In greenhouses heated artificially temperature is partly controllable by thermostat. However, greenhouse temperature control depends on other factors such as the amount of sunlight entering the greenhouse, ventilation, and the cooling effect of moisture evaporating from the greenhouse. It should be realised that temperature is controllable by varying these other factors. Which to vary, so that the best conditions for the plants prevail, depends on the nature of the plants.
A thermostat can of course be used to control various other forms of automation affecting temperature. Generally the rod type already described is the best. However, there are other designs on the market. The moving air type-is also accurate. It consists of a tiny fan blowing a gentle current of air over the thermostat control device which gives greater accuracy in operation and a better sampling of air temperature. Unfortunately, in constantly running fan heaters, this principle is not an advantage in some cases. For fan ventilation a special thermostat is necessary.
When possible it is always best to site a thermostat as though it were a plant. That is, with respect to the vicinity of the plants to the glass or the coolest position in the greenhouse.
For fan heaters and other electrical heaters, it is often recommended that the thermostat should be sited about one-third along the greenhouse, one-third down from the roof ridge, and about 7 inches from the glass. It should not be put in draughts or near the source of heat. When used to operate ventilation or humidity in summer it should not be put in direct sunlight.