Determining Heating Requirements for the Greenhouse

Successfully heating the greenhouse means that you ensure the temperature inside the greenhouse does not fall below a specified level, despite the vagaries of winter weather. To achieve this, one has to calculate the expected heat loss (e.g., in BTU/hr) for a worst-case outdoor temperature, and then provide a heater than can keep adding heat to counteract this maximum heat loss on a continuous basis for as long as that worst-case temperature and/or other conditions (e.g., unusually high winds) persist.

Gas heaters are usually rated in BTU/hr, but electric heaters are generally rated in watts. Both ratings are units of “power,” or amount of energy used per unit of time. The standard conversion factor is: 3.41214 BTU/hr = 1 watt. One has to be careful to distinguish between heater ratings that specify the amount of energy used (i.e., input) versus the amount of heat created for use in the greenhouse (i.e., output). For electric resistance heaters, these amounts are very close, but for gas heaters, they can vary considerably, because some of the heat generated may be going up the chimney, if there is one.

To calculate the worst-case heat loss, I assumed that the temperature in my area would not fall below negative 10 degrees Fahrenheit (i.e., 10 degrees below zero). That is the worst-case outside temperature used by the U.S. Department of Agriculture for specifying plant hardiness zones. I am in Zone 6a (in Mercersburg, PA).

The heat loss depends on the difference between this worst-case temperature and the temperature desired to be maintained - in this case it is a temperature difference of 70 F degrees.

The heat loss also depends on the prevailing wind conditions, since heat exchange is more efficient at a barrier where the escaping heat is constantly removed; the construction of the barrier between the plants and the outside environment (e.g., the polycarbonate glazing); and details of the way the heat is provided (e.g., radiant vs convection heating). I used a formula developed by the Canadian Government, as discussed in detail in a separate document. The results of my calculations are just summarized here. To keep the inside temperature at 60 degrees F, my heater must continually replace heat energy lost at the rate of 3800 watts.

If we would assume an electric heater using mostly convection operates at 100% efficiency (which I understand is pretty much the case), this is the size of electric resistance heater we would need. (If a radiant, or infrared, heater, rather than a convection heater, were used, a different heating system factor would be used and the result might be slightly different. Convection heaters heat and circulate the air, while radiant heaters heat the objects in the greenhouse, but not the intervening air.) My calculation of heat loss includes an additional 20% as a safety margin, to account for possible errors, mis-approximations, etc., as well as the possibility that the temperature might actually drop somewhat below -10 degrees F.

If the greenhouse is to be heated with liquid propane (LP) gas, this wattage amount must be converted to BTU/hr, which gives us a heat loss value equal to approximately 13,000 BTU/hr heat output. If we assume our LP gas heater operates at only 70% efficiency, the heater input rating must be at least 18,600 BTU/hr.