It is vital to maintain a temperature within the specified upper and lower limits for the species of plant being grown. This temperature may need to vary between dark and light periods, again dependant upon species.The desired results are usually obtained by either fans, to lower the ambient room temperature, or heat sources to raise it.

Reducing the room temperature can be done in various ways. For example, if your grow room is quite small and the air input via say a 4 inch diameter ducting system, then you might decide to use another slightly larger duct with an in-line fan to extract the warm air and send it to the outside atmosphere. This can be supplemented by the use of free standing oscillating fans circulating the grow room air. Should more cooling be needed,in some hot climates for example,then an air cooler can be used. This can be easily made by passing the air over a refrigerated surface like a car radiator with ice cold water running through it.

It goes without saying that when the lights are turned off the temperature will reduce quite rapidly. If your grow room is not very well insulated, this is when you may need to turn on your heaters. Most growers find that it is more efficient to increase the insulation to a point where the heat loss is minimal, rather than spend money on raising the ambient temperature too much.

It is important to remember that any heat source used must not be placed so close to the plants as to burn them. Oil filled radiators and similar systems can be used with good effect.

In order to monitor the system some kind of sensor and switching device will be needed. These control systems are readily available at most suppliers. You will need a sensor for temperature linked to a switch for powering the cooling systems. Do not however rely upon them to never fail. I know of growers who have left their plants unattended for several days, only to return to a dead crop of nearly mature plants. The culprit being a defunct control unit that allowed the lights to stay on and did not turn on the fans when the temperature climbed.

It is always wise to monitor your plants on a regular daily basis.