Rules of thumb, load estimators, and duct sizing — all in one place. Use these as a quick sanity check in the field or during early-stage estimating.
| Rule | Value | Code / Source |
|---|---|---|
| Cooling capacity (residential) | 1 ton / 400–500 sf | ASHRAE HoF / field rule |
| Cooling capacity (commercial office) | 1 ton / 250–350 sf | ASHRAE 90.1 |
| Cooling capacity (retail/restaurant) | 1 ton / 150–200 sf | Field rule |
| Dehumidification latent load | 20–30% of total load | ASHRAE HoF |
| Sensible Heat Ratio (office) | SHR ≈ 0.75–0.85 | ASHRAE 62.1 |
| Chilled water ΔT (central plant) | 10–12°F (5.5–6.7°C) | ASHRAE HoF Ch.43 |
| Rule | Value | Code / Source |
|---|---|---|
| Heating load (well-insulated house) | 20–25 BTU/hr·sf | HRAI Manual J |
| Heating load (avg Canadian house) | 25–35 BTU/hr·sf | HRAI Manual J |
| Heating load (older/poorly insulated) | 35–50 BTU/hr·sf | HRAI Manual J |
| Hot water boiler sizing (residential) | 50 BTU/hr per sf | Field rule |
| Hot water ΔT (baseboard/radiators) | 20°F (11°C) | ASHRAE HoF |
| Heat pump COP (cold climate, -15°C) | ≥ 1.7 COP (CSA C656) | NBC 2020 / CSA C656 |
| Source | BTU/hr | Reference |
|---|---|---|
| Person — seated/office (sensible) | ~250 BTU/hr | ASHRAE HoF Table 1 Ch.18 |
| Person — seated/office (total) | ~450 BTU/hr | ASHRAE HoF Table 1 Ch.18 |
| Person — standing/walking | ~550 BTU/hr sensible | ASHRAE HoF |
| Person — heavy work / gym | ~900–1,500 BTU/hr | ASHRAE HoF |
| Desktop computer + monitor | ~400–600 BTU/hr | ASHRAE HoF |
| Laptop | ~100–200 BTU/hr | ASHRAE HoF |
| Refrigerator (residential) | ~600–800 BTU/hr | ASHRAE HoF |
| Commercial oven (gas) | ~10,000–30,000 BTU/hr | ASHRAE 62.1 App C |
| LED lighting | ~3 W/sf → ~10 BTU/hr·sf | ASHRAE 90.1 Table 9.5 |
| Fluorescent T8 | ~6 W/sf → ~20 BTU/hr·sf | ASHRAE 90.1 |
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