Standard Sizing Range
AFUE Efficiency Comparison
| AFUE | Required Input | Annual Gas | Annual Cost | vs 80% AFUE |
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How to size a furnace correctly
The right furnace size starts with an accurate heat load calculation — not a rule of thumb like "40 BTU per square foot." A 2,000 ft² home in Vancouver needs roughly 40,000 BTU/hr. The same home in Edmonton needs 70,000 BTU/hr. Square footage alone tells you nothing about what the furnace actually has to do.
Output BTU/hr vs. Input BTU/hr
Furnaces are sold by input BTU/hr — the gas they burn. What heats your home is the output: input × AFUE. A 100,000 BTU/hr input furnace at 95% AFUE delivers 95,000 BTU/hr to the house. Always size to output, not input. The AFUE calculator converts between input and output for any efficiency level.
Standard equipment sizing practice
Standard equipment sizing practice specifies furnace output must be between 100% and 140% of the design heat loss. Going below 100% means the furnace can't keep up on the coldest days. Going above 140% causes short-cycling and all the problems that come with it. This calculator flags both conditions clearly. Most permit applications in Canada require sizing documentation showing compliance with these standard ratios.
92% vs 95% AFUE: is the upgrade worth it?
On a 60,000 BTU/hr heat load in Toronto (4,000 HDD), the difference between 92% and 95% AFUE saves roughly $80-120/year in gas depending on local prices. With a $300-500 premium for the higher efficiency unit, payback is 3-5 years. In colder cities with more HDD the savings are proportionally larger. Use the AFUE calculator for a precise payback analysis.
Frequently Asked Questions
There's no reliable answer without knowing your city, insulation levels, window area, and ceiling height. A 2,000 ft² home in Vancouver with good insulation might need 40,000 BTU/hr. The same floor plan in Edmonton with average insulation could need 70,000 BTU/hr. Use the heat load calculator to get the actual number for your home, then come back here to size the furnace. The BTU-per-square-foot rule of thumb is unreliable enough that contractors who use it are guessing.
Standard sizing practice allows furnace output up to 140% of design heat loss. So if your heat loss is 80,000 BTU/hr, the maximum allowed output is 112,000 BTU/hr. A 120,000 BTU/hr output furnace (about 126,000 BTU/hr input at 95% AFUE) would exceed this limit and is technically oversized. An oversized furnace short-cycles, causes temperature swings, and wears out faster. Go with the correctly sized unit — it's better on every measure.
Yes, in most cases. A two-stage furnace runs at 65% capacity most of the time and only fires at 100% on the coldest days. This extends run times, improves temperature distribution, reduces noise, and improves humidity control in winter. A modulating furnace adjusts continuously between roughly 40-100% — even better comfort and efficiency. The premium over single-stage is $300-600. In a cold Canadian climate with a well-sized load calculation, a two-stage or modulating 95-97% AFUE furnace is the standard recommendation.