I am getting bids to replace a 24 year old gas furnace. two firms told me I need 60,000 BTU and two quoted me 80,000. the current furnace us 80,000 BTUs and the two firms say it is too big. I have tried plugging info into some BTU calculators but am getting mixed results. The house is 1765 sq foot split entry with good insulation and double pane windows and i live in the Puget Sound Region of Washington state.