I got bids from five vendors to change my furnace. But their size estimates range from 170,000 BTU input to 250,000 BTU input. They each claim their sizing calculations to be correct. how can there be such a wide variation? Is there some formula I can use to make an independent ballpark calculation to size my furnace given house parameters? I don't want to pay more for an unnecessarily large furnace. on the other hand, i don't want to freeze in winter either. thank you for your answers.