| Property | Stream 1 | Stream 2 | Mixed |
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Mixed air calculations for AHU design in Canada
Every air handling unit that brings in outdoor air mixes two airstreams at its inlet: return air from the conditioned space and outdoor air required for ventilation. The mixed air condition is the entering condition for the cooling or heating coil, and it determines the coil's required capacity. Getting the mixed air calculation right is the foundation of accurate coil sizing.
Mixed air properties follow a simple mass-weighted average. If you mix 8,000 CFM of return air at 75°F/50% RH with 2,000 CFM of outdoor air at 95°F/55% RH, the total is 10,000 CFM. The mixed dry-bulb is (8000×75 + 2000×95) / 10000 = 79°F. Humidity ratio mixes the same way. Enthalpy also follows this rule, making it easy to calculate the total coil load: total CFM times 4.5 times the enthalpy difference between mixed air and leaving air conditions.
Economizer cycles and mixed air in Canadian climates
Canadian shoulder seasons — spring and fall — offer significant economizer opportunities. When outdoor air is cool enough, increasing the outdoor air fraction provides free cooling without running the refrigeration system. A dry-bulb economizer activates when outdoor temperature is below roughly 16-18°C. At 100% outdoor air, the mixed air equals outdoor conditions and no mechanical cooling may be needed. This calculator lets you check mixed air conditions at any outdoor air fraction to determine whether the coil will be in cooling or heating mode.
In Alberta and Saskatchewan, outdoor air in spring and fall is often both cool and dry — ideal for economizers. In Vancouver, outdoor air is frequently near the dewpoint limit, so enthalpy-based economizer controls are preferred over dry-bulb-only controls. Use the enthalpy calculator to compare outdoor air enthalpy against return air enthalpy and determine the economizer changeover point.
Frequently Asked Questions
Mixed air temperature = (CFM1 × T1 + CFM2 × T2) / (CFM1 + CFM2). Apply the same formula to humidity ratio for mixed air moisture content. Mixed air enthalpy = (CFM1 × h1 + CFM2 × h2) / (CFM1 + CFM2). The mixed air condition is the entering coil condition — feed it into the cooling coil calculator with your leaving air design condition to size the coil. For the full psychrometric state point at mixed conditions, use the psychrometric calculator.
During economizer operation, increasing outdoor air fraction provides free cooling when outdoor conditions allow. Dry-bulb economizers activate below roughly 16-18°C outdoor temperature. At 100% outdoor air, mixed air equals outdoor conditions. The mixed air enthalpy determines whether the coil adds or removes heat. In Canadian spring and fall, cool dry outdoor air often means mixed air enthalpy is already below supply air setpoint — no mechanical cooling needed. Use the enthalpy calculator to compare outdoor and return air enthalpy for economizer changeover analysis.