How many BTUs does a funace need to heat a 1500 Sq ft home?

Where? Florida or North Dakota? It matters... along with other criteria. Proper way to size heat & ac is to get a Manual J heat load calculation done. You can do this yourself but it is tedious for a beginner. Chances are your present equipment is oversized and you can save yourself $ by going down a size or maybe even two with a Hi Eff replacement. google ACCA Manual J and download a copy, even if you decide not to do this yourself at least youll be more informed regarding the process and whats involved. Either way have one done.