What size furnace do you actually need?

How the math works

This calculator uses a simplified version of Manual J — the residential heat-loss calculation methodology used by HVAC contractors and approved by the Air Conditioning Contractors of America (ACCA). The full Manual J accounts for over 30 variables; this tool uses the 5 that drive the largest portion of total heat loss for typical Halton Hills homes.

The simplified math

• Halton Hills design temperature: -22°C. This is the temperature your furnace needs to maintain comfort against — not the coldest day ever, but the 99% design temperature published by Environment Canada and used by ACCA. About 87°F temperature difference between indoor (21°C / 70°F) and design outdoor.
• Base heat loss per sqft ranges from ~25 BTU/sqft (excellent insulation) to ~55 BTU/sqft (poor insulation) at our design temperature.
• Window adjustment adds 10-25% depending on window quality, since windows are the highest heat-loss surfaces in most homes.
• Ceiling height adjustment increases air volume that needs heating — 9 ft adds ~12%, 10 ft+ adds ~25%.
• Output = Input × AFUE. A 96% AFUE furnace with 80,000 BTU input delivers ~76,800 BTU output. Sizing recommendations target output capacity.

Why oversizing is the most common mistake

Most contractors size furnaces with rule-of-thumb (typically 1 ton / 12,000 BTU per 600 sqft) which produces oversized equipment in roughly 70% of Halton Hills homes — particularly homes that have had insulation, window, or air-sealing upgrades since the original install. An oversized furnace short-cycles, doesn't dehumidify properly, wears mechanical components faster, and often costs more to operate than a properly-sized smaller unit.

For more detail on furnace selection, see our furnace buying guide.

This is a sizing estimate, not a quote. A proper Manual J calculation factors in home orientation (south-facing windows gain solar heat), infiltration (how leaky the building envelope is), occupancy patterns, and other variables this tool doesn't ask about. We do free in-home Manual J calculations as part of every quote — book one for an exact recommendation.