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Psychrometrics in Canadian HVAC: why moist air properties matter
Psychrometrics is the science of moist air — the thermodynamic relationships between dry air, water vapour, temperature, and pressure. Every HVAC system that conditions air interacts with psychrometric properties: cooling coils remove both sensible heat (temperature) and latent heat (moisture), heating coils add sensible heat, and ventilation systems mix outdoor and indoor air streams with different moisture levels. Getting these numbers right is the foundation of accurate coil sizing, dehumidification design, and energy analysis.
In Canada, outdoor psychrometric conditions vary more than almost anywhere else. Vancouver's marine climate brings cool, moist winters and mild summers. Calgary sits at 1045 metres elevation, which reduces atmospheric pressure and changes all psychrometric calculations — the same air at Calgary reads different humidity ratio values than at sea level. Toronto summers hit 35°C with high humidity, driving large latent loads on cooling systems. This calculator accounts for elevation-corrected atmospheric pressure in all calculations.
The 7 psychrometric properties explained
Dry-bulb temperature is the familiar air temperature. Wet-bulb temperature is the temperature air reaches when water evaporates into it adiabatically — always lower than dry-bulb except at 100% RH. Dew point is the temperature at which air becomes saturated and condensation begins. Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure, expressed as a percentage. Humidity ratio (specific humidity) is the mass of water vapour per unit mass of dry air in gr/lb or g/kg. Enthalpy is the total heat content of moist air in BTU/lb or kJ/kg, combining sensible and latent components. Specific volume is the volume per unit mass of dry air in ft³/lb or m³/kg, used to convert between mass flow and volumetric flow.
How to use psychrometric calculations for coil sizing
To size a cooling coil, calculate the enthalpy of entering air (typically summer outdoor conditions mixed with return air) and the enthalpy of leaving air (design supply air condition). The difference in enthalpy multiplied by the mass flow rate gives total coil capacity in BTU/hr. The sensible component uses the dry-bulb temperature difference, and the latent component uses the humidity ratio difference. Use the cooling coil calculator to run this analysis directly from entering and leaving conditions.
For winter heating coil sizing in Canada, humidity ratio stays nearly constant across the coil (heating coils add sensible heat only). Use this calculator to find the humidity ratio of design winter conditions, then use the heating coil calculator to size the coil from entering and leaving dry-bulb temperatures.
Frequently Asked Questions
A psychrometric calculator computes all thermodynamic properties of moist air from any 2 known values. Given dry-bulb temperature and relative humidity, it calculates wet-bulb temperature, dew point, humidity ratio in gr/lb or g/kg, enthalpy in BTU/lb or kJ/kg, specific volume in ft³/lb or m³/kg, and vapour pressure. HVAC engineers use it to analyze coil performance, size dehumidification systems, determine ventilation energy recovery requirements, and verify system performance. See the enthalpy calculator and cooling coil calculator for applied analysis.
Dry-bulb is the standard air temperature. Wet-bulb is always lower than dry-bulb (except at 100% RH) — the larger the wet-bulb depression, the drier the air. Dew point is where condensation begins; at dew point, all 3 temperatures are equal. In Canadian winters, knowing indoor dew point is critical for preventing condensation on cold windows and within wall assemblies. Use the dew point calculator and condensation risk calculator for building envelope analysis.