Furnace and heat pump sizing in Ontario

Toronto runs about 3,434 heating degree-days a year, which puts it in the moderate band at roughly 40 BTU per square foot of heating. But Ontario does not sit in one band — the table below shows the spread.

By Francesco Zinghinì

Ontario is two provinces in heating terms. Windsor, Toronto, London and Ottawa all fall in the same band, but that hides a wide spread inside it: Ottawa needs about a third more heating over a year than Windsor does. North of the Canadian Shield the picture changes outright — Sudbury sits a full band colder than any city in the south. Sizing off “Ontario” as a unit is the common mistake here.

Heating degree-days and January temperatures in Ontario

Degree-days below 18 °C and January temperatures from the Environment and Climate Change Canada Canadian Climate Normals 1991–2020. The band column applies the mapping described on the Canada index.

City	Heating degree-days (18 °C)	January daily mean	January daily min	Band	BTU/sq ft
Toronto	3,434	-3.5 °C	-6.7 °C	Moderate	~40
Ottawa	4,434	-10.0 °C	-14.3 °C	Moderate	~40
London	3,872	-5.4 °C	-9.1 °C	Moderate	~40
Windsor	3,364	-3.6 °C	-7.1 °C	Moderate	~40
Sudbury	5,114	-12.7 °C	-17.5 °C	Cold	~50

How the province compares to itself

Worked example

Take a 1,500 sq ft home in Toronto. At the moderate band of 40 BTU per square foot:

1,500 sq ft × 40 BTU/sq ft = 60,000 BTU/h

A gas furnace is rated by input, so divide the output by the AFUE to get the rating you shop for: at 95% AFUE that is about 63,000 BTU/h of input. That falls between the standard 60,000 and 80,000 BTU/h sizes. Which one you should buy is exactly the question a load calculation answers and a rule of thumb cannot — rounding down leaves the house short on the coldest nights, and rounding up brings the short-cycling problems of an oversized furnace. Run your own numbers in the furnace-size calculator, and use AFUE typical bands if you do not have the nameplate.

Heat pumps in Ontario

A heat pump loses capacity as the outdoor temperature falls, exactly when the house needs the most heat. The temperature where its output equals the house load is the balance point; below that, backup heat makes up the difference. Toronto averages a January daily minimum around -6.7 °C, and the design condition is colder still.

A cold-climate air-source heat pump is a reasonable primary heat source at these degree-days, with backup sized to cover the coldest stretch. The question worth asking a contractor is what capacity the unit still delivers at your design temperature — not its rated capacity.

The heat-pump-size calculator works from the same band, the balance-point calculator finds the crossover temperature. Dual fuel and the balance point explains where backup heat takes over.

A note on units

Canadian homes are almost always described in square feet and equipment is rated in BTU/h or tons, but outdoor temperature is in °C and degree-days are on the 18 °C basis rather than the American 65 °F. The two degree-day scales are not interchangeable: an American HDD65 figure is about 1.8× the Canadian HDD18 number for the same place. Every degree-day figure on this page is the Canadian 18 °C basis. If you have a floor area in square metres, multiply by 10.76 to get square feet before using the calculators.

Before you buy equipment: these bands are rule-of-thumb estimates from degree-days, not a load calculation. Canadian equipment sizing is done against the January 2.5% design temperature for your municipality, published in Appendix C of the National Building Code of Canada (and reproduced in provincial codes), and against a CSA F280 or Manual J load calculation for your specific house. Ask your contractor for theirs. Gas, electrical and refrigerant work must be done by a licensed professional.

Frequently asked questions

What size furnace do I need in Ontario?

As a first estimate, multiply your heated floor area in square feet by 40 BTU/h per square foot — the moderate band that Toronto’s 3,434 heating degree-days fall into. A 1,500 sq ft home works out to about 60,000 BTU/h of output. That is a sanity check, not a specification: the size you buy should come from a CSA F280 load calculation.

Why not use a US climate zone for Ontario?

IECC climate zones are defined for the United States and most Canadian provinces fall outside or at the extreme edge of the range. Heating degree-days are published for Canadian stations by Environment and Climate Change Canada and describe the same thing more directly, so these pages use them instead.

Does one band apply to all of Ontario?

No. The cities in the table above fall into more than one band, which is why this page lists them separately. Use the row closest to where you actually live; using a provincial average would oversize equipment in the mild part and undersize it in the cold part.

Is this the design temperature my contractor uses?

No. Canadian equipment sizing is done against the January 2.5% design temperature for your municipality, published in Appendix C of the National Building Code of Canada. The January normals on this page describe an average January, not the design condition, and are here to place the province in a band — not to replace the code table.