Standard and code basis
Residential cooling load calculations use the ASHRAE simplified cooling load method, adapted for Canadian conditions:
- Outdoor design temperatures come from NBCC 2020 Appendix C — specifically the 2.5% July cooling design temperature (the temperature exceeded on only 2.5% of summer hours). This ensures the AC is sized for realistic Canadian summer conditions, not extreme US peak temperatures.
- Indoor setpoint: 24°C (75.2°F) — the standard Canadian residential cooling setpoint used in load calculations.
- Solar heat gain coefficients (SHGC) follow ASHRAE standards for single, double, and triple glazing.
Only cities where the NBCC Appendix C summer design temperature is ≥ 24°C appear in the calculator — because below that threshold, no mechanical cooling is needed by definition.
Your inputs — what each field means
| Input | What it represents | Effect on result |
|---|---|---|
| City | Your location | Determines the outdoor summer design temperature. Toronto (33°C design) vs Windsor (32°C) — similar. Calgary (28°C) — significantly lower peak cooling load. |
| Floor area (m²) | Total cooled floor area | Used to estimate wall area and as the basis for lighting and appliance load. |
| Wall insulation (R-value) | Thermal resistance of the wall assembly | Higher R-value walls reduce conductive heat gain. For cooling, walls matter less than for heating — solar gain and internal loads dominate in summer. |
| South-facing windows (m²) | Total glazed area facing south | South windows receive direct sun all day in summer. They carry a higher solar gain multiplier (600 W/m² base irradiance vs 300 W/m² for other orientations). |
| Other windows (m²) | Total glazed area on all other orientations | East/west windows peak at sunrise/sunset respectively. North windows receive no direct sun in Canadian latitudes. Combined as a conservative estimate at 300 W/m² base irradiance. |
| Window type | Single, double, or triple glazing | Determines SHGC (solar heat gain coefficient): single = 0.60, double = 0.40, triple = 0.25. Lower SHGC = less solar gain. |
| Shading | External shading from overhangs, trees, or blinds | Shade multiplier: none = 1.0, partial = 0.6, full = 0.3. Overhangs are the most effective passive cooling strategy — reducing south window solar gain by 40–70%. |
| Occupants | Number of people typically in the home during peak hours | Each person generates approximately 73W of sensible heat. |
The calculation — step by step
- Determine ΔT (temperature difference):
ΔT = outdoor summer design temperature − 24°C (indoor setpoint)
Example: Toronto summer design = 33°C → ΔT = 9°C. - Wall conduction heat gain:
Q_wall = (wall area m²) ÷ (R-value × 0.176) × ΔT
Wall area = 4 × √(floor area) × 2.7m ceiling height − window area. - Ceiling/attic heat gain:
Q_ceiling = (floor area m²) ÷ (50 × 0.176) × (ΔT + 10)
The +10°C accounts for solar gain in the attic raising the attic temperature above the outdoor air temperature on a sunny day. - Window conduction heat gain:
Q_window_cond = (total window area m²) ÷ (WINDOW_R × 0.176) × ΔT - Solar heat gain through windows:
Q_solar = south area × 600 W/m² × SHGC × shade + other area × 300 W/m² × SHGC × shade
SHGC values: single = 0.60, double = 0.40, triple = 0.25. Shade multiplier: none = 1.0, partial = 0.6, full = 0.3. - Internal gains — occupants, lighting, appliances:
Q_occupants = occupants × 73 WQ_lighting = floor area × 8 W/m²Q_appliances = 1,000 W (fixed average for Canadian home) - Apply a 10% safety factor:
total W = (sum of all gains) × 1.10 - Convert to BTU/hr and tons:
BTU/hr = W × 3.412 · tons = BTU/hr ÷ 12,000
Select the smallest standard AC size: 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or 5.0 tons.
Reading your result
- Cooling load (W and BTU/hr): The total peak sensible heat gain your AC must remove.
- Tons: Industry-standard unit for AC capacity. 1 ton = 12,000 BTU/hr.
- Recommended AC size (tons): The smallest standard size that meets the peak load. Right-sized AC runs longer cycles, which is better for dehumidification than an oversized unit that short-cycles.
Assumptions and limitations
- Sensible cooling loads only — does not calculate latent (dehumidification) load. In humid climates (southern Ontario, Atlantic Canada), latent load can add 20–30% to the total capacity requirement.
- Uses peak-day, midday design conditions. In practice, AC loads vary by hour, room orientation, and occupancy pattern — a full Manual J calculation accounts for these variations.
- Lighting estimate of 8 W/m² assumes a mix of LED and fluorescent fixtures. Modern all-LED homes may be closer to 4 W/m².
- Does not differentiate between ductless mini-split systems and central ducted AC — both are sized in tons by the same load calculation. Duct losses (typically 10–20% in unconditioned spaces) would add to the required capacity for ducted systems.
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