Heat pumps have exterior units, use a separate measure of efficiency, and don’t have an AFUE rating. However, most heat pumps have a back-up heat source, usually a gas or electric furnace ...

expressed by its annual fuel-utilization-efficiency (AFUE) rating,” Stout says. “The AFUE shows what percentage of fuel a furnace converts into heat. So, a furnace with a 97 rating is 97% ...