What AFUE means on a furnace efficiency label

What does the AFUE rating measure?

AFUE compares the heat a furnace delivers with the energy in the fuel it burns, expressed as a percentage across a standard season of starts, stops and steady running. It is a laboratory figure, set by a test method, so that models can be compared on the same basis. Higher means less fuel wasted for the same heat. A furnace rated at 90 percent loses about a tenth of the fuel's energy as exhaust and cabinet losses.

The rating applies to furnaces and boilers that burn fuel. Heat pumps are rated in different terms because they move heat rather than burn fuel, and electric resistance furnaces turn nearly all electricity into heat, so their rating looks excellent even where electricity is the costliest way to heat. Fuel price matters more than the percentage in that comparison; see natural gas versus electric heating.

What is a good AFUE rating?

The efficiency ratings of furnaces fall into broad bands. Older furnaces were often in the range of the sixties or seventies, mid-efficiency models are around the eighties, and condensing high-efficiency models are in the nineties. Condensing furnaces get there by cooling the exhaust enough to turn its water vapour back into liquid, which releases extra heat. That is why they need plastic venting and a condensate drain.

Canada regulates the minimum efficiency of furnaces sold, and the bar for gas models has risen in recent years, so most new gas furnaces on the market are high-efficiency ones. The current minimum is set in federal energy efficiency regulations, so check the Natural Resources Canada pages if the exact figure matters to you. For a cold climate with a long heating season, a higher rating pays off more than in a mild region, and the new furnace guide includes a worked example of what a gap in AFUE means in fuel terms.

What does AFUE not tell you?

AFUE describes the furnace, not the house. It does not include heat lost through leaky or uninsulated ducts, from a wrongly sized unit, or from a home that leaks air. A 96 percent furnace that short cycles or feeds ducts running through a cold attic will deliver less than its label suggests. It also leaves out the electricity used by the blower, which is why variable-speed motors can make a measurable difference in running cost.

Comfort is not part of the rating either. Two furnaces with the same AFUE can feel different: a single-stage unit blasts and shuts off, while a two-stage or modulating one runs longer at lower output. The two-stage and variable-speed comparison explains that difference. The best savings often come from sizing the furnace right, sealing the ducts and improving the envelope, which an energy audit can identify.

How do you use AFUE when buying?

Use it as one input. Look for the rating on the product label, the manufacturer's data sheet and the quote, and ask the contractor to state it in writing. Estimate savings by comparing the fuel burned by the current furnace and the new one for your house, using your fuel bills and local prices. The gain from moving from a rating in the eighties to the nineties is real, but the dollar value depends on your fuel cost and how much you heat.

Then weigh the extra cost of a high-efficiency model, including venting and the drain, against the saving. If the furnace also affects rebate eligibility, read the program rules, since some set a minimum rating. The furnace rebates guide shows how to check them, and the furnace replacement guide puts efficiency in context with age, size and installation.

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