You may find some info telling you that all you need to do find out the right furnace size is multiply x number of square feet by x number of BTUs to get your size.  While some of these formulas may take into consideration where you live, they do not factor in your house.  Which is what you want to heat, right?  And there is far more to your house than the number of square feet.  All of the following must be included in any calculation to efficiently size your furnace.  What direction do your windows face?  That will impact the amount of heat your home already gets from the sun.  How old are your windows?  Double/single paned?  Are they insulated?  How well insulated is your house?  Walls, attic and basement?  That’s just a few of the things you need to factor into your equation.  You can download a demo of the HVAC software mentioned above and find out more.

What about just buying the same size furnace you already have?  In some circumstances that may be fine-assuming it was sized right.  If you are replacing a furnace that is 15 or more years old, I wouldn’t recommend it though.  Furnace efficiencies have changed significantly.  Chances are that clunker in your basement is way over sized.

I’ll leave you with an irony.  While the above mentioned method is the only proper way to size a furnace, it’s generally very difficult to find an HVAC contractor that will do it.  It’s a time consuming job, and/or they may not have the software that will help them do it.  But the fact that you are here means you are a smart consumer.  You’re doing your research.  Now don’t settle for anything but the best!