Lets do the math, 1 gal of Lp is 91,500 btus, Your furnace is usually 30,000btu per hr full flame so if your furnace BURNS for 3 hours you will use approx 1 gallon of gas. A full 20# cylinder holds 4 gallons, 30# 7 gallons and a 40# 9.25 gallons. Keep in mind if you keep the tstat low it will not burn as much. If you store it inside a building it exhausts co the owner may or may not appreciate you doing that.
Jim & Debbie England
Do you have Gas? 2015 F350 6.2L CCLB DRW 4.30 axle. 2012 Yellowstone Ridgeline 34RLT Fifth Wheel