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HVAC Design Temperatures Ontario by City 2026: 99% Heating and 1% Cooling Design Conditions

If a contractor sizes your furnace or heat pump against the wrong outdoor temperature, everything downstream is wrong: equipment capacity, ductwork, expected fuel use, even how comfortable your house feels in February. The right numbers are not opinions. They are published values from the ASHRAE Handbook of Fundamentals, built from decades of Environment Canada climate data, and referenced by CSA F280-12 and the Ontario Building Code. This guide lists them for more than 20 Ontario cities and explains how Manual J, CSA F280, and the OBC use them.

By The Get a Better Quote Research Team

Key Takeaways

  • The 99% heating design temperature is the outdoor dry-bulb value that a city is colder than only 1% of annual hours. The 1% cooling design temperature is the value exceeded only 1% of annual hours.
  • ASHRAE publishes these values in the Handbook of Fundamentals (2021 edition is current for 2026 Ontario work).[1]
  • Toronto is roughly -22 C heating and 31 C cooling. Ottawa is -25 / 31. Sudbury is -30 / 29. Thunder Bay is -32 / 29.
  • CSA F280-12 is the Canadian standard for residential load calculations and is referenced by the Ontario Building Code. Manual J is the US equivalent and is commonly used in Ontario for equipment compatibility.[3]
  • Dew point drives the latent cooling load. Southern Ontario sees 1% dew points of 21 to 24 C during humid summers. Sizing on dry-bulb alone produces clammy houses.
  • Sizing to the record low instead of the 99% value wastes money and degrades runtime efficiency. Short-cycling oversized systems are the single most common Ontario HVAC installation mistake.

What the 99% Heating Design Temperature Actually Means

The 99% heating dry-bulb temperature is the outdoor air temperature that a location falls below only 1% of the hours in a typical year, averaged over a long baseline (25 to 30 years of Environment Canada station data). For a typical year of 8,760 hours, that means the outdoor air is colder than the 99% value for roughly 87 hours. Everything else is warmer.[1]

ASHRAE also publishes a 99.6% value (colder than this for only 0.4% of hours, about 35 hours per year) and a 98% value (colder than this for 2% of hours). Most Ontario residential work uses the 99% value. Critical or sensitive facilities occasionally use 99.6% for extra margin, but that decision should be deliberate, not casual.

The 99% value is not the record low. Toronto has hit below -30 C in extreme winters, but its 99% value is close to -22 C. The gap exists because sizing to a once-a-decade spike wastes capacity on hours that almost never happen, and oversized systems short-cycle, run inefficiently, and dehumidify poorly.[6]

What the 1% Cooling Design Temperature Means

The 1% cooling dry-bulb is the mirror image: the outdoor temperature exceeded only 1% of the annual hours, which for most of southern Ontario lands around 29 to 31 C. ASHRAE also publishes a mean coincident wet-bulb temperature, which is the wet-bulb reading typically observed when the dry-bulb is at the 1% value. Both matter. Dry-bulb drives the sensible load (raising air temperature). Wet-bulb drives the latent load (stripping humidity).[1]

A cooling system sized to the 1% dry-bulb but ignoring wet- bulb can hit the thermostat set point on the hottest afternoon and still leave the house clammy because it dehumidified too little. This is especially relevant for modulating heat pumps and variable-capacity AC systems, which behave very differently from old single-stage units when latent load is mis-estimated.

Comprehensive Ontario City Table (ASHRAE 2021)

The table below lists 99% heating and 1% cooling design temperatures for 20-plus Ontario cities, drawn from ASHRAE's 2021 Handbook of Fundamentals (Chapter 14, Climatic Design Information), which is built from Environment Canada climate station normals over a 25 to 30 year baseline.[1][2] Values are rounded to the nearest whole degree Celsius. For permit-level work, always pull the full set (99.6%, 99%, 98% heating; 0.4%, 1%, 2% cooling; wet-bulb; dew point) for the specific weather station closest to the building site.

City99% Heating (C)1% Cooling (C)Climate Zone (OBC)
Toronto (Pearson)-2231Zone 5
Toronto (City / Island)-1830Zone 5
Mississauga-2131Zone 5
Hamilton-2031Zone 5
Burlington-2030Zone 5
Oakville-2030Zone 5
Barrie-2530Zone 6
Oshawa-2230Zone 5
Ottawa-2531Zone 6
Kingston-2430Zone 6
Peterborough-2530Zone 6
Kitchener-Waterloo-2130Zone 5
Guelph-2230Zone 5
London-2030Zone 5
Windsor-1832Zone 5
Sarnia-1931Zone 5
St. Catharines / Niagara-1830Zone 5
Sudbury-3029Zone 6
North Bay-3029Zone 6
Sault Ste. Marie-2728Zone 6
Thunder Bay-3229Zone 7A
Timmins-3429Zone 7A
Kenora-3329Zone 7A
Kapuskasing-3528Zone 7A

A few patterns jump out. Southern Ontario cities along Lake Ontario and Lake Erie are the mildest heating climates in the province (Windsor, Toronto Island, Niagara all around -18 C). Northern cities (Timmins, Kapuskasing) are 15-plus degrees colder at the 99% value, which doubles or triples the heating load for the same envelope. Cooling spreads are narrower: the hottest city (Windsor at 32) is only 4 degrees warmer than the coolest (Sault Ste. Marie and Kapuskasing at 28).

Using Design Temperatures for Manual J (and CSA F280)

Manual J, the ACCA residential load calculation standard, and CSA F280-12, the Canadian equivalent referenced by the Ontario Building Code, both use the same approach: compute the heat flow through every envelope element (walls, ceilings, windows, doors, floors, infiltration) at the design temperature difference between indoor set point and outdoor 99% or 1% value, then sum the result.[3][6]

For a Toronto home, a simplified Manual J / F280 heating calculation looks like this:

Using -30 C instead of -22 C for Toronto inflates the delta-T to 52 C, an 18% jump. The whole-home load jumps by the same factor. The equipment gets oversized by 18%, runs shorter cycles, and burns through its modulation range less efficiently. Multiply that across tens of thousands of Ontario installations and the waste is enormous. This is why the standard exists and why it matters which outdoor temperature shows up on the load calculation.

For a deeper walk-through of the Manual J methodology specifically, see our Manual J Load Calculation Ontario 2026 guide, and for heat pumps specifically (where low-ambient capacity derating interacts with the 99% value), see Heat Pump Sizing Ontario 2026.

How the Ontario Building Code References Design Conditions

The Ontario Building Code (OBC, O. Reg. 332/12) incorporates CSA F280-12 by reference for residential space heating and cooling appliance sizing. CSA F280-12 in turn pulls its climate inputs from the same Environment Canada data that ASHRAE uses in its Handbook of Fundamentals.[4][3] In practice this means:

For homeowners, the practical signal is to ask the contractor for a written CSA F280 or Manual J report with the specific city and design temperatures shown on the first page. If they cannot produce one, they are not meeting the OBC requirement, even if the install ends up passing inspection. For more context on how sizing ties into the broader Ontario permit landscape, see HVAC Sizing Ontario.

Dew Point and Latent Cooling in Ontario

Dry-bulb temperature tells you how hot the air is. Dew point tells you how much moisture is in it. In southern Ontario, the 1% design dew point typically sits between 21 and 24 C during humid summer months, which is high enough that latent load (humidity removal) becomes a meaningful fraction of total cooling energy.[1]

An oversized air conditioner hits the thermostat set point quickly, shuts off, and never runs long enough to strip humidity out of the air. The result is a house that is 22 C but feels clammy and uncomfortable. An undersized AC runs constantly and may fail to hit the set point on a hot afternoon, but often feels better because it is dehumidifying continuously. A correctly sized AC (from a proper Manual J or CSA F280 calculation that accounts for both sensible and latent loads at design conditions) runs long enough to strip humidity and short enough to hit the set point.

For variable-capacity heat pumps and modulating inverter- driven AC systems, this matters even more. These systems reach their highest efficiency when they can modulate down during mild weather, but they still need enough design capacity to handle a hot, humid 1% day with latent load included. Sizing only on dry-bulb and ignoring dew point is one of the most common reasons new heat pumps get negative comfort reviews in Ontario.

Practical Takeaways

Related Guides

Frequently Asked Questions

What does 99% heating design temperature mean?

It is the outdoor dry-bulb temperature that a location is colder than only 1% of the hours in a typical year, based on long-term climate records. Put another way, the outdoor temperature equals or exceeds the 99% value 99% of the time during a normal heating season. ASHRAE publishes these values in the Handbook of Fundamentals and they are the basis for sizing residential heating equipment under CSA F280 in Ontario and Manual J in the United States. Sizing to this value means the system is designed to handle nearly every real winter hour, not the single coldest spike of the decade.

What does 1% cooling design temperature mean?

It is the outdoor dry-bulb temperature that a location exceeds only 1% of the hours in a typical year. For Ontario, these values cluster around 29 to 31 degrees Celsius depending on the city. Sizing air conditioning to the 1% value, rather than the hottest day on record, prevents the oversized short-cycling systems that dominated the market before Manual J adoption. ASHRAE also publishes a coincident wet-bulb temperature alongside the 1% dry-bulb value, which is what drives the latent cooling load.

Where do ASHRAE design temperatures come from?

The values are computed from Environment Canada climate station data over a long baseline period, typically 25 to 30 years, and published in the ASHRAE Handbook of Fundamentals. The 2021 edition is the current reference for 2026 Ontario work. Each weather station has a full set of design conditions: 99.6%, 99%, and 98% heating; 0.4%, 1%, and 2% cooling; plus coincident wet-bulb, dew point, and wind values. CSA F280-12 and the Ontario Building Code both point back to these same underlying climate records for load calculation inputs.

Can I use the record low for my city instead of the 99% value?

You can, but you will oversize the system and waste money. Furnaces and heat pumps sized to the record low spend their entire lives short-cycling because the load they were designed for almost never occurs. CSA F280-12 and Manual J both require the 99% value precisely because it balances capacity against runtime efficiency. If a contractor is using the record low or a round number like -30 degrees for Toronto (where the 99% value is closer to -22), the system is being oversized for no real benefit.

Why does the Ontario Building Code care about design temperatures?

The Ontario Building Code (OBC) references CSA F280-12 as the required method for determining residential heating and cooling loads, and CSA F280 uses ASHRAE design conditions as a primary input. When an HVAC permit application is submitted, the load calculation supporting the equipment selection must be based on the correct municipal design temperatures. A contractor who cannot produce a CSA F280 calculation with the right design temperatures for the specific municipality is not meeting the OBC requirement, even if the job passes a rough-in inspection.

How does dew point factor into cooling design?

Dry-bulb temperature tells you the sensible cooling load. Dew point tells you the latent load, which is the energy needed to remove humidity. ASHRAE publishes a 1% dew point value for each weather station, and in southern Ontario that value is typically 21 to 24 degrees Celsius during the humid summer months. A heat pump or air conditioner sized only on dry-bulb temperature can meet the thermostat set point but still leave the house clammy if the latent load is higher than the equipment can strip. This is why proper Manual J and CSA F280 calculations always include wet-bulb and dew point inputs, not just dry-bulb.

  1. ASHRAE 2021 ASHRAE Handbook of Fundamentals (Chapter 14: Climatic Design Information)
  2. Environment and Climate Change Canada Canadian Climate Normals 1991-2020
  3. CSA Group CSA F280-12: Determining the Required Capacity of Residential Space Heating and Cooling Appliances
  4. Government of Ontario Ontario Building Code (O. Reg. 332/12)
  5. Natural Resources Canada Keeping the Heat In (Home Energy Retrofit Guidance)
  6. ACCA Manual J 8th Edition: Residential Load Calculation (ANSI/ACCA 2 Manual J - 2016)