The size of the heater your greenhouse requires is determined by the lowest expected outside temperature, the maximum inside temperature desired, and the greenhouse surface area. There are many types of heaters and heating systems that are satisfactory for small greenhouses if your home heating system can't handle the job. The best heating system will depend on size of the greenhouse, the availability of fuel, economy of operation, and possibly the initial cost. Electric heat is generally more expensive, but very convenient to control automatically. A warm air overhead heating system equipped with plastic distribution tubes and automatic control using natural gas fuel, will generally be more economical and have the best air distribution.
Adequate ventilation is essential for healthy plant growth.
The combined use of an intake or exhaust fan and a perforated transparent plastic tube makes an ideal method of introducing fresh air into a greenhouse in the winter without a cold draft. Development of this principle has produced automatic climate control systems which can heat, dehumidify, ventilate, or recirculate the air in a greenhouse for proper climate control in the fall, winter, and spring seasons. Excessively high summer temperatures are a serious problem to greenhouse owners across most of the United States. This problem can be reasonably solved either by conventional shading or by whitewashing the greenhouse, but the purpose of growing plants in natural daylight then becomes partially defeated. There's a better way.