Building Services, HVAC and Electrical Energy Systems
Domain 5 tests the building-services calculations that sit behind energy bills: cooling loads, coil and pump duties, chiller efficiency, ventilation and electrical distribution. You should be able to check a design number, spot an unrealistic part-load claim and explain the electrical consequences.
Core rules
- Cooling load is the sum of conduction, solar, internal, infiltration and ventilation loads; latent load is separate. Use psychrometrics: coil duty = mass flow × enthalpy difference, and sensible heat ratio = sensible/total.
- Chiller efficiency is expressed as COP or kW/TR; COP = 3.517/(kW/TR). Part-load performance is represented by IPLV or NPLV, not a single full-load COP. Chilled-water ΔT, condenser-water temperature and cooling-tower approach all affect performance.
- Pump power = ρgQH/η, with Q in m³/s and H in m. Fan power follows airflow × total pressure/efficiency. Use total pressure, not static pressure alone, for fan input power.
- Electrical design distinguishes connected load, maximum demand, demand factor and diversity factor. Transformer losses have no-load and load components; power-factor correction reduces reactive current and losses; harmonics can overheat neutral conductors and transformers.
Worked example
A chilled-water pump handles 15 L/s against 35 m total head at 72% efficiency. P_hydraulic = 1,000 × 9.81 × 0.015 × 35 = 5,150 W. Shaft power = 5,150/0.72 = 7,150 W. If the motor efficiency is 90%, input power = 7,150/0.90 = 7,950 W, or about 7.95 kW. Over 5,000 hours, energy = 39,750 kWh. At HK$1.20/kWh, annual cost = HK$47,700. A 5 m head reduction would save roughly 14% of pump power - often cheaper than a controls upgrade.
Traps
Using arithmetic mean temperature difference for a coil with condensation; mixing COP and kW/TR; treating total and static pressure as interchangeable; ignoring motor and drive efficiency; confusing demand factor with diversity factor; and assuming power-factor correction reduces kWh.