People love square-foot rules because they are fast. Contractors love them because they let them say something confident before anyone has done any real work. Homeowners love them because they feel simple. Unfortunately, heat pumps do not care how emotionally satisfying the shortcut felt.

Two houses with the same floor area can have very different heating loads. One may have large north-facing glass, average insulation, and a cold unfinished basement. Another may have better windows, better air sealing, a warmer basement condition, and a tighter envelope overall. Same square footage. Completely different heat loss profile.

That is why a real CSA F280 heat loss calculation matters. You are not trying to estimate a generic house. You are trying to size a heating system for this house — with its actual plans, glazing, insulation, room volumes, and local Ontario design temperature. Use our free design temperature lookup to confirm your municipality's value before any conversation about equipment starts.

Square-foot rules also make oversizing sound safe. Homeowners hear "better a little too big than too small." But oversized heat pumps short-cycle, cost more up front, and behave less gracefully than a well-matched system. Undersized systems have their own problems, especially if nobody thought clearly about auxiliary heat. Guesswork is bad in both directions. Our cold climate heat pump guide covers exactly what happens to output at -22°C and -28°C when the sizing started from the wrong number.

Ontario's code language points to CSA F280 for determining heating and cooling loads. That is why we keep coming back to it — this is not some optional consultant flourish. It is the disciplined way to figure out what the house actually needs.

A proper F280-based sizing path looks at the building envelope, window areas, room conditions, ventilation assumptions, local design data, and the real geometry of the house. That is why our process begins with a CSA F280 heat loss calculation before we talk seriously about equipment selection.

It is also why permit-ready heat pump sizing often belongs inside a bigger package. When the house needs more than a simple equipment recommendation, the right next step may be a full HVAC design package or ventilation design alongside. Sizing is not isolated from the rest of the mechanical design — it drives it.

If you are comparing heating strategies for a high-performance home, two useful reads: geothermal vs air-source heat pump in Ontario and best heating system for an ICF home.

Good sizing starts with good inputs. That usually means plans, elevations, insulation details, window information, municipality, and enough mechanical context to understand what the house is trying to do. The cleaner the information, the better the sizing result.

What we usually want to see:

• Floor plans for all levels
• Elevations and sections if available
• Window and door sizes or schedules
• Envelope and insulation details
• Project location in Ontario
• Whether the system is intended to be ducted, ductless, or hybrid

That is one reason the heat pump sizing page lives so closely beside our heat loss calculation and permit HVAC design pages. These are connected services, not random pages holding hands for marketing. For ICF builds specifically, standard inputs underestimate the insulation performance — the 2026 Ontario heat loss guide covers how ICF construction changes the calculation.