| Parameter | Metric | Imperial |
|---|
Specific Volume Reference — Common HVAC Conditions
| Condition | Temp / RH | Sea Level | Edmonton 671 m | Calgary 1045 m |
|---|
All values in m³/kg (dry air basis). Standard HVAC reference: 0.830 m³/kg at 20°C, 0% RH, sea level = 1.204 kg/m³.
Specific volume of moist air: what it is and how to use it in Canadian HVAC
Specific volume is the volume per unit mass of dry air, including the moisture it carries. You'll see it expressed as m³/kgda (cubic metres per kilogram of dry air) or ft³/lbda in imperial. It's the direct reciprocal of air density, so higher specific volume means less dense air. ASHRAE psychrometric tables use specific volume rather than density because dry-air mass stays constant as moisture and temperature change, making it the cleaner basis for psychrometric calculations.
The ASHRAE formula is v = Rda × T / P × (1 + 1.6078 × W), where Rda is 0.28705 kJ/(kg·K), T is absolute temperature in Kelvin, P is atmospheric pressure in kPa, and W is humidity ratio in kg/kgda. Temperature, humidity, and pressure each influence the result independently, so accurate site conditions matter for any precision calculation.
Converting between volumetric flow and mass flow
Fan and duct tables use volumetric flow in L/s or CFM. Coil load calculations use mass flow in kg/s or lb/hr. Specific volume bridges the two: mass flow = volumetric flow / specific volume. At standard sea-level conditions (20°C, 0% RH, 101.3 kPa), v = 0.830 m³/kg, so 1000 L/s represents 1.000/0.830 = 1.205 kg/s of dry air. In Calgary at 20°C and 50% RH, v = 0.954 m³/kg, so the same fan delivers only 1.000/0.954 = 1.048 kg/s. That 13% mass reduction means coil capacity and latent load calculations come up short if you skip altitude correction. Pair this with the enthalpy calculator to get accurate coil loads at any site condition.
Altitude correction for high-elevation Canadian cities
HVAC projects in Calgary, Edmonton, Red Deer, Kelowna, and Lethbridge require altitude corrections for fans, coils, and filters. Lower atmospheric pressure increases specific volume proportionally. Calgary's atmospheric pressure of about 89.4 kPa is 88% of sea-level pressure, so specific volume at any given temperature is about 13 to 14% higher than at sea level. Fan manufacturers publish altitude correction curves, but calculating actual specific volume from your operating air conditions gives you a more precise correction factor than a simplified rule of thumb. Use the branch duct sizer with corrected air density when designing ducts for elevated Canadian sites.
Density correction factor and fan performance
The density correction factor (CF) is the ratio of standard specific volume to actual specific volume: CF = 0.830 / vactual. Fans deliver essentially constant volumetric flow regardless of air density, so a fan rated at 2000 L/s at standard conditions still delivers 2000 L/s at altitude. But the mass flow and the static pressure capability both drop in proportion to air density. Any resistance in the system that's driven by velocity pressure (filters, coils, terminal units) also drops at altitude. Always verify altitude-corrected fan curves with your equipment supplier for projects above 500 m. The air density calculator gives you the complementary density figure for grille and diffuser sizing.
Frequently Asked Questions
Specific volume is the volume per unit mass of dry air, in m³/kg or ft³/lb. It's the reciprocal of air density. Fan curves are published at standard conditions (0.830 m³/kg at sea level, 20°C). When your actual specific volume is larger, the same fan delivers the same volumetric flow but less mass flow and generates less static pressure than the catalog data suggests. Use specific volume to convert duct airflow readings in L/s to mass flow for coil load calculations, and to apply the density correction factor when selecting fans at non-standard temperature or elevation. Pair it with the humidity ratio calculator to get W from RH.
Higher altitude reduces atmospheric pressure, which increases specific volume proportionally. Calgary at 1045 m has atmospheric pressure of about 89.4 kPa versus 101.3 kPa at sea level. At 20°C and 50% RH, specific volume in Calgary is roughly 0.954 m³/kg versus 0.840 m³/kg at sea level, a 13.6% increase. A fan delivering 1000 L/s in Calgary carries about 13.6% less dry-air mass than the same fan at sea level. Equipment sized from sea-level performance data without altitude correction delivers less heating and cooling capacity than specified. Always apply a specific volume correction when sizing coils, heat exchangers, and VAV boxes for Canadian sites above 500 m.