How to Determine the Propane Burn Rate for a Furnace
The propane burn rate of a furnace is an important factor in making the decision about which model to buy or the size of tank to fit it with. All gas appliances come standard with the input British Thermal Units (BTU) per hour, or BTUH, clearly stated. This figure may be used to calculate the length of time it takes for the appliance to consume a certain amount of propane. Propane, often referred to as liquid propane (LP), is sold by the pound or gallon depending on where you live.

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 40lb. 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 40lb. portable tank can fuel the sample furnace for 17.5 hours (3.5 * 5 = 17.5). The 40lb. stationary tank will last 33.25 hours (3.5 * 9.5 = 33.25).
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Tip
 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.