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OBC DIVISION B · PART 9 · SECTION 9.33 — HEATING AND AIR-CONDITIONING

What indoor temperatures must a home heating system be designed to maintain in Ontario?

Heating systems must be capable of maintaining 22 C (72 F) in living spaces, 18 C (64 F) in unfinished basements, and 15 C (59 F) in heated crawl spaces, at the outdoor winter design temperature for the building's location.

Code reference
Heating facilities are mandatory in dwellings and must be designed to hold prescribed indoor design temperatures: 22 C in living areas, 18 C in unfinished basements and 15 C in heated crawl spaces, measured against the local outdoor January design temperature taken from the Code's climatic data for the municipality.
OBC Division B, Subsection 9.33.3.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

These are design capacities, not thermostat mandates: the system must be able to hit 22 C on the coldest design day, which for much of southern Ontario means outdoor design temperatures around minus 20 C and colder northward. The numbers anchor the heat-loss calculation, so undersized systems fail this test even if they feel adequate in mild weather. Additions and finished basements are the common trouble spots; extending ducts without checking capacity can leave both old and new spaces unable to reach design temperature. Cooling, where installed, has its own summer design conditions.

Plain language

Your furnace or heat pump must be strong enough to keep the living areas at 22 C even on the coldest design night of an Ontario winter. Basements get slightly lower targets, but nothing in the house may freeze.

Related questions

How must heating and cooling systems be sized for houses in Ontario?What are the requirements for heating and air-conditioning ducts in Ontario homes?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.