What is an ASHRAE design temperature?

An ASHRAE design temperature is a statistical outdoor temperature used for HVAC sizing calculations. The 99% heating design temperature means outdoor temperatures are at or above this value 99% of all hours in a year — so heating equipment only needs to handle colder conditions 1% of hours (about 88 hours/year). The 1% cooling design temperature means outdoor temperatures are at or below this value 99% of summer hours.

Why should I not use the record low temperature for heating design?

Using the record low would massively oversize HVAC equipment. Record lows occur perhaps once per decade and last only a few hours. ASHRAE 99% design temperatures are specifically chosen so that equipment sized to this temperature will maintain comfort 99% of all hours — only falling short during the most extreme 1% of weather, which is an acceptable engineering tradeoff that prevents oversizing.

What is the difference between 99% and 99.6% design temperatures?

ASHRAE publishes both 99% and 99.6% heating design temperatures. The 99.6% value is colder and more conservative — outdoor temperatures are below this value only 0.4% of hours (about 35 hours/year). Standard residential load calculations use 99% (less conservative). Commercial and critical applications sometimes use 99.6%. This table provides the 99% values, which are standard for residential HVAC sizing.

How do I use the design temperature in a heat load calculation?

The heating design temperature is your outdoor winter design temperature in the heat load calculator. Subtract it from your indoor setpoint (typically 70°F or 21°C) to get the heating ΔT used in the formula Q = U × A × ΔT. For example, Toronto's 99% heating design temperature is 5°F (-15°C). Indoor 70°F minus outdoor 5°F = ΔT of 65°F (36°C). Enter this in our Heat Load Calculator.

🌍 ASHRAE Design Temperatures

Design Temperature Lookup

Find ASHRAE 99% heating and 1% cooling design temperatures for 200+ Canadian and US cities. Required for every heat load and cooling load calculation. Click any city to use its temperatures directly in your calculations.

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🌡 Use in Heat Load Calculator → ❄ Use in Cooling Load Calculator → 📋 Use in Heating & Cooling Load Estimator →

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City ↕	Province/State ↕	Country ↕	Heating 99% ↕	Cooling 1% DB ↕	Cool WB ↕	NBC Zone ↕

ASHRAE Design Temperatures for HVAC Load Calculations

ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes design weather data for thousands of locations worldwide in its Handbook of Fundamentals. This data is the foundation of every HVAC load calculation — without accurate design temperatures, equipment sizing is a guess.

What is the ASHRAE 99% Heating Design Temperature?

The ASHRAE 99% heating dry-bulb temperature is the outdoor temperature equalled or exceeded by 99% of all hours in the heating season (typically October through April in Canada). This means outdoor temperatures fall below the 99% value only 1% of hours — approximately 88 hours per year. Equipment sized to this temperature will maintain indoor comfort during 99% of cold weather conditions. Using the record historical low would massively oversize equipment since those temperatures occur extremely rarely.

What is the ASHRAE 1% Cooling Design Temperature?

The ASHRAE 1% cooling dry-bulb temperature is the outdoor temperature equalled or exceeded by only 1% of all hours in the cooling season. Cooling equipment sized to this temperature will maintain indoor comfort during 99% of hot weather. The coincident wet-bulb temperature (also tabulated here) is needed for latent cooling load calculations. Use both values in our cooling load calculator and our psychrometric calculator.

How to Use Design Temperatures

In the heat load calculator, enter the heating 99% value as your outdoor winter design temperature. In the cooling load calculator, enter the 1% dry-bulb as your summer outdoor design temperature and the coincident wet-bulb for latent load calculations. The ΔT (temperature difference) between indoor setpoint and outdoor design temperature drives all envelope heat loss calculations: Q = U × A × ΔT.

NBC Canada Climate Zones

The National Building Code of Canada assigns buildings to climate zones based on heating degree days. Zone 4 covers the mildest regions (coastal BC), Zone 5 most of southern Canada, Zone 6 the prairies and northern Ontario, Zone 7 includes subarctic regions, and Zone 8 covers the Arctic. Minimum insulation R-values in the NBC vary by zone — see the NBC Canada guide for requirements. Note that US climate zones follow a different ASHRAE 90.1 classification from 1 (hot) to 8 (subarctic).

Frequently Asked Questions

Record lows are historical extremes that may occur once per decade for a few hours. Using them for HVAC sizing would result in massively oversized equipment that short-cycles constantly during normal weather. ASHRAE 99% design temperatures are statistically derived values — outdoor temperatures are below this value only 1% of hours (88 hours/year). Equipment sized to the 99% value maintains comfort in 99% of conditions, which is an excellent engineering tradeoff. See our heat load calculator for how design temperature is used.

Use the design temperature of the nearest listed city with a similar climate. If you are between two listed cities, interpolate conservatively (use the colder heating value). For remote locations, consult the full ASHRAE Handbook of Fundamentals or Environment Canada's Design Climate Data. For heating, erring on the slightly colder side is always the safer choice. Note that elevation differences can significantly affect design temperatures — high-elevation locations are colder.

The coincident wet-bulb (WB) temperature occurs simultaneously with the 1% dry-bulb cooling design temperature. It is used to calculate the latent (moisture) component of the cooling load and to determine outdoor air enthalpy for ventilation load calculations. In very humid climates (southeastern US, coastal regions), the coincident WB is close to the dry-bulb, indicating high moisture content. Use it in the cooling load calculator and psychrometric calculator.