HVAC Fundamentals and Load Calculations
Design heating and cooling loads are the numerical foundation of every later decision - equipment capacity, duct and pipe size, terminal selection and energy compliance. The exam tests whether you can build a load from its components, choose the correct outdoor design conditions for the project's climate zone, and carry a psychrometric state through to a supply-air quantity.
Design conditions and the load equation
- Use the winter and summer outdoor design dry-bulb and wet-bulb values given in GB 50736-2012 for the specific city; do not substitute a neighbouring city or a national average.
- Room cooling load = envelope and glass conduction + solar gain through glass + internal gains (people, lighting, equipment) + infiltration, each at the coincident design hour.
- Room load excludes the fresh-air load; coil load includes it. Mixing the two is the most common arithmetic error on the paper.
- Sensible heat factor SHF = sensible/(sensible + latent) fixes the supply-air state and lets you check whether the coil can handle the latent portion.
Psychrometric chain
Supply airflow from sensible heat: L = Qs / [ρ·cp·(tN − tS)]. Moisture from latent heat: Δd = Ql / (m × 2500). Verify both, and verify that the apparatus dew point is below the required supply dew point.
Worked example
An office has room sensible 8.4 kW and latent 2.1 kW; indoor 26 °C, supply 16 °C. SHF = 8.4/10.5 = 0.80. Airflow = 8400/(1.2 × 1.006 × 10) = 696 L/s ≈ 2506 m³/h. Mass flow = 0.835 kg/s; Δd = 2100/(0.835 × 2500) = 1.01 g/kg. The supply moisture content must therefore be the indoor value minus 1.0 g/kg - a coil leaving condition you must prove, and the check that the chilled water temperature can actually reach it.
Traps
Summing individual room peaks and calling it the block load; ignoring orientation-dependent solar through glass; applying diversity to people but not to lighting; removing fan heat gain from the room load when the fan sits downstream of the coil; forgetting that the ventilation load grows if return air is ducted through an unconditioned ceiling void.