Providing just enough warmth to keep out frost greatly increases the possibilities of the greenhouse, and a little more widens the scope still further. To heat a greenhouse need not be an expensive business provided the right equipment is used and every effort is made not to waste heat. In fact, this is rarely the case, and much of the equipment on the market could do with considerable improvement. Here an attempt will be made to suggest heating apparatus that will prove economical, and the correct way of using it will be explained as well as ways to avoid heat waste – and hence reduce fuel bills.
ASSESSING GREENHOUSE HEATING REQUIREMENTS
It is of the utmost importance that you try to make a rough estimate of the amount of heat your greenhouse will require. When the exterior is at a lower temperature than the required minimum, heat will of course be gradually lost to the outside. A heater must replace this heat as it is lost and so maintain the temperature desired. It should not raise the temperature unnecessarily since this will result in waste, but it should be able to supply heat in sufficient quantity to cope with the most severe winters. It should be appreciated that the greater the temperature gradient between the interior and the exterior, the faster heat will be lost. Every attempt must be made to maintain the lowest temperature necessary to ensure the well-being of the plants.
The heat lost from a greenhouse can be calculated in terms of British thermal units per hour (BTU/ hr). The heat yield of fuels and the heat output of heaters can also be assessed in terms of the same units. It follows that if you know the amount of heat lost from your greenhouse at the coldest time of the year, you can choose a heater that will have sufficient output to make good the loss.
The heat lost will of course depend on the size of the greenhouse, the temperature you want to maintain, the outside lowest possible temperature for the area in which you live, and the materials of construction. Absolutely exact estimates are rarely possible, but the assessment is extremely useful and it will save installing heaters that are totally inadequate or with outputs far in excess of what is necessary.
Since different materials of construction and the surface area they cover will affect the rate at which heat is lost, the calculation involves factors for glass, timber, brick, and so on, and most ordinary gardeners find it somewhat tedious. It is always better to make a small over-estimate, since if the recommendations given later are followed there is no need to run a heater at the maximum output all the time. For this reason it is convenient to make the calculations assuming that the greenhouse is an all-glass structure which means the maximum heat loss.
For greenhouses with base walls and lean-to types, the figures already worked out for you in the table given here will be lower, but they will still be a good guide to the size of heater or its heat output that should be installed, and you will certainly have no need to worry about problems during an exceptionally severe winter.
All good paraffin heaters and hot water heaters fired by solid fuel or oil should have their BTU/hr output ratings stated by the manufacturers.
If your greenhouse size does not exactly correspond with the approximate dimensions given in the table it is good enough to choose the nearest. You can also add figures. If, for example, you had a large greenhouse 10 feet by 30 feet long, you can add the BTU/hr rating given for 10 by 20 feet to that given for 10 by 10 feet. For a minimum temperature of 32°F (a frost-free greenhouse) you would therefore need 11,800+7,300 = 19,100 BTU / hr, or about 5,600 watts if electricity is used. In a case like this two 3,000-watt electric heaters could be installed, for example.
If we consider a small greenhouse, say 5 by 6 feet, and we again use electricity, it will be seen that a 1,000-watt heater will prove more than adequate. Actually if the heater has an efficient thermostat it would not matter if a 2,000-watt heater was used: it would only be switched on by the thermostat when the temperature fell to the figure at which the thermostat is set.
No heat or electricity would be wasted. What happens is that if a heater with a higher output is used, it will be switched on less frequently. However, for technical reasons it is less economic to have unnecessarily excessive ratings.
In case some people are not familiar with the thermostat and its operation it may be worth explaining more fully. Of vital importance for economic greenhouse heating and for effective temperature control for the plants well-being, the thermostat is any device that controls temperature by regulating the fuel supply to the heater. It usually works by certain metals expanding or contracting with temperature change, and the movement operating the flow of fuel, the flow of air to solid fuel so that the speed with which it burns is regulated, or electrical contacts. In the case of electricity very accurate thermostatic control is possible, and in some types of heater using other fuels electricity is used only so that its thermostatic accuracy can be taken advantage of, and the current may be used only to operate motors or electromagnets to govern the flow of oil, air, or solid fuel.
Accurate and reliable thermostatic control is a feature to look for when buying a greenhouse heater, but more information will be found under the various heater type headings in the following s. A thermostat will usually have a dial graduated in degrees and cover a range from just above freezing to about 90°F (32°C). The indicator on the knob should be set to the temperature required. Sometimes,the actual temperatures are not indicated, in which case the temperature must be set in the nrst place by reference to a thermometer. In all cases it is wise to check correct maintenance of the temperature desired by referring to an accurate thermometer, and to then adjust the pointer setting if necessary.
COST OF GREENHOUSE HEATING
Nowadays it is impossible to put any accurate cost to heating since the prices of fuels are changing rapidly–unfortunately most often rising. Fuel prices may also vary from place to place and according to the quantity in which they are bought. Bulk buying may cut costs dramatically. If, then, a particular fuel like anthracite or paraffin is being used for domestic heating, it may be worth choosing a similar form of heating for the greenhouse.
If the BTU heat output of a fuel is known, it is possible to make a rough assessment of cost from its current price. This assumes you know the rating of your heater and the amount of heat needed in terms of BTU which you can get from the table already given. The BTU heat output of various fuels is given below:
Parafiin: 157,000 BTU/ gallon
Solid fuels: 12,000/ 14,000 BTU / lb Electricity: 3,412 BTU/ unit
Fuel oil: 165,000 BTU/ gallon
Liquid propane or butane: 21,500 BTU/lb Gas fuels: 100,000 BTU/ therm
These output figures all assume efficient use of the fuel. Poor heating apparatus and thermostatic control, and bad installation, can waste at least half the possible output. Although the figures may seem to differ widely, in practice there is not an enormous difference in the price of fuels. At the present time, paraffin oil is probably the cheapest, and bottled gas such as propane or butane the dearest. It should, however, be realised that some fuels that may appear expensive may lend themselves to more efficient combustion or use than cheaper kinds, and thus in the long run cancel out the apparent higher cost.
If you know the price of your fuel per gallon, pound, unit, or therm, and the amount of heat you require per hour for your greenhouse, it is only a matter of simple arithmetic to arrive at a rough cost. don’t forget that the heater will be probably out of use during the warmer months of the year. Also costs will vary depending on where you live in the country. In the colder north the heaters may be in greater use than in the south, and the severity of the winter will have a considerable influence.
VARIOUS FORMS OF GREENHOUSE HEATING
These are still the most widely used. They have the advantage of being inexpensive, of using paraffin which is relatively cheap compared with other fuels, and very convenient to use. All greenhouse owners should have a good paraffin heater handy in case of emergency, whatever other form of heating may be installed. Breakdowns, and particularly strikes, can make electricity and fuel deliveries unreliable. Abnormally cold weather may also make an extra paraffin heater useful.
Unfortunately there are disadvantages, but some can be overcome. Firstly a wick heater is difficult to control by thermostat.
There are models available, but they still need improvement.
The main problem at present is to achieve a sufficiently low heat output at the off or pilot light stage. A flue is a desirable feature, since thermostatic models are at present likely to produce fumes harmful to plants.
The ordinary oil heater of course needs frequent attention so that the wick can be adjusted as the weather fluctuates; otherwise there may be much waste of paraffin. It is not the ideal choice if you have to be away from your greenhouse for long periods.
Another important point is that when paraffin burns it produces about its own volume of water released as vapour, and much carbon dioxide. The carbon dioxide doesn’t matter it can be beneficial to plants-but the water vapour can be a nuisance. In winter it is best to keep the greenhouse atmosphere on the dry side, or at any rate not encourage excessive humidity, so ventilation is necessary. Ventilation is also essential to let in air for combustion of the paraffin. If insufficient air gets to the burner, fumes will be evolved.
The ventilation means entry of cold air so that the full BTU output of the paraffin is not really used to greatest advantage.
Many people seem to have trouble with paraffin heaters, but it is nearly always their own fault. We have already seen that care must be taken over adequate ventilation. The burner must also be kept clean and the wick trimmed according to the makers recommendation or again there will be fumes. don’t spill paraffin on the burner when filling so that it can be volatilised by the warmth of the heater, and never use crude paraffin-only the domestic grades are suitable. Crude paraffin such as washings from garages may contain sulphur compounds and other chemicals that burn to produce gases extremely poisonous to plants. Some plants are extremely sensitive to fumes from paraffin heaters. Tomato seedlings, schizanthus, and some orchids, are examples. Such plants will usually show browning or blackening of the leaf edges and yellowing of foliage. On orchids and thick-leaved plants this may not appear for some time after initial exposure to the fumes.
When buying an oil heater it is specially important to see that you get one with an adequate BTU/ hr heat output. Many oil heaters are far too small for the job they are expected to do.
It is not good enough to know that a heater will burn for so many hours on so much paraffin: the heat it yields must be sufficient and you don’t get this for nothing. For example, to keep a 6×10 feet greenhouse frost-free you will have to have an oil heater burning about a quarter of a pint of paraffin per hour, bearing in mind the BTU output of paraffin is about 157,000 BTU / gallon.
Do not use domestic oil heaters for the greenhouse. There are specially designed models. The blue flame type is recommended. Owing to an efficient air supply to the burner the paraffin is more efficiently oxidised and there is less risk of fumes. Catalytic-type oil heaters have been known to evolve fumes in some circumstances, but these are again mostly employed for domestic heating. Greenhouse oil heaters often have pipes or ducts to spread the heat and this is a good feature to look for. Some may have hot water pipes attached as well as hot air ducts. Often a humidity trough is fitted which is intended to be filled with water, but this is not a desirable practice in most cases. As already pointed out, paraffin burns to evolve much water vapour and there is rarely need to increase the humidity still further. Such troughs can, however, sometimes be used for vaporising pesticides.
Other good design features to look for in oil heaters include stainless steel lamp chimney, oil level indicator, and separate oil tank connected to give automatic filling via a constant level device. In any case, an oil reservoir of fair size is desirable to reduce the chore of filling. Copper tanks are best since they are less likely to rust and lead to leaks.
Electricity has already proved itself to be perhaps the most efficient and effective greenhouse heating fuel. Modern technology has made it extremely reliable and there is rarely trouble from breakdown for long periods. It is regrettable that strikes have tended to bring electricity into disfavour, but we can only hope that the future will resolve such difficulties.
With world shortages of other fuels it seems that electricity will have to become the main fuel eventually, its source being atomic energy. Electrical heating may therefore be a wise investment for new installations. Moreover, electricity can be used in so many other ways that it is of great benefit to lead a power cable to the greenhouse. Advice notes on electrical installation are given by suppliers.
Undoubtedly the most efficient and convenient form of heater for the average home greenhouse is the fan heater. This, used in conjunction with an accurate thermostat, will control temperature to within about one or two degrees and there is virtually no waste of heat or fuel. The plants also enjoy the air circulation that the fan gives and excellent healthy growing conditions prevail. There is no contamination of the air with products of combustion with any form of electrical heating, and there is no need for any attention whatsoever for very long periods. The air circulation reduces trouble from fungoid diseases dramatically.
Some greenhouse fan heaters have a fan that is constantly running and a built-in thermostat that only switches on the heat when necessary. With this type the warmed air is consequently kept vigorously stirred after the thermostat switches off the heat. This is not an advantage, since the air whirling around the cold greenhouse sides will quickly lose its heat. Still air is a poor heat conductor, but set in motion the heat transfer is rapid. The insulating properties of static air are taken adva11tage of in double glazing where the trapped air forms the heat barrier. The best fan heater should consequently have a separate thermostat that controls both fan and heat. This way warm air is supplied as needed. When the fan is off the air remains relatively motionless -although there will still be some movement caused by convection. The arrangement gives the beneht of intermittent air circulation, but quick heat loss due to constant circulation is avoided.
Because there is no contamination of the air and no need for extra ventilation, a fan-heated greenhouse can be left practically sealed for several months during severe weather, which further cuts waste. Another important point is that the greenhouse can be lined with polythene to give a double glazing effect. With a lining it does not matter if a continuously running fan is used because the warm air cannot come into direct contact with the glass. In large greenhouses it is also possible to use a continuously running fan without fear of too much waste of heat, since the greater volume of air will be circulated less quickly. This of course assumes that the fan is of normal proportions.
The usual rating for fan heaters is from about one to three kilowatts. Switching to allow different levels of heat output is a desirable feature. More than one heater can be used if necessary. The fans are usually designed to render lubrication very infrequent, and maintenance is negligible.
Convection heaters are sometimes used but seem to be uncommon. These consist of a cabinet with holes at the top and bottom. Heating wires inside warm the air which rises to flow out at the top. Cold air is drawn in at the bottom.
Another popular electric heater is the tube. Heating tubes are used in a similar way to the old hot water pipes. However, since they are hollow, and have only a heating wire inside, they hold little heat when switched off. Fan heaters and convection heaters share this excellent characteristic. It means that they all respond instantly to a thermostat and accurate temperature control is possible.
Ordinary domestic electric fires of any kind are not suitable for the greenhouse and may be very dangerous where there is water and moisture. The radiant heat some give is damaging to plants.
When it is possible to choose your own thermostat, always select the rod type with variable temperature control. A thermostat should always be checked by reference to a thermometer regardless of whether it has the degrees marked or not. The scale is often inaccurate. Also check after a thermostat has been out of use for some time, before using the heating system again. 37.)
Hot water pipes have been used for many years. In their modern form they are still useful, especially when high temperatures are to be maintained. For technical reasons they are more efficient and economical when operating at higher temperatures.
Owing to their great heat-holding capacity they respond slowly to thermostatic control, but they react faster at higher temperatures.
Electricity can be used for hot water pipes by means of an immersion heater. This, however, is not recommended because it tends to work out very expensive. Generally, solid fuel or oil-fired boilers are preferred. Natural gas could also be used. There are a variety of boilers on the market. The modern ones are very easy to install and can be put in and operating in a matter of hours. No major alterations to the greenhouse are necessary.
Modern solid fuel boilers are designed to reduce stoking to the minimum and also the clearing of ash. Most have a reasonable thermostatic control, but obviously this cannot be so accurate as with more easily controlled fuels. Oil-Bred boilers are of course semi-automatic and need little attention. Small installations use paraffin as fuel, but larger ones may use fuel oil with an electric pump to supply the fuel to the boiler. Oil is usually better controlled thermostatically.
In all hot water pipe systems the makers instructions regarding installation must be followed exactly. The pipes must rise gently from the boiler. Nowadays aluminium alloy pipes are preferable to heavy cast iron. Boilers are always rated in terms of heat output as BTU/hr, but manufacturers will advise if given details of your greenhouse. The pipe length necessary to give out sufficient heat must be considered.
It is of no use having a large boiler if there is insufficient surface area of pipe to distribute and radiate the heat. Only the fuels recommended by the boiler maker must be used, The pipes are best filled with rain water or other soft water initially. They may have to be topped up via the expansion chamber situated at the far end of the pipe run from time to time. Large installations may have a mains constant level system of the water tank and ball valve type.
Coal gas is not easily adapted for greenhouse heating because both it, and its products of combustion, are poisonous to plants.
Natural gas is perfectly safe and on combustion it gives the same products as paraffin-carbon dioxide and water vapour.
For natural gas heaters, the comments regarding ventilation and humidity made under Parajlin-oil wick type heaters apply, assuming that the natural gas heater is allowed to pass its combustion products into the greenhouse atmosphere.
It is usually more convenient to use a piped natural gas supply. A special natural gas greenhouse heater is available.
This is neat, portable to some degree, and thermostatically controlled, but there could be a better arrangement for heat distribution. Bottled natural gas propane or butane can be used but tends to be expensive. Where gas cannot be piped these can, however, prove convenient. Propane is advisable when the storage bottle is to be kept outside the greenhouse. Butane may not volatilise sufficiently quickly in very cold weather. The larger the bottles or cylinders, the more economical these liquefied gases become.
CONSERVING GREENHOUSE WARMTH
Obviously ill-fitting doors and vents and broken glass will let in much unwanted cold air and shoot up fuel bills enormously.
All such sources of draught must be dealt with. Many people have found that lining the greenhouse with polythene cuts fuel bills dramatically (as much as 40 per cent is theoretically possible). As mentioned above, electrically heated houses respond well to double glazing of this sort. Use the thinnest polythene available. It must be as clear as possible.
Remember that it is the static air trapped between the plastic and the glass that forms the insulation, not the polythene. It must be put up to avoid gaps and it should not be possible for air to flow freely between the plastic and glass. In timber houses the lining is easily put up with drawing pins. A half-inch to one-inch space between the glass and plastic is ideal. In metal houses it is usually necessary to fasten pieces of timber batten to the interior of the glazing bars. An adhesive like Evostick will do the trick if used when the surfaces are dry.
The drawing pins can then be pressed into the wood. With some aluminium greenhouses that have an inner flange on the glazing bars it is often possible to fasten the polythene with clothes pegs. Special suckers are also available for making the job of lining easier. The edges of polythene sheet can be held together with a smear of glycerine. Correct positioning of heaters will do much to avoid waste.
No part of the greenhouse structure should be allowed to become heated to a temperature higher than necessary. don’t put heating tubes too near the glass. If possible spread them around the greenhouse to even out heat distribution. Unfortunately they are often sold mounted in banks. A fan heater should go at a central position at one end, and a convection heater slightly to one side at a central position in the greenhouse. Oil heaters should also go slightly to one side to give better heat circulation. During a very severe winter or in an emergency, old blankets or sacking can be thrown over the roof at night to conserve heat. Such covering must be removed for daylight. Dirty glass will radiate heat more readily, so see that the glass is kept sparkling clean in winter. This will also let in as much of the suns radiation as possible.
Unless dirt has got between the glass and polythene in a lined greenhouse, and provided the polythene is clear and that light entry is good, a lining can often be left up for the summer months. Much depends on how much light the plants you are growing require.
Some people have used storage heaters with an off-peak tariff for greenhouse heating. However, this type is difficult to control thermostatically. There may be too much heat at times and not enough when it is wanted. They are best used as background warmth with another heat source maintaining the maximum thermostatically.
Oil heaters with electric heaters
As with storage heaters, an accurately controllable electric heater can be used to keep the maximum level of heat with an economical paraffin heater for background warmth. This way the more expensive fuel has less work to do. However, be sure that if fan heating is used with paraffin lamps there is no current of air likely to upset combustion or interfere with a lamps working.