (British Thermal Units) in a watt, viz. 1.33 divided by 3.412 = 0.39.)
3. Multiply the result of this by the ‘temperature lift’ (the difference between the inside temperature to be maintained and the 20° F. outside temperature).
TYPES OF HEATER
The output of a heater is determined by its electrical loading. Any two heaters of the same loading will produce the same amount of heat, while consuming identical amounts of electricity, but some types distribute the heat more efficiently than others. The number of units per hour consumed by a heater is equal to the kilo wattage of the heater, so that a 34-kilowatt heater will consume 3l units of electricity per hour.
Tubular Heaters are normally used in small; there is a special waterproof, horticultural grade, available in lengths ranging from 2 to 10 ft., which should be used under greenhouse conditions. The electrical loading is usually 60 watts per foot of length.
The best place for mounting a tubular heater is low down on the side walls of the house. If this happens to be underneath a solid bench, a gap of at least 3 in. must be left between the back of the bench and the side of the house to allow a free passage of air.
Convector Heaters are the most satisfactory form of portable heater. They are reasonably cheap to buy, but only the ones specially designed for the greenhouse should be used.
A convector heater consists of a galvanized-iron cabinet containing a heating element. The air enters at the bottom, passes over the element and is warmed before being discharged at the top. Heat distribution is not quite so good as with tubular heaters. Sizes range from 500 to 3,000 watts.
Fan-assisted Heaters work on the same principle as convectors, but the air is forced over the element by a fan and after being warmed circulates throughout the greenhouse.
Fan Unit Heaters are another type of fan-assisted heater, but the fan is mounted immediately behind the element and the air is blown out at a fairly high velocity and at a comparatively high temperature. These heaters are particularly useful in large greenhouses for providing frost protection, but the hot air could cause damage if allowed to blow directly on to the foliage of tender plants. If used in a small greenhouse, a suitable mounting point is above the door.
Fan heaters may be used in conjunction with specially-designed perforated plastic tubing which distributes the warm air throughout the house; but such installations are more suitable for the medium to large greenhouse, as 4 kilowatts is the smallest size available. Fan unit heaters are quite robust and should have a fairly long life.
A fairly new development for heating greenhouses is the use of copper-covered cables that are mineral-insulated—that is, insulated with magnesium oxide instead of plastic or rubber. These make a neat, attractive installation, fixed to the wall beneath the bench in the same place as the tubular heaters. This system has the advantage of being low in initial cost, but has the disadvantage of running at fairly high surface temperatures.
ELECTRIC WATER HEATING
An existing hot water system can be converted to electricity by fitting an immersion heater into the pipes. The solid fuel boiler is disconnected and the flow and return pipes are connected together to form a closed loop.
The cost of conversion is not high, and it is particularly useful for replacing a worn-out coke boiler where the heating pipes are still in good condition.
Whichever type of electric heating is in-stalled, a reliable thermostat will be essential to its economic operation, and the expenditure on a good-quality 24-in. Rod-type thermostat with a waterproof head will be saved quite easily during the first winter. The thermostat should not be set too high or unnecessary electricity will be consumed. The best position for the thermostat is one-third of the way along the house from the door, mounted horizontally about 8 in. from the glass, one-third of the way down from the ridge.
High night temperatures in a greenhouse are usually quite unnecessary, and for the gardener who is using his greenhouse in a general way, a night temperature of 45° F. (7° C.) will be high enough during the winter months. Consumption of fuel will depend on the temperature maintained, and if it is raised by only 5° F. the fuel consumption will be doubled. If the temperature should be raised to 60° F. (15° C), the fuel consumption would be four times greater than it would be at-15° F.
In addition to providing heat for the greenhouse, electricity can play a vital role in keeping it cool by controlling the ventilation. By the use of thermostatically-controlled extractor fans, ventilation of small to medium greenhouses can be entirely automatic. The fan is fitted into one end of the house, preferably as high as possible in the end opposite to the door. When the optimum temperature is exceeded the thermostat operates and the fan switches in and runs until the temperature falls by a few degrees. This, of course, is working in the reverse direction to the heating thermostat.
The following gives a rough guide to the size of fan required for 30 air changes per hour:
A 7-½-in. diameter fan for houses up to 280 cu. ft. capacity.
A 9-in. diameter fan for houses up to 500 cu. ft. capacity.
A 12-in. diameter fan for houses up to 1,000 cu. ft. capacity.
For larger greenhouses, the automatic operating of traditional ventilators may be more suitable; there are now several types of equipment on the market for doing this.
The gardener who wishes to mix his own compost and must sterilize the loam will find an electric soil sterilizer most useful. This works by heating the soil to 180° F. (82° C), which destroys insect pests, diseaseand weed seeds. There are several different models on the market.
Pests in the greenhouse can be dealt with by means of a small thermostatically-controlled, electrically-heated unit which volatilizes insecticides, causing a continuous (low of minute particles into the atmosphere.