The furnace CANNOT use excess LP. If you went thru 20 gallons of LP in 3 or 4 days, you must have a massive leak by the tanks, which you should hear or smell. Until you figure it out, I would get the tanks refilled and observe HOW MANY gallons of LP are actually in the tanks. If the furnace used excess LP(Impossible) you would have black soot all over the exterior vent. I have 43 years as a RV tech and NEVER have I had a LP appliance use EXCESS LP. Doug
How long will a 20lb propane tank last on a 30000 BTU heater?
14.34 hours-----20 hours on a 30 lb
A pound of propane has 21,591 btu. So if you had a 21,500 btu heater it would run basically 20 hours on a true 20lb bottle. With a 30,000 btu heater you multiply 21,500x20(pounds)=430,000 and divide by your heater size of 30,000btu. Which gives you 14.34 hours if you run it at full all the time.