HVAC DESIGN CHEAT SHEET
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HVAC DESIGN CHEAT SHEET
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1) HEAT LOAD & AIRFLOW (rules of thumb – verify to code)
• Interior airflow: 3.5 L/s per person
• Perimeter airflow: 7.5–10 L/s per person
• Outside air (office typical): 10 L/s per person*
• Interior zone load: 80–100 W/m²
• Perimeter zone load: 150–200 W/m²
• High people loading: 200–250 W/m²
*Actual OA must follow AS 1668.2 by occupancy; filtration can alter values.
2) COIL & WATER/PIPE VELOCITIES
• Return-air filter face velocity: ≤ 2.5 m/s
• Cooling coil face velocity: ≤ 2.5 m/s
• CHW/CDW pipe velocity: ≤ 2.5 m/s
3) DUCTWORK VELOCITIES
Flexible duct:
– Max 3 m/s
Sheet-metal duct:
– 7 m/s (acoustics/noise limit)
– 10 m/s (friction-loss upper bound)
4) CONVERSIONS (exact factors)
• Tons refrigeration × 3.517 = kW (refrig.) (1 TR = 3.517 kW)
• BTU/h × 0.293071 = W
• CFM × 0.471947 = L/s
• L/s = m³/s × 1000 (and m³/s = L/s ÷ 1000)
• Water heatflow (kW) = 4.186 × L/s × ΔT (°C) ← units are kW
5) AIR & WATER HEAT CALCS (deliver W unless noted)
• Air sensible: Qs (W) = 1.20 × L/s × ΔT (°C) (1.20 is ρ×cp ≈ 1.2 kJ/m³·K)
• Air latent: Ql (W) = 3.00 × L/s × ΔW (g/kg) (≈ 2500 kJ/kg latent)
• Air total: Qt (W) = 1.20 × L/s × Δh (kJ/kg)
• Water heating/cooling: Q (kW) = 4.186 × L/s × ΔT (°C)
6) DUCT INSULATION (guide – check NCC/BCA)
• Exposed to direct sun: ~R3 (≈100 mm)
• In conditioned area: ~R1.2 (≈38 mm)
• Other areas: ~R2 (≈75 mm)
7) TYPICAL ΔT VALUES (project-specific)
• Cooling coil air ΔT: 10–12 °C
• Heating coil air ΔT: 10–20 °C
• Chilled water ΔT: 5–6 °C
• Hot water ΔT: ~10 °C
8) QUICK POWER FORMULAS
• Air: kW = 0.00120 × L/s × ΔT (°C)
• Water: kW = 4.186 × L/s × ΔT (°C)
• Convert W to TR: TR = Watts ÷ 3516
Notes:
– “Face velocity” applies at the net free area of coils/filters.
– OA separation: keep outdoor-air intakes ≥6 m from exhaust/relief (per AS 1668.2 guidance).
