Printable cheat sheet
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HVAC quick reference (IMC + ASHRAE)
CFM ↔ BTU/h
Sensible heat: BTU/h = 1.08 × CFM × ΔT. Solve for CFM: CFM = BTU/h ÷ (1.08 × ΔT). Common ΔT: 18-22°F cooling, 60-80°F heating.
Duct friction-loss targets
- Low-velocity (commercial): 0.08 in WC / 100 ft
- Medium-velocity: 0.10-0.15 in WC / 100 ft
- High-velocity (VAV runs): 0.20+ in WC / 100 ft
ΔP/L = 0.109 × Q^1.9 / D^5.02 (round-duct approximation).
ASHRAE 62.1 ventilation rates
| Occupancy | Rp (CFM/person) | Ra (CFM/sf) |
|---|---|---|
| Office | 5 | 0.06 |
| Conference room | 5 | 0.06 |
| Classroom (K-12) | 10 | 0.12 |
| Lecture hall | 7.5 | 0.06 |
| Retail sales | 7.5 | 0.12 |
| Restaurant dining | 7.5 | 0.18 |
| Healthcare exam | 10 | 0.18 |
| Lobby / corridor | 5 | 0.06 |
HVAC load rules of thumb
- Cooling BTU/SF: sun-heavy 32, average 25, shaded 20
- Climate-zone multiplier: zone 1 = 1.15, zone 4 = 1.0, zone 8 = 0.80
- Heating BTU/SF: 20 (zone 1) to 55 (zone 8)
- Tons cooling = BTU/h ÷ 12,000
- 1 ton ≈ 400 CFM