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A greenhouse affords plants protection from the elements, and tor this reason they usually grow better than they would out ot doors in the same location. Unhealed glasshouses provide a little protection from frost (ofthe order of 2 to _V C), but uuheated polythene

Rtghl above Temporary slatted shelving should not be used, as here, for storing Dots or other equipment When not required it should be dismantled to admit mote light. Right below Permanent shelving provides useful extra space, but it limits use of the benching below it.

Greenhouse Growing

greenhouses under a clear sky give no protection at all.

Unhealed greenhouses consequently have tlieir effective cropping use limited to the warmer months of the year. In the south of England an eight-month period of use is possible from March to November - but in the north the period is somewhat shorter. This is not to say that winter cropping is impossible, but the range is strictly limited and a high percentage of plants may succumb to disease. Some greenhouse heating is therefore well worthwhile. Heating the whole greenhouse can be expensive; alternatives are to divide the greenhouse into different compartments with permanent glass divisions, or to erect a temporary screen of a double layer of polythene. Yet another way of saving money is to localize the heating - warming only a propagating frame, for instance, or soil-warming a border or greenhouse bench.

Before buying heating equipment it is as well to think carefully about the minimum temperature that is to be aimed for. It is natural to think that if frost can be kept out by maintaining temperatures above t to 2 C semi-hardy plants will overwinter safely, In practice this is not so, for the relatively humid conditions under glass that so often accompany low outdoor temperatures create conditions in which plant diseases become rampant. There are three basic heating regimes in the greenhouse [see Chapter s). Possibly most suitable for the majority of amateur growers, and giving considerable scope, is .1 minimum temperature ofi C (45 F) overall, with higher local temperatures for plant propagation. Safe overwintering of a wide range of plants is possible, and many house and decorative plants will grow satisfactorily provided you exercise some care over watering and ventilation. Increasing the overall minimum temperature to 12C (54 F) raises the heating bill considerably; and maintaining it at l8°C (f>i"F) would put heating costs out of the reach of most amateur gardeners.

expense As fuel prices have soared horticulturists have adapted their growing techniques. Specialist plant propagators are now offering a wide range of young rooted plants raised in high temperatures but ready for grow ing on in lower-temperature regimes, and this is one way in which the amateur can save on heating bills. The fitting of thin polythene of 150-gauge part way up the sides of the greenhouse for a few weeks in the spring wiil provide worthwhile additional protection for plants. But, as in the home, attention should first be paid to eliminating draughts, and the fitting of doors and ventilators should be checked as well as the join between the greenhouse and its foundations.

Heating costs vary enormously, depending, lor example, 011 the region in which one lives, whether the greenhouse is in an exposed or protected situation, its height above sea level, whether it is covered with glass or polythene, the age of the structure, and so on. The following will serve as a rough guide, however, glasshouse heating by electricity

Approximate number of umti mmumrd per week m mainuininR a minimum temperature of f C Uj F) during the v/intcf month*

Flntif .1 rr.i of Mild ;reRein;under ot greenhouse oi En^l.md hn^l.ind ,nid

Flntif .1 rr.i of Mild ;reRein;under ot greenhouse oi En^l.md hn^l.ind ,nid

and WJIO



2.SX 1,8 m

{8 X 6 ft)




3.7x2.5 m

(12x8 ft)




JX 2-5

(16X8 ft)




6 X } m

(20 x ¡0 ft)


I 20


Unit consumption will approximately double if a minimum temperature of 10 C (51 F) is required and will treble if 15 C (60 F) is required. Equipment suppliers or your local electricity board will usually be able to provide fairly accurate running costs for a particular district and type of greenhouse, and the board will also advise on the most suitable tariff Because it can be thermostatically controlled, electric-heating running costs compare favourably with those ot paraffin heaters if high minimum temperatures are not required.

systems Having made the decision to insta! a heating system, you then have ro decide on the type. First, consider whether an existing system can be used In the case of lean-to structures and conservatories, you may be able to extend the home central-heating system without great expense, although the heating demands differ - the home requires much of its heat during the day, whereas the greenhouse needs much of its heat at night. Those with a heated swimming pool may be able to adapt its heating equipment to serve the two purposes, pool and greenhouse heating.

Choice of fuel is wide: oil, town gas, propane, electricity, coal, anthracite, wood - even straw. Each has its advantages and disadvantages.

The commonest system is a small heater fuelled by paraffin, gas, or electricity, For the bigger greenhouse a boiler fired by oil, gas, or solid fuel from which so mm (2 in) hot-water pipes arc run will usually provide the most economical source of heat, taking both fuel and labour costs into consideration. In such systems the heat source can be situated at ground level outside the structure, and the smoke stack should be placed so that the prevailing winds take the products of combustion away from the glass and not over it.

Paraffin heaters have a wide appeal and many good designs are available. Some have built-in water troughs to increase greenhouse humidity, but such refinements are not essential, and to my mind easy servicing is more important to ensure that the heater burns cleanly at all times, because the fumes are vented into the greenhouse. It is essential to use a high grade of paraffin;

A paraffin heaier provides one of the cheapest sources of artificial heal

A paraffin heaier provides one of the cheapest sources of artificial heal

inferior grades may give off sulphur, which can damage the plants.

Gas heaters using natural or bottled gas are available and while initial costs of equipment arc somewhat higher than that for paraffin or electric heaters of similar output, running costs may work out cheaper.

Greenhouse Gas Heat
Gas fueled heaters can be run from bullied gas as in this example or from town gas

The simplicity of electric heating lias wide appeal m spite of the high cost per unit compared with other fuels. Tubular heaters were a favoured form of heating lor many years, but nowadays the horticultural fan heater appears to have gained in popularity. Fan heaters distribute their heat well around the greenhouse and the gentle air movement through the plain foliage is beneficial. Both tubular and I.hi heaters can

A turbo-Mow fan healer The fan also helps to move air within the greenhouse in summer

A turbo-Mow fan healer The fan also helps to move air within the greenhouse in summer

Greenhouse Heating Machine

be thermostatically controlled, and some equipment has built-in thermostats.

Off-peak electricity is somewhat cheaper and some greenhouse gardeners have a night-storage heater installed along the central path. This type of heater requires the laying of a solid concrete path several millimetres thick through which electric warming wires are run. The depth of concrete and the number of wires required have to he calculated for each situation; your local electricity board will be able to advise you on this.

thermostats Accurate thermo-static control of heating equipment is in the interests both of economy and nt" the plants. Rod thermostats are commonly used, as are room thermostats of the type used in the home, Inexpensive thermostats of this kind may not give very accurate temperature control and a more expensive temperature controller might well prove cheaper in the long run. Aspirated screens protect the thermostat from radiation and at the same time provide a steady movement ofairoverit. Whatever type of thermostat is installed it is advisable to check it from time to time, for a faulty thermostat can quickly run up an enormous heating hill. A maximum/minimum thermometer housed in a wellvented box within the greenhouse is a worthwhile investment that will allow you to make spot checks on the thermostat. If it is faulty, have it repaired immediately.

Soil warming wires provide bottom heat for this bench Note soil thermometer

Soil warming wires provide bottom heat for this bench Note soil thermometer

Bottom Heat For Greenhouse

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Building Your Own Greenhouse

Building Your Own Greenhouse

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