HVAC & Indoor Environmental Quality

Keeping buildings comfortable and healthy. Learn thermal comfort (PMV, adaptive comfort, ASHRAE 55/IMAC), psychrometrics (dry bulb, humidity, enthalpy), cooling load components (envelope, glazing, people, lighting, equipment, ventilation), sizing in TR, HVAC systems (split, VRF, chilled water with AHUs/FCUs, district cooling), ventilation rates and IAQ (CO₂, PM2.5), energy efficiency (COP, IKW/TR, economisers, heat recovery), and passive/mixed-mode design — with Python for an office cooling load and fresh-air rate.

HVAC (heating, ventilation and air conditioning) is typically the largest energy user in commercial buildings in India — 40–60% of electricity. Correct load estimation and system selection matter for comfort, cost and emissions.

Indoor air quality is equally important: adequate fresh air and filtration protect health and productivity.

An analogy

A building's cooling load is like a leaking bucket you must keep empty: heat pours in through walls, windows, people and equipment, and the air conditioner must pump it out as fast as it enters.

Thermal comfort

Comfort depends on air temperature, radiant temperature, humidity, air speed, clothing and activity. Typical AC setpoints: 24–26 °C, 50–60% RH (BEE recommends 24 °C default). Adaptive comfort (IMAC for India) shows people in naturally ventilated buildings accept wider ranges; ceiling fans extend comfort by 2–3 °C.

Psychrometrics and loads

Moist air properties (dry bulb, wet bulb, dew point, humidity ratio, enthalpy) are read from the psychrometric chart. Cooling loads are sensible (temperature) and latent (moisture from people, fresh air). Peak loads are calculated hour-by-hour (CLTD/RTS methods or simulation) because solar gains peak at different times on different façades.

HVAC systems

Common systems:

IAQ and efficiency

Ventilation per ASHRAE 62.1/NBC (e.g. ~10 L/s/person in offices); CO₂ below ~1000 ppm indicates adequate ventilation; MERV 13+ filtration for PM2.5, UV-C and demand-controlled ventilation. Efficiency measures: high-COP chillers (0.55–0.65 kW/TR), VFDs on pumps/fans, economisers, heat recovery, higher chilled-water temperatures, BMS optimisation and continuous commissioning.

Formulas

Worked example: Sensible load of supply air

Given: 2 m³/s air, ΔT 10 K

Find: Q_s.

Answer: 24 kW

Worked example: ACH

Given: 1800 m³/h fresh air, room 2000 m³

Find: ACH.

Answer: 0.9 ACH

Worked example: Chiller power

Given: 300 TR at 0.6 kW/TR

Find: Power.

Answer: 180 kW

Office cooling load and fresh air

# Cooling load estimate for an office floor and ventilation
area, people = 500.0, 50
Q_env = 500 * 0.9 * 12 / 1000             # kW: wall/roof UA ~0.9 W/m2K per m2 floor * 12 K
Q_glass = 120 * 0.35 * 450 / 1000         # 120 m2 glass, SHGC 0.35, 450 W/m2 solar
Q_people = people * (75 + 55) / 1000      # sensible + latent W per person
Q_light = area * 8 / 1000                 # 8 W/m2 LED
Q_equip = area * 12 / 1000                # 12 W/m2 plug loads
fresh = people * 10 / 1000                # 10 L/s per person -> m3/s
Q_vent = 1.2 * fresh * (60 - 45)          # kW via enthalpy difference kJ/kg (outdoor 60, indoor 45)
total = Q_env + Q_glass + Q_people + Q_light + Q_equip + Q_vent
for name, q in (("envelope", Q_env), ("glazing solar", Q_glass), ("people", Q_people), ("lighting", Q_light), ("equipment", Q_equip), ("fresh air", Q_vent)):
    print(f"{name:14s} {q:5.1f} kW")
print(f"total {total:.1f} kW = {total/3.517:.1f} TR ({area/(total/3.517):.0f} m2 per TR)")
print(f"fresh air {fresh*1000:.0f} L/s = {fresh*3600:.0f} m3/h")
# envelope         5.4 kW
# glazing solar   18.9 kW
# people           6.5 kW
# lighting         4.0 kW
# equipment        6.0 kW
# fresh air        9.0 kW
# total 49.8 kW = 14.2 TR (35 m2 per TR)
# fresh air 500 L/s = 1800 m3/h

Solar gain through glazing is the largest single load (38%) — façade design matters more than anything else. Fresh air adds 9 kW because humid outdoor air must be cooled and dehumidified; energy-recovery wheels can recover much of this. 35 m²/TR is a typical efficient office (old rules of thumb of 15–20 m²/TR lead to oversizing).

Common mistakes

Applications

Key takeaways

Going further