💨 Outdoor Air Share of Supply
⚡ Estimated Fan Power
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
|---|
How to Use the AHU Sizing Calculator
Choose the closest match for default supply air temperature difference and duct static pressure, or select Other / Custom to enter your own project values.
Enter the zone sensible cooling load from your load calculation, plus the winter heating load for the same area. These come from the commercial load calculator if you have not run one yet.
Enter the minimum outdoor airflow from your ventilation calculation. This sets a floor on total supply airflow even when the cooling load alone would call for less.
Click Calculate AHU Sizing to see supply airflow, cooling coil load, heating coil capacity, outdoor air percentage, and an estimated fan power for equipment schedule purposes.
Understanding Air Handling Unit Sizing
An air handling unit moves conditioned air to one or more zones through a cooling coil, a heating coil, filters, and a supply fan. Sizing one correctly means finding the airflow that satisfies three things at once: the sensible cooling load, the minimum outdoor air requirement, and a supply air temperature difference that keeps occupants comfortable without oversized ductwork.
Airflow Is Set by the Larger of Two Requirements
Cooling airflow comes from dividing the sensible load by the supply air temperature difference. Ventilation airflow comes straight from your outdoor air calculation. In a densely occupied space like a classroom or conference room, ventilation airflow can exceed cooling airflow, which means the AHU ends up oversized for cooling alone. Run both numbers and size the unit for whichever is larger, not just the cooling number. The commercial ventilation calculator gives you the outdoor air side of that comparison.
Why Coil Load Is Higher Than Zone Load
The cooling coil does not just offset the zone sensible load. It also has to cool the outdoor air component down to supply temperature, and it absorbs heat added by the supply fan motor sitting in the airstream. On a system with a high percentage of outdoor air, like a dedicated outdoor air unit or a lab exhaust makeup unit, the coil load can run well above the sum of the zone sensible loads alone. This calculator adds an estimated outdoor air cooling penalty and fan heat gain on top of the zone load to give a more realistic coil sizing number.
Heating Coil Sizing for Canadian Winters
The heating coil has to bring outdoor air, mixed with return air, up to the design supply air temperature at the coldest winter design condition for your city. Calgary and Edmonton design around -23C to -27C, while Toronto sits closer to -16C and Vancouver rarely sees design temperatures below -4C. A system with a high outdoor air percentage in Winnipeg needs a much larger heating coil than the same airflow in Vancouver, even with identical zone loads. Pair this with the design temperature lookup tool for your specific city.
Fan Power Is a Budget Estimate, Not a Selection
The fan power shown here uses a simplified relationship between airflow, total static pressure, and combined fan-motor efficiency. It is useful for early equipment schedules, energy models, and electrical load estimates, but actual fan selection depends on the specific fan curve, filter and coil pressure drops, and duct system design. Confirm final fan horsepower against the manufacturer's certified performance data before specifying equipment.
Single AHU vs. Multiple Smaller Units
Larger central AHUs generally cost less per unit of airflow and simplify maintenance, but a single large unit puts every zone it serves at risk during a shutdown. Many Canadian commercial buildings split floors or wings across two or more AHUs specifically to limit that exposure, especially in hospitals and buildings with critical spaces. This calculator sizes one AHU at a time. Run it once per air handling zone if your building uses multiple units.
Frequently Asked Questions
Supply airflow is driven by whichever of two requirements is larger: the airflow needed to remove the sensible cooling load at the design supply air temperature difference, or the minimum outdoor air and ventilation airflow required for the zones served. In metric units, airflow in L/s equals sensible load in watts divided by 1.23 times the supply air temperature difference in degrees Celsius. Compare that to the ventilation requirement from the commercial ventilation calculator and size the AHU for the larger value.
Most Canadian commercial AHUs are designed around a 10 to 12 degree Celsius temperature difference between room air and supply air for comfort cooling applications. A smaller temperature difference of 8 to 9 degrees is common for spaces sensitive to cold drafts, while a larger difference of 14 to 17 degrees reduces fan energy and duct size but increases the risk of draft complaints if diffusers are not selected carefully.
The cooling coil load includes the zone sensible cooling load plus the additional load from cooling outdoor air down to the supply air temperature, plus fan heat gain across the supply fan. In a system with significant outdoor air, the coil load can be noticeably higher than the sum of just the zone loads, which is why outdoor air percentage matters so much in coil selection. See the fan coil calculator for perimeter zone terminal units instead of central AHU coils.
A common preliminary estimate for a variable air volume AHU with standard filtration and coils uses a total static pressure of 750 to 1,000 Pa and a combined fan and motor efficiency of about 65 percent. Fan power in kW equals airflow in L/s times total pressure in Pa, divided by the efficiency, divided by 1,000. This is a budget estimate only. Final fan selection should come from the manufacturer's certified fan curve.
Related HVAC Tools
Zone cooling and heating loads
Minimum outdoor air requirement
Downstream terminal unit sizing
Packaged rooftop alternative
Dedicated outdoor air sizing
Perimeter zone terminal units
Reduce coil sizing with recovery
City-specific design conditions