It's important to keep as much heat inside as you can, so start with insulation. After that, any form of heat can be used. In large greenhouses, hot water is most common. Known as a 'hydronic system', hot water is heated in a boiler and then piped around the greenhouse and between plants via multi-purpose tube rail systems. The temperature and flow of the water can then be accurately controlled with automated pump and valve systems. For smaller greenhouses, warm air is an economical option. The warm air is created by heaters run on natural gas, petrol or diesel—which also generate CO 2  that can be used to enhance plant growth—and then distributed with a fan and a heat exchanger. Remember, it’s important in a climate-controlled greenhouse project that heat is distributed evenly for optimum crop production.