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A greenhouse is all about heat and the conservation thereof. The design of a greenhouse should be such that heat retention is at a maximum and the materials used such that there is little heat loss to the outside. In summer a greenhouse will gain most of its heat directly from the sun, and as such should be placed in a position where it receives direct sunlight in the morning, i.e. South facing, and if possible some shade in the afternoon when the sun is at its hottest.
In the Northern hemisphere the sun is relatively low in wintertime, and it is then that one must think about artificial means of heating the greenhouse and of some form of insulation on the sides of the greenhouse facing North or facing prevailing cold winds. It will probably be necessary to ventilate a greenhouse on a hot summer's day, and allowance should be made for this in its design. This ventilation may be in the form of roof vents, roof panels or mechanical ventilation using fans to exhaust the air. In summer these should be of sufficient power as to be able to replenish the total air in the greenhouse in one minute.
Heating systems are extremely varied, ranging from central heating - radiating hot water pipes heated by a gas, oil, wood or coal burners, to free-standing portable heaters and to the forced circulation of warmed air. Great care should be taken in the choice of a free standing heater, especially if it is a gas or oil burner, for the gases it gives off may upset the fine balance of gases needed by the growing plants.
The heating system for a greenhouse in one location will almost certainly not be the same for an identical greenhouse situated one hundred miles away - or even five miles away! The heating requirements of a greenhouse will also depend on the type of plants to be grown or propagated, some plants being extremely susceptible to minor heat or humidity changes.
Simple formulae may be used to calculate the heat loss of any greenhouse, and this may be used to give an indication of the heating requirements of that greenhouse. Simple electronic thermometers are available to measure inside and outside air temperature and humidity. On a larger scale the temperature and humidity of a commercial greenhouse may be controlled completely automatically.