Gas and Oil Home Heating Furnaces - sizing a gas furnace

We are currently in the market for a new gas furnace (to replace our old oil one) and have had several quotes...but they all have different size BTU's. We are looking for a formula to help us determine the best size ourselves. Does anyone have one? Our house is a one and a half story wood frame home (built in 1867). It's about 2,600 sq. ft. with four chimneys (only one still works) and a crawl space instead of a basement. It is generally cool inside year round. We just put on a new roof and most of our windows face the west (our prevailing wind is from the northwest)...could someone help us please. K

Hi Kater:


Go to www.hvaccomputer.com (http://www.hvaccomputer.com) where for $39 you can utilize a program to do a room by room heating/cooling load calculation for your house. It's a bargain. Thanks. John.

Thanks bigjohn...I actually found the site after I posted the question...it's amazing what you can find on the web these days. Cheers. K