[AutumnRidge]

Lots of things come into play when figuring heating loads in Greenhouses. Basically you must have enough BTUs to maintain your desired temperature in a given enviroment. Heat loss in Greenhouses can vary greatly such as use of single film or double inflated film, Infrared or conventional film, structure orientation, air distribution, type of floor, irrigation and drainage systems. Double inflated film will save you 45 to 50% on heat loss. another 15 to 20% savings if you choose Infrared film. Orientation is a catch 22. East / West allows more solar energy during daylight hours but cooling expense goes up. North / South equals less Solar energy collection but is easier to cool. Correct air distribution is critical to maintaining even heating. Bare ground is a very good heat sink. Its better to have gravel and a ground cloth to mitigate heat loss. Of course if crops are to be grown in the ground within the structure this is not an option. If irrigation systems uses water that is too cold and is allowed to wet surfaces other than what is intended, it takes a large percenage of the heat you have produced out through your drainage system. Just a rough rule of thumb, a 24 ft wide X 72 ft long X 10 ft tall greenhouse with double Infrared film inflated, takes approx. 175,000 BTUs to keep it at 70 degrees with outside air temperature of 32 degrees and no wind. Below 32 and wind another 175, 000 BTUs and it will be good down to negative 10 with 20 mph wind. One of our 24 X 72 X 10 Greenhouses is set up with two 175,00 BTU heaters. we only use the second one when temps drop below freezing to maintain 70 degrees. In our area we figure 25 BTUs per cu. ft. to mainatin70 degrees at negative 10 degrees. Hope this helps some to figure what you need.