AFUE explained: what the efficiency number really means for Canadian furnaces

AFUE is a laboratory measure of how much of the fuel's energy becomes useful heat over a typical heating season, including the losses that happen at start-up and shut-down that simple combustion-efficiency numbers ignore. A furnace certified at 96% AFUE turns about 96 cents of every dollar of gas into heat; the rest leaves up the vent or through the jacket.

The certification follows standardized test procedures, and manufacturers publish the certified figure by size and, often, by installation position. That is why this site's model pages carry figures like 96.7% AFUE upflow and 95.0% downflow and horizontal: the number on the rating plate is the one that applies to how your furnace is installed.

Since July 3, 2019, residential gas furnaces manufactured for Canada must meet at least 95% AFUE under the Energy Efficiency Regulations, with narrow exceptions for through-the-wall units with integrated cooling and furnaces for manufactured homes. The practical meaning: every standard furnace offered today condenses, needs plastic venting and a condensate drain, and sits somewhere between 95% and 98.5% AFUE.

The 80%-class furnace you may still have in the basement is not illegal to operate; the regulation governs manufacture. When it is replaced, the new furnace is condensing by law.

Between 95% and 98.5%, the difference is about 3.5 gas dollars per hundred: roughly $35 to $100 a year for a typical Lower Mainland house, depending on consumption. Between an 80% furnace and a 95% furnace the difference is 15 to 18 cents per dollar, which is the efficiency leap that actually shows on bills.

The marketing tier above 95% earns its premium on staging and blower behaviour, not on the last points of AFUE. A 97% two-stage furnace and a 96% two-stage furnace are separated by about one gas dollar in a hundred; the comfort difference between tiers is larger than the efficiency difference.

Many furnaces certify different AFUE by installation position. Carrier's Performance 97, for example, certifies 97.0% upflow, 95.0% downflow and 96.1% to 96.7% horizontal depending on size; the Comfort 96 family certifies one figure upflow and horizontal and 95% downflow. The certified figure for your installation is printed on the rating plate, and that is the number to use in comparisons.

The rating plate inside the blower door states the model number, the input in BTU/h, the certified output and the AFUE for the installed configuration. Comparing a quoted furnace to its published data starts there: model number first, then input and output, then AFUE by position. The plate also carries the electrical figures and the date of manufacture, all useful at service time.