Air density, altitude and HVAC design in Calgary and the Rockies

Most HVAC equipment data, rules of thumb and textbook equations assume standard air at sea level. Calgary sits at roughly 1,000 to 1,100 m (about 3,300 to 3,600 ft) above sea level, and mountain communities such as Canmore, Banff and Lake Louise are higher still. At these elevations, air is noticeably less dense, and that affects almost every air-side calculation.

The corrections are not difficult, but they are easy to forget, especially when equipment is selected by someone working from sea-level catalogue data. Ignoring altitude can lead to undersized airflow, misread balancing measurements, overheated motors and poorly performing combustion equipment. This guide explains what changes and what to do about it.

How altitude changes air

Atmospheric pressure falls with elevation. Because air density is proportional to pressure (at a given temperature), density falls too. Using the standard atmosphere model:

Actual barometric pressure varies with weather, but the standard value for the site elevation is used for design. Standard air density at sea level is about 1.2 kg/m³ (0.075 lb/ft³). In Calgary, the equivalent is roughly 1.06 kg/m³ at the same temperature.

What changes in HVAC calculations

Heat transfer per unit of airflow

The familiar sensible heat equation, Q (W) = 1.23 x L/s x delta t, contains air density. At Calgary's elevation, the constant drops to roughly 1.08 (in imperial, 1.08 x cfm x delta t becomes roughly 0.95 x cfm x delta t). Put simply, each litre per second of air carries about 12% less heat.

To meet the same sensible load, a system in Calgary needs about 12 to 14% more volumetric airflow than the same system at sea level. The latent and total heat equations change in the same proportion. Using sea-level constants undersizes airflow.

Psychrometrics

At lower pressure, the humidity ratio corresponding to a given dry-bulb and relative humidity is higher, and wet-bulb relationships shift. Use a psychrometric chart or software set to local pressure. See psychrometrics for HVAC engineers.

Fans

A fan is a constant-volume device at a given speed: it moves the same volumetric flow regardless of air density. What changes is pressure and power.

• Static pressure developed by the fan is proportional to density.
• Fan power is proportional to density.
• System resistance (duct and component pressure drop at a given volume flow) is also proportional to density.

Selecting a fan at altitude

Fan curves are published at standard air density. The usual procedure:

1. Calculate the required volumetric airflow using altitude-corrected heat equations.
2. Calculate the system static pressure at actual density. Pressure drops from manufacturer data for coils and filters are often at standard air and should be corrected.
3. Convert the actual static pressure to an equivalent standard-air value by dividing by the density ratio.
4. Select the fan from standard-air curves at the required volume and the equivalent static pressure.
5. Calculate actual brake power by multiplying the standard-air power by the density ratio.

Many fan selection programs handle this automatically if elevation is entered. The fan laws include density terms for this reason, and fan selection at altitude should always state the design density.

Coils, condensers and cooling equipment

Air-side heat transfer depends on the mass of air passing over the coil. At lower density, a coil moving the same volume of air transfers less heat. Manufacturer selection software can correct for altitude, but only if the elevation is entered.

Air-cooled condensers, air-cooled chillers, rooftop units and dry coolers are particularly sensitive, because their capacity depends on rejecting heat to outdoor air. Ask manufacturers for altitude-corrected capacities. Cooling towers are also affected, though the effect is usually modest.

Motors and drives

Electric motors rely on air for cooling. NEMA MG 1 defines usual service conditions for motors, including altitudes up to 1,000 m (3,300 ft). Calgary is at or just above that elevation, and mountain sites are well above it. Above the rated altitude, motors may need to be derated or selected with a higher service factor margin, unless the ambient temperature is low enough to compensate.

Variable frequency drives and other electronic equipment similarly often have an altitude limit, above which output current is derated. Check manufacturer data for both motors and drives on projects at elevation.

Combustion equipment

Burners need oxygen, and at altitude each cubic metre of air contains less of it. Gas-fired appliances must be adjusted or derated so that combustion remains complete and safe. In Canada, gas appliance installation is governed by CSA B149.1, and appliances carry certified elevation ranges. Many appliances certified for the Canadian market are rated for elevations up to about 1,370 m (4,500 ft), which includes Calgary but not all mountain communities. Above the certified range, the manufacturer's high-altitude instructions apply. Always confirm with the appliance documentation and the authority having jurisdiction.

Make-up air units, boilers, furnaces and unit heaters should be commissioned with combustion analysis at the actual site.

Measuring airflow at altitude

Air balancing instruments must be corrected for density.

• Velocity calculated from a pitot tube reading depends on density. Velocity equals the square root of two times velocity pressure divided by density. Using sea-level density understates velocity at altitude.
• Flow hoods and electronic manometers often have a density correction setting. It must be set for the local barometric pressure and temperature.
• Airflow stations and VAV box flow sensors are calibrated with a flow coefficient and may need field adjustment.

Testing, adjusting and balancing agencies working in Calgary routinely apply these corrections, and the TAB report should state whether airflows are reported as actual or standard. See air balancing procedures and instruments.

Actual versus standard airflow

Specifications and schedules can express airflow as actual (volume at site conditions) or standard (equivalent volume at standard density). The two differ by the density ratio, about 12% in Calgary. Mixing them causes confusion between designer, supplier and balancer. Good practice is to state on drawings which basis is used and to keep it consistent from design to TAB report.

Frequently asked questions

Does a fan move less air in Calgary?

A fan moves the same volume of air at the same speed, but that volume weighs less and carries less heat. It also develops less static pressure and uses less power.

Do I need to correct for altitude in Edmonton?

Edmonton's density is about 8% below sea level. The effect is smaller than in Calgary but still worth including for fan, coil and combustion selections.

Who is responsible for altitude correction?

The designer should state the design elevation and density. Equipment suppliers should provide altitude-corrected selections, and balancers should correct instrument readings. Problems arise when one party assumes another has made the correction.

Key takeaways

• Air in Calgary is about 12% less dense than at sea level; mountain sites are lower still.
• Use altitude-corrected heat equations and psychrometrics, which require more airflow for the same load.
• Fans move the same volume but develop less pressure and use less power.
• Check motor, drive and combustion equipment ratings for elevation.
• State actual or standard airflow basis clearly and correct balancing instruments for density.