So a gallon of Propane has 91,452 BTUs / hr. Our Atwood furnace uses 16,000 BTU / hr input. The normal 20# tank holds about 4.2gal.
91452 x 4.2 ÷ 16,000 (use your number here) = 24 hours of running time heater only. Your mileage may vary!
We also use ours for cooking and hot water. With no...