Humidity & Ventilation Load Calculator
Account for West Virginia's variable humidity levels in HVAC system sizing. Calculates latent and sensible ventilation loads per ASHRAE 62.1 and Manual J principles.
Space & Occupancy
WV Climate Conditions
Indoor Design Conditions
Ventilation Parameters
Formulas Used
Humidity Ratio (W):
W = 0.62198 × (φ × Pws) / (Psite − φ × Pws) [lb water / lb dry air]
where Pws = saturation pressure (Magnus formula), φ = relative humidity fraction,
Psite = barometric pressure corrected for WV elevation.
Enthalpy of Moist Air:
h = 0.240·T + W·(1061 + 0.444·T) [BTU/lb dry air]
Sensible Ventilation Load:
Qs = 1.1 × CFM × ΔT [BTU/hr]
(1.1 = 60 min/hr × 0.075 lb/ft³ × 0.240 BTU/lb·°F)
Latent Ventilation Load:
Ql = 0.68 × CFM × ΔWgrains [BTU/hr]
(0.68 = 60 × 0.075 × 1076 / 7000; 1076 BTU/lb = latent heat of vaporization)
ASHRAE 62.1 Ventilation Rate:
CFM = (Occupants × Rp) + (Floor Area × Ra)
ERV/HRV Recovery:
Tsupply = Tout + εs·(Tin − Tout)
Wsupply = Wout + εl·(Win − Wout)
Altitude Correction (WV elevations 565–4,863 ft):
Psite = 14.696 × (1 − 6.8754×10⁻⁶ × elevation)5.2559 psia
Assumptions & References
- ASHRAE Fundamentals Handbook — psychrometric equations and moist air properties.
- ASHRAE Standard 62.1-2022 — Ventilation for Acceptable Indoor Air Quality (ventilation rate procedure).
- ASHRAE Standard 62.2-2022 — Ventilation for Residential Buildings (0.35 ACH minimum).
- ACCA Manual J (8th Ed.) — Residential Load Calculation; latent/sensible load methodology.
- WV design conditions from ASHRAE Climate Design Data (99.6% / 1% design values).
- Standard air density 0.075 lb/ft³ at sea level, corrected for WV regional elevations.
- Latent heat of vaporization assumed 1,076 BTU/lb at typical HVAC conditions.
- ERV/HRV effectiveness values per AHRI Standard 1060 typical performance ranges.
- Moisture removal calculated assuming all condensed moisture is liquid water (0.1043 lb/pint).
- Results represent ventilation loads only — add envelope (wall, roof, window) loads for total system sizing.
- WV Climate Zone: predominantly Zone 4A (mixed-humid) with mountain areas in Zone 5A.