ID HVAC cheat sheet

Heating, Ventilation and Air Conditioning (HVAC)

• Sensible heat formula: Qs = 1.08 × CFM × ΔT (Btuh, sea level, standard air); latent heat: QL = 0.68 × CFM × ΔW (grains/lb); total heat: Qt = 4.5 × CFM × Δh.

• Psychrometric relations: dew point is the saturation temperature at constant humidity ratio; relative humidity = partial vapor pressure ÷ saturation pressure at dry-bulb; wet-bulb equals dry-bulb at 100% RH.

• Cooling coil apparatus dew point (ADP): bypass factor BF = (leaving DB − ADP) ÷ (entering DB − ADP); coil sensible heat ratio SHR = Qs ÷ Qt.

• Airflow for sensible capacity: CFM = Qs ÷ (1.08 × ΔT); a 3-ton (36,000 Btuh) coil at 20°F ΔT requires about 1,667 CFM nominal.

• Duct sizing: Q = V × A; friction chart design typically 0.08–0.10 in. w.c. per 100 ft for supply ducts; velocity pressure VP = (V ÷ 4,005)² in. w.c. with V in fpm.

• Fan laws: CFM varies directly with RPM, static pressure with RPM², and brake horsepower with RPM³; for a fixed system, BHP also varies with CFM³.

• Refrigeration cycle: superheat = suction temp − saturation temp at suction pressure; subcooling = saturation temp at liquid pressure − liquid line temp; typical targets 8–12°F SH, 8–15°F SC.

• Static pressure budget: typical PSC blower rated at 0.5 in. w.c. external static; ECM/X13 blowers often 0.5–1.0 in. w.c.; total external SP = supply + return + filter + coil drops.