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Sizing heating coils for Canadian winter design conditions
Heating coils add sensible heat only, raising air temperature without changing humidity ratio. This makes the calculation simpler than cooling coils: capacity (BTU/hr) = 1.08 × CFM × ΔT (°F), or in metric, capacity (kW) = 1.23 × L/s × ΔT (°C) at standard density. Because the process is purely sensible, relative humidity drops sharply as the air warms — this is why Canadian winters with heated indoor air feel so dry despite outdoor air carrying reasonable RH.
Preheat coils are the most demanding heating coil application in Canadian climates. They raise outdoor ventilation air from the winter design temperature (often -20°C to -35°C depending on location) to a safe temperature, typically 0-10°C, before mixing with return air or passing through downstream cooling coils. This prevents the cooling coil from freezing and protects downstream equipment.
Choosing the right design temperature
ASHRAE climate data provides heating design temperatures at 99% and 99.6% annual percentiles — meaning outdoor temperature is colder than this value for only 1% or 0.4% of the year, respectively. Sizing for the absolute coldest recorded temperature wastes capital and capacity for nearly the entire heating season. Using the 99% design temperature is standard practice for most Canadian commercial and institutional buildings. Critical applications like hospitals may use 99.6% or even lower percentiles for added margin.
Canadian winter design temperatures vary enormously by region: Vancouver's mild marine climate sits around -2°C while Winnipeg and Edmonton plunge to around -29°C. Always pull your design temperature from current ASHRAE climate data or your local building code's climatic design data table rather than using a generic assumption.
Reheat coils and terminal unit sizing
Reheat coils in VAV systems add heat to already-cooled supply air to prevent overcooling in zones with reduced cooling load. Because the entering air is typically already at supply air temperature (around 13-16°C), the required temperature rise is much smaller than a preheat coil — often only 5-15°C. Reheat coil capacity is usually a fraction of the cooling coil's nominal capacity. Use the cooling coil calculator alongside this tool when sizing matched VAV terminal units with reheat.
Frequently Asked Questions
Q (BTU/hr) = 1.08 × CFM × (T_leaving − T_entering) in °F. Heating coils add sensible heat only, so humidity ratio stays constant while RH drops as temperature rises. For 2000 CFM heated from -20°C to 18°C, capacity is roughly 164,000 BTU/hr at sea level. At elevated sites, multiply by the density ratio for accuracy — this calculator applies that correction automatically based on your elevation input.
Preheat coils typically raise outdoor air from the winter design temperature to 0-10°C, low enough to avoid overheating but high enough to prevent freezing of downstream cooling coils. Design temperature varies by city: Vancouver around -2°C, Toronto around -16°C, Winnipeg and Edmonton around -29°C. Use the 99% or 99.6% ASHRAE design temperature for your city, not the extreme minimum, which wastes capacity for most of the season. Use the city presets above for quick reference values.