How important is AFUE?
The least efficient gas furnace available today has a AFUE of 78%. All furnace manufacturers today build two efficiency ranges of gas furnaces. Their standard gas furnace will be between 78%-81% efficient (called “80% AFUE”) and their high efficiency gas furnace will be between 90%-96% efficient (called “90% AFUE). To be practical, most homes will have some factor that will justify one efficiency range or the other. In very few homes are both efficiency ranges equally practical. The 80% AFUE gas furnaces require some type of masonry or metal vertical chimney. A home with an existing chimney is usually a candidate for the 80% AFUE furnace. A home that currently has no chimney (ie, existing heat pump home that may be switching to natural gas) or a home that has a very old defective chimney, probably would be the best candidate for a 90% AFUE gas furnace. Other factors also play into the choice.
