Determine the British Thermal Unit per Hour (BTUH) rating of the furnace. This is located in the user manual or on a placard attached to the appliance. For this example, say your furnace has a BTUH of 26,000.
Convert the BTUH of the furnace into liquid propane pounds and gallons. 1 gallon of liquid propane weighs approximately 4.23 lbs. 1 gallon of liquid propane contains approximately 91,500 BTUs. A furnace with a rating of 26,000 BTUH would consume 1 gallon of liquid propane every 3.5 hours (91,500 / 26,000 = 3.5).
Calculate the length of time your propane tank can fuel the furnace. A standard portable 40-lb. propane tank holds approximately 5 gallons, or 22 lbs., of propane -- the rest of the weight is the tank itself. A standard stationary tank rated at 40 lbs. holds 9.5 gallons, or 40 lbs., of liquid propane. Multiply the burn rate calculated in the previous step (1 gallon every 3.5 hours) by the amount of propane in the tank. The 40-lb. portable tank can fuel the sample furnace for 17.5 hours (3.5 * 5 = 17.5). The 40-lb. stationary tank will last 33.25 hours (3.5 * 9.5 = 33.25).
Things You Will Need
- Stationary propane tanks are rated by the amount they hold. Portable propane tanks are rated by the total weight of the tank plus the contents when full.