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Humidity ratio: the foundation of latent load calculations
Humidity ratio is the actual mass of water vapour in air, expressed per unit mass of dry air. Unlike relative humidity — which changes with temperature even when the actual moisture content stays constant — humidity ratio is an absolute measure. When you heat cold dry outdoor air in a Canadian winter, the relative humidity plummets but the humidity ratio stays the same. This makes humidity ratio the correct variable for calculating moisture mass balances and latent energy flows in HVAC systems.
The humidity ratio is the vertical axis on every psychrometric chart. Moving horizontally means changing sensible heat at constant moisture content. Moving vertically means adding or removing moisture. A cooling coil that dehumidifies moves air diagonally down and to the left — both sensible cooling and latent dehumidification occurring simultaneously. The latent portion is calculated directly from the humidity ratio change.
Latent load formula using humidity ratio
The latent cooling load in BTU/hr for a given airstream is: Q_latent = 0.68 x CFM x (W1 - W2), where CFM is airflow in cubic feet per minute and W1 and W2 are entering and leaving humidity ratios in gr/lb. The constant 0.68 incorporates air density at standard conditions and the latent heat of vaporisation of water. At elevations above 1000 feet — Calgary, Edmonton, and many BC interior cities — air density is lower and you should use an elevation-corrected density factor rather than 0.68. This calculator computes the corrected factor automatically.
For ventilation latent loads, subtract the supply air humidity ratio from the outdoor air humidity ratio and multiply by outdoor air CFM and 0.68. In Canadian summers, outdoor air at 35°C and 60% RH carries about 110 gr/lb, while a typical supply air condition is 52 gr/lb at 55°F leaving coil temperature. Each CFM of outdoor air contributes about 39 BTU/hr of latent load — significant in high-occupancy or high-ventilation buildings like schools and arenas. See the load calculations category for complete latent load analysis tools.
Humidity ratio in Canadian winter humidification design
Winter outdoor air in Canada carries almost no moisture. At -20°C and 80% RH, the humidity ratio is only about 5 gr/lb. When that air infiltrates a building and is heated to 22°C, relative humidity drops to around 12% — well below the ASHRAE 55 minimum of 30% for occupant comfort. Steam or evaporative humidifiers must add moisture to raise the humidity ratio to a target of about 40-55 gr/lb. The mass of water to add per hour equals airflow in lb/hr multiplied by the humidity ratio difference in lb/lb. This calculator gives you the starting and target humidity ratios needed for humidifier sizing.
Frequently Asked Questions
Humidity ratio is the mass of water vapour per unit mass of dry air — gr/lb in imperial, g/kg in metric. HVAC engineers use it to calculate latent cooling loads: Q_latent (BTU/hr) = 0.68 x CFM x (W_entering - W_leaving), where W is in gr/lb. It's the vertical axis on the psychrometric chart and stays constant during sensible-only heating or cooling. Use the psychrometric calculator for the full state point and the enthalpy calculator to find total heat content at any humidity ratio.
Comfortable indoor humidity ratio in Canada is typically 40-65 gr/lb (5.7-9.3 g/kg), giving 40-55% RH at 72°F. In Canadian winters, outdoor air at -20°C and 80% RH has only about 5 gr/lb — when heated to 22°C, RH drops to 12%. Humidification to 40-55 gr/lb target is standard in Canadian buildings. ASHRAE 55 recommends maintaining humidity ratio below 0.012 kg/kg (84 gr/lb) for comfort. Use the dew point calculator to check condensation risk at your target indoor humidity ratio.