Calculate mixed air temperature, humidity, enthalpy and dew point when two airstreams combine in HVAC systems, with mixing ratio sweep table.
In HVAC systems, two airstreams — typically outside air (OA) and return air (RA) — are blended in a mixing box before conditioning. The resulting mixed air conditions determine the cooling or heating load the coils must handle. Accurate mixed-air calculation is essential for energy-efficient system design and proper ventilation.
Unlike temperature, which mixes linearly by flow proportion, relative humidity must be recalculated from the mixed humidity ratio and the saturation pressure at the mixed temperature. This means the mixed RH is not simply an average of the two input values, and condensation can occur even when both incoming streams are below saturation.
This Mixed Air Calculator handles two airstreams with independent temperature, relative humidity, and airflow rate. It computes the mixed air temperature, humidity ratio, relative humidity, enthalpy, dew point, and approximate wet bulb. Altitude correction adjusts atmospheric pressure for high-elevation sites. A sweep table shows how mixed conditions change across outside air fractions from 0% to 100%, useful for economizer setpoint selection. Presets cover common HVAC scenarios from winter mixing to data center cooling.
Use this page to estimate mixed-air temperature, moisture content, and coil load when outside air and return air blend in an AHU or mixing box. It gives you a quick psychrometric check before you size coils or set economizer controls. It also keeps the mixed state and outside-air fraction together so you can compare several mixing ratios consistently.
Mixed Temperature: Tmix = (Q₁ × T₁ + Q₂ × T₂) / (Q₁ + Q₂) Humidity Ratio: W = 0.622 × Pv / (Patm − Pv) Saturation Pressure: Ps = 610.78 × exp(17.27T / (T + 237.3)) Mixed Humidity Ratio: Wmix = f₁ × W₁ + f₂ × W₂ Mixed Enthalpy: hmix = 1.006 × T + W × (2501 + 1.86 × T) kJ/kg Altitude Correction: Patm = 101325 × exp(−alt / 8500)
Result: Tmix = 16.5°C, RHmix = 42.3%, Wmix = 5.1 g/kg
Mixing 500 CFM of 0°C/50% RH outside air with 1500 CFM of 22°C/40% RH return air produces a mixed air temperature of 16.5°C with 42.3% relative humidity — a 25% outside air fraction.
Mixed-air conditions are the starting point for coil sizing, economizer logic, freeze protection, and ventilation checks. If the blend is wrong, the rest of the HVAC load calculation is wrong as well.
Dry-bulb temperature can be averaged by flow weighting, but relative humidity cannot. The correct path is to mix humidity ratio or enthalpy first, then derive the resulting RH and dew point from the mixed state.
This kind of calculation is most useful when comparing outdoor-air fractions, checking winter mixing-box temperatures, and estimating whether a coil or duct surface risks condensation. It is a psychrometric screening tool, not a full controls sequence by itself.
RH depends on both moisture content and temperature. Since saturation pressure is nonlinear with temperature, the mixed RH must be recalculated from Wmix and Ps at Tmix.
The percentage of total airflow that comes from outside. ASHRAE Standard 62.1 sets minimum values for adequate ventilation, so the fraction matters as much as the temperature.
Yes. If the mixed dew point exceeds a surface temperature (like cold ductwork), condensation can form even though neither incoming stream is saturated.
An economizer uses cool outside air to reduce mechanical cooling. The OA fraction sweep table helps find the optimal ratio for energy savings.
Higher altitude means lower atmospheric pressure, which changes humidity ratios and saturation behavior. At 1500 m, Patm drops about 16%.
Mixed-air enthalpy is used to estimate total coil load because it combines sensible temperature effects with latent moisture effects in a single energy quantity. That makes it a convenient single-number check for HVAC load comparisons.