HVAC Fundamentals

HVAC (Heating, Ventilation, and Air Conditioning) fundamentals cover the principles and systems used to control indoor environmental conditions for thermal comfort, indoor air quality (IAQ), and process requirements. The goal is to maintain appropriate temperature, humidity, air movement, and air cleanliness while minimizing energy consumption. Heat transfer in buildings occurs through conduction (through walls, roofs, windows), convection (air films on surfaces), radiation (solar gain, longwave exchange), and latent heat (moisture addition/removal). Cooling loads consist of: (1) Conduction through building envelope. (2) Solar radiation through windows and opaque surfaces. (3) Infiltration (uncontrolled air leakage) and ventilation (controlled outdoor air). (4) Internal loads from occupants, lighting, and equipment. Heating loads are dominated by envelope conduction and infiltration/ventilation losses. Psychrometrics describes moist air properties: dry-bulb temperature (T), wet-bulb temperature (T_wb), dew point (T_dp), relative humidity (RH), humidity ratio (W), enthalpy (h), and specific volume (v). The sensible heat equation is Q_s = ṁ × c_p × ΔT, where c_p ≈ 1.005 kJ/kg·K for air. The latent heat equation is Q_l = ṁ × h_fg × ΔW ≈ ṁ × 2501 × ΔW kJ/kg, where h_fg is the latent heat of vaporization and ΔW is the change in humidity ratio (kg water/kg dry air). Total cooling load is Q_total = Q_s + Q_l = ṁ × (h_out − h_in). HVAC systems are classified by the medium used to transfer heat: (1) All-air systems (VAV, constant volume, dual-duct) use conditioned air to meet loads. (2) All-water systems (fan-coil units, radiant panels) use chilled or hot water. (3) Air-water systems (induction units, fan-coil with outdoor air) combine both. (4) Direct expansion (DX) systems (package units, split systems, VRF) use refrigerant evaporating directly in the space coil. Ventilation provides outdoor air for IAQ, typically 5–10 L/s per person or based on floor area (ASHRAE 62.1). Minimum outdoor air must be heated, cooled, humidified, or dehumidified, adding to the load. Energy recovery ventilators (ERV/HRV) reduce this penalty by transferring heat (and moisture for ERV) between exhaust and outdoor air. Efficiency metrics include: COP (coefficient of performance) = Q_out / W_in for heat pumps; EER (energy efficiency ratio) = cooling capacity (Btu/h) / electrical input (W) at rated conditions; SEER (seasonal energy efficiency ratio) = seasonal cooling output / seasonal electrical input; AFUE (annual fuel utilization efficiency) for furnaces; kW/ton for chillers (lower is better).