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Updated June 2026What is cooling load?
Cooling load is the rate at which heat must be removed from a conditioned space to maintain a comfortable indoor temperature during peak summer conditions. It is measured in BTU/hr or tons of refrigeration (1 ton = 12,000 BTU/hr). Like heat load in winter, the cooling load determines the minimum capacity your air conditioning system must provide.
Cooling load has two components: sensible load (heat that raises the air temperature — sun through windows, conduction through walls, internal equipment) and latent load(moisture that must be removed to control humidity — occupants breathing, cooking, infiltration of humid outdoor air). In Canada's humid summer climate, the latent load is significant and is a major reason why oversizing causes comfort problems.
Why AC sizing matters in Canada
Canada's humid continental climate (eastern provinces) and semi-arid climate (Prairies) create different sizing challenges. In Ontario and Quebec, summer humidity is high — a properly sized AC runs long cycles, removing moisture and maintaining dry, comfortable air. An oversized unit blasts cold air, satisfies the thermostat temperature setpoint in minutes, and shuts off — leaving humidity high and air clammy.
In Alberta and Saskatchewan, summers are hot but dry. Cooling load is dominated by sensible heat (temperature) rather than latent heat (humidity). This means AC sizing in Calgary is more straightforward — oversizing is still wasteful but the humidity problem is less pronounced.
In coastal BC, summers are mild and the cooling season is short. A properly sized unit — often just 1–1.5 tons for a typical home — handles the modest summer heat without the oversizing trap that affects eastern Canada.
How cooling load is calculated
The CSA F280 and ASHRAE simplified method accounts for the following heat gain sources:
- Conduction through envelope: Heat flowing in through walls, ceiling, floor, windows. Q = U × A × ΔT, where ΔT = (outdoor design temp) − (indoor setpoint, typically 24°C)
- Solar gain through windows: Direct solar radiation through glazing — highest on west and south faces in afternoon. This is often the largest single cooling load component in modern homes with large windows
- Infiltration: Hot, humid outdoor air leaking in through cracks and gaps, adding both sensible and latent load
- Internal gains: Occupants, lighting, appliances, electronics — all contribute heat to the conditioned space
- Latent load: Moisture from occupants and infiltration that the AC must condense and remove
Worked examples
Example 1 — 140 m² home in Toronto (Ontario):
- Summer design: 33°C dry-bulb, 24°C wet-bulb. ΔT = 33 − 24 = 9°C
- Envelope conduction (walls + roof + floor): approximately 1,500 W
- Window solar gain (20 m² of glazing, south and west): approximately 3,500 W peak
- Infiltration + occupants + appliances: approximately 2,000 W
- Total sensible: ~7,000 W; add latent ~2,000 W
- Total cooling load: ~9,000 W = ~30,700 BTU/hr ≈ 2.5 tons
Example 2 — 140 m² home in Calgary (Alberta):
- Summer design: 28°C dry-bulb, 17°C wet-bulb (low humidity). ΔT = 28 − 24 = 4°C
- Lower ΔT reduces conduction gains by ~55% vs Toronto
- Minimal latent load due to low humidity
- Total cooling load: approximately 15,000–18,000 BTU/hr ≈ 1.5 tons
Summer design temperatures by Canadian city
| City | Province | Summer Design Temp | Wet Bulb | Typical 150m² home |
|---|---|---|---|---|
| Windsor | ON | 33°C | 25°C | 2.5–3.0 tons |
| Toronto | ON | 33°C | 24°C | 2.0–2.5 tons |
| Ottawa | ON | 31°C | 23°C | 2.0–2.5 tons |
| Montreal | QC | 30°C | 23°C | 2.0–2.5 tons |
| Halifax | NS | 28°C | 22°C | 1.5–2.0 tons |
| Calgary | AB | 28°C | 17°C | 1.5–2.0 tons |
| Edmonton | AB | 27°C | 19°C | 1.5–2.0 tons |
| Vancouver | BC | 29°C | 21°C | 1.5–2.0 tons |
| Victoria | BC | 26°C | 19°C | 1.0–1.5 tons |
| Winnipeg | MB | 30°C | 23°C | 2.0–2.5 tons |
Frequently asked questions
What size air conditioner do I need for a 1,500 sq ft home in Canada?
A 1,500 sq ft (139 m²) home in Ontario typically needs 1.5–2 tons (18,000–24,000 BTU/hr). In a hotter location like Windsor (33°C design) or with large south-facing windows and poor insulation, size up to 2.5 tons. In Calgary (28°C design) or Vancouver (29°C), a 1.5-ton unit is often sufficient for the same size home.
How many BTU per square foot in Canada?
A rough rule is 150–200 BTU/hr per m² (14–19 BTU/hr per sq ft) for average Canadian conditions. This translates to roughly 1 ton per 65–85 m². However, this rule is highly sensitive to climate zone, solar exposure, insulation quality, window area, and internal heat gains. This calculator refines the estimate based on your specific city and home details.
Why does oversizing an AC cause problems?
An oversized air conditioner cools the space quickly but shuts off before completing a full refrigeration cycle. This short-cycling means the unit never runs long enough to dehumidify the air — the result is a cold, clammy space with high relative humidity. It also causes rapid on/off cycling that wears out the compressor faster. In Canada's humid summer climate, dehumidification is as important as temperature control.
Do I need air conditioning in Canada?
AC adoption in Canada has grown dramatically. In southern Ontario (Toronto, Windsor), summer design temperatures reach 33°C with high humidity — air conditioning is considered essential. In Calgary and Edmonton, summers are hot (28°C) but dry, making AC common but less critical. In Vancouver and Victoria, summers are mild and many homes still lack AC, though heatwave events have increased demand significantly.
What is a ton of air conditioning?
One ton of air conditioning equals 12,000 BTU/hr of cooling capacity. The term comes from the cooling effect of melting one ton of ice over 24 hours. A typical Canadian home needs 1.5–3 tons (18,000–36,000 BTU/hr). Central air conditioning units are available in increments of 0.5 tons — 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons are common residential sizes.
Does a heat pump replace both the furnace and air conditioner?
A heat pump can provide both heating and cooling, replacing a central AC entirely and supplementing or replacing a furnace. In moderate climates like coastal BC, a heat pump alone may provide all the heating needed. In colder Canadian climates (Ontario, Prairies), a cold-climate heat pump can operate efficiently down to −25°C or lower, significantly reducing dependence on gas heating.
How does attic insulation affect cooling load?
The attic is the largest source of cooling load in most Canadian homes during summer — sun heats the roof to 60–70°C and conducts heat into the living space below. Increasing attic insulation from RSI-5 (R-28) to RSI-8.6 (R-49) can reduce the cooling load through the ceiling by up to 40%, potentially allowing a smaller AC unit and significantly reducing summer electricity bills.
What is the summer design temperature used for cooling calculations?
The summer design temperature is the outdoor dry-bulb temperature at the 97.5th percentile — meaning it is exceeded only 2.5% of summer hours. For Toronto, this is 33°C dry-bulb with a wet-bulb of approximately 24°C (which characterizes humidity). NBCC Appendix C provides these values for Canadian cities and they form the basis of all CSA F280 cooling calculations.
Code reference
Based on CSA F280-12 (R2017) (cooling load methodology) and ASHRAE Handbook of Fundamentals simplified cooling load method, adapted for Canadian climate data from NBCC 2020 Appendix C. Have cooling system sizing performed or verified by a licensed HVAC contractor.