We are replacing a 13 year old heat pump system and considering a duel fuel system(heat pump as primary with propane backup). We already have a propane cooktop and clothes dryer. We live in the Washington DC area and our house is roughly 3200 sq ft above grade. Our current gas consumption is only 4 gal/month (47 gal/yr). The system we are considering is a Carrier 38YXA030 2.5 ton 14 SEER Puron system with a 58MVP060, 94% AFUE 2-stage variable speed furnace and CK5PX030 evaporator coil. We will also be using the Infinity controller. Note that there will be a separate straight heat pump system for the upstairs.
My question is this: Can anybody estimate how much propane we will consume in an "average" Mid-Atlantic winter? The only outstanding issue is whether 2 100 gal propane tanks will be big enough, because we can not use a 500 gal tank and don't want to install a 3rd 100 gal tank. If we can't reasonably get away with using just 2 100 gal tanks, we will probably forgo the propane backup and go straight heat pump, although this will make me very unhappy.