Propane Heater Sizing Formula

To determine how long a specific propane powered heater will operate on a bottle of propane, use the following formula: 

****NOTE: Propane burns at the rate of approximately 21,000 BTU per pound.**** 

21,000 (times) bottle capacity in pounds (divided by) heater rating in BTU's.

For example:

21,000 x 100 lb / 250,000 BTU = 2,100,000 / 250,000 (or) 8.4 hours of run time.
21,000 x 20 lb / 35,000 BTU = 420,000 / 35,000 (or) 12 hours of run time. 

It is important to size a heater and bottle properly. Propane stores in a bottle as a liquid, converting to a gas as it is dispensed. This conversion causes a huge temperature drop in the liquid as it evaporates into a gas inside the bottle. A 250,000 BTU construction heater connected to a 20 lb capacity Bar-B-Q bottle will attempt to draw fuel from the bottle so fast that this vaporization will freeze the liquid propane into a solid, thus stopping the conversion from liquid to gas. This can happen in very little time. It is much less likely to occur in a larger 100 lb capacity construction bottle, due to the fact that the larger thermal masss of the liquid in a full cylinder tends to hold it's temperature better.