HVAC Formulas Reference
Sensible Heat
Q = 1.08 × CFM × ΔT
Sensible heat (BTU/hr) from airflow
CFM = Q / (1.08 × ΔT)
Required airflow from heat load
Q = U × A × ΔT
Heat transfer through surfaces
Latent Heat
Q = 0.68 × CFM × ΔW
Latent heat (BTU/hr), ΔW in gr/lb
Q = 4.5 × CFM × Δh
Total heat, Δh in BTU/lb
SHR = Qs / Qt
Sensible Heat Ratio
Air Properties
CFM = Velocity × Area
V in ft/min, A in sq ft
Velocity = 4005 × √(VP)
V in ft/min, VP in inches WC
lb/hr = CFM × 60 × ρ
Mass flow rate (ρ ≈ 0.075 lb/ft³)
Duct Sizing
deq = 1.3 × (a×b)0.625 / (a+b)0.25
Equivalent round diameter
ΔP = f × (L/D) × (V²/2g) × ρ
Darcy-Weisbach equation
ΔP = 0.109136 × f × Q1.9 / d5.02
Friction loss (in WC/100 ft)
Fan Laws
CFM2/CFM1 = RPM2/RPM1
Flow varies with speed
SP2/SP1 = (RPM2/RPM1)²
Pressure varies with speed²
HP2/HP1 = (RPM2/RPM1)³
Power varies with speed³
Refrigeration
Superheat = Tline - Tsat
Evaporator superheat
Subcooling = Tsat - Tline
Condenser subcooling
Target SH = (3 × WB + OD)/2 - 80
Superheat target (TXV bypass)
Psychrometrics
W = 0.622 × Pw / (Patm - Pw)
Humidity ratio (lb/lb)
h = 0.24T + W(1061 + 0.444T)
Enthalpy (BTU/lb)
RH = Pw / Pws × 100
Relative humidity (%)
Equipment Sizing
1 Ton = 12,000 BTU/hr
Cooling capacity
CFM per Ton ≈ 400
Typical airflow (350-450)
EER = BTU/hr / Watts
Energy Efficiency Ratio
Quick Reference Constants
1.08
Sensible heat factor
0.68
Latent heat factor
4.5
Total heat factor
0.075 lb/ft³
Air density (std)