Mixed Air Calculator

How Mixed Air Calculator Works?

The Mixed Air Calculator applies steady-state psychrometric mass and energy balances to combine outdoor air and return air. It uses the outdoor airflow, return airflow, dry-bulb temperatures, relative humidities, and barometric pressure to determine the condition of the resulting air stream.

For each air stream, saturation vapor pressure is determined from temperature. The water-vapor partial pressure is then calculated from relative humidity:

Here, $\phi$ is relative humidity expressed as a fraction, $P_{\mathrm{sat}}$ is saturation pressure, and $P_v$ is vapor partial pressure. Humidity ratio, defined as the mass of water vapor per mass of dry air, is calculated by:

where $P$ is barometric pressure. The dry-air density associated with each stream can be estimated using the ideal-gas relationship:

The dry-air mass flow for each inlet is obtained by multiplying dry-air density by volumetric airflow. The combined dry-air mass flow is:

Moisture is conserved during adiabatic mixing, so the mixed humidity ratio is the dry-air-mass-weighted average of the two inlet humidity ratios. Enthalpy is balanced in the same way:

For ordinary HVAC psychrometric calculations, moist-air enthalpy is approximated by:

The mixed dry-bulb temperature is recovered from mixed enthalpy and humidity ratio:

Finally, the mixed vapor pressure is obtained from the mixed humidity ratio, and it is used with the mixed saturation pressure to calculate relative humidity. Dew point is the temperature at which the mixed vapor pressure equals saturation pressure. Mixed airflow is calculated from the mixed moist-air density and the combined dry-air mass flow.

Practical Use Cases

HVAC engineers can use the calculator to estimate the entering-air condition at an air-handling unit when outdoor ventilation air combines with return air. The results support coil-load calculations, equipment selection, and evaluation of heating or cooling requirements.

Building designers can assess how different outdoor-air and return-air proportions affect mixed temperature, humidity ratio, enthalpy, and dew point during design conditions.

HVAC technicians can compare measured outdoor and return conditions with expected mixed-air properties when troubleshooting dampers, mixing boxes, economizer operation, or abnormal coil performance.

Building operators can evaluate ventilation strategies by observing how changes in outdoor airflow influence indoor air-conditioning demand and mixed-air humidity.

Energy analysts can use mixed-air enthalpy and dry-air mass flow to estimate the sensible and latent conditioning burden imposed on heating and cooling equipment.

Industrial and process-air users can examine the combined moisture and temperature condition before air enters a drying, storage, manufacturing, or environmental-control process.

Accuracy and Limitations

The calculator is suitable for steady-state HVAC mixing estimates when outdoor and return streams are well mixed, pressure is uniform, and no moisture condenses or evaporates during mixing. Its results depend on the accuracy of the entered airflow, temperature, relative humidity, and barometric pressure; relative-humidity errors can substantially affect humidity ratio and dew point, especially near saturation. The psychrometric relationships use ideal-gas behavior and standard moist-air enthalpy approximations, so results may differ slightly from detailed psychrometric software. The calculation does not model duct heat transfer, fan heat, damper leakage, stratification, coil contact effects, condensation, or transient changes.

Quick Examples