Mixing Outdoor Air & Return Air

How to compute mixed air properties (T, RH, W, h) by mass-weighted mixing of outdoor air and return air using psychrometrics.

HVAC systems often mix outdoor air (OA) and return air (RA) before conditioning. The mixed air state determines coil loads, dehumidification potential, and comfort outcomes.

Key idea: mix on a dry-air basis

Psychrometric “mixing” is a mass balance problem. For best accuracy, mix properties using the dry-air mass flow as the weighting basis (and assume both streams are at roughly the same pressure).

The two values to mix

Humidity ratio (W): absolute moisture content (kg/kg_da).
Enthalpy (h): useful energy variable for HVAC loads (kJ/kg_da).

Mixing formulas (mass-weighted)

Let α be the outdoor air fraction on a dry-air basis (0–1). Then:

Wmix = α · WOA + (1 − α) · WRA
hmix = α · hOA + (1 − α) · hRA

After you have Wmix and hmix, you can compute the rest of the mixed air properties (Tdb, RH, Tdp, Twb, …) from that state.

What it looks like on a psychrometric chart

The mixed air point lies on the straight line between the OA and RA points on the chart. Its location along the line depends on the mixing fraction α.

Using FluidTool (practical workflow)

Compute WOA and hOA from your outdoor air measurements (e.g., Tdb + RH).
Compute WRA and hRA from your return air measurements.
Choose a mixing fraction α (outdoor air fraction on a dry-air basis) and compute Wmix/hmix using the formulas above.
In the humid air tool, switch inputs to W + H, enter Wmix and hmix, and read the resulting mixed air Tdb/RH/dew point/wet-bulb.

Open humid air tool

Common pitfalls

Mixing RH is wrong: don’t average RH directly. Mix W and h, then compute RH from the mixed state.
Pressure/altitude: OA and RA should be evaluated at the same pressure for consistency.
Using volume fractions: if you only know volumetric flow rates, convert to a dry-air mass basis (density depends on T and humidity).

HVAC systems often mix outdoor air (OA) and return air (RA) before conditioning. The mixed air state determines coil loads, dehumidification potential, and comfort outcomes.

Key idea: mix on a dry-air basis

The two values to mix

Humidity ratio (W): absolute moisture content (kg/kg_da).
Enthalpy (h): useful energy variable for HVAC loads (kJ/kg_da).

Mixing formulas (mass-weighted)

Let α be the outdoor air fraction on a dry-air basis (0–1). Then:

Wmix = α · WOA + (1 − α) · WRA
hmix = α · hOA + (1 − α) · hRA

After you have Wmix and hmix, you can compute the rest of the mixed air properties (Tdb, RH, Tdp, Twb, …) from that state.

What it looks like on a psychrometric chart

The mixed air point lies on the straight line between the OA and RA points on the chart. Its location along the line depends on the mixing fraction α.

Using FluidTool (practical workflow)

Compute WOA and hOA from your outdoor air measurements (e.g., Tdb + RH).
Compute WRA and hRA from your return air measurements.
Choose a mixing fraction α (outdoor air fraction on a dry-air basis) and compute Wmix/hmix using the formulas above.
In the humid air tool, switch inputs to W + H, enter Wmix and hmix, and read the resulting mixed air Tdb/RH/dew point/wet-bulb.

Open humid air tool

Common pitfalls

Mixing RH is wrong: don’t average RH directly. Mix W and h, then compute RH from the mixed state.
Pressure/altitude: OA and RA should be evaluated at the same pressure for consistency.
Using volume fractions: if you only know volumetric flow rates, convert to a dry-air mass basis (density depends on T and humidity).

Key idea: mix on a dry-air basis

The two values to mix

Mixing formulas (mass-weighted)

What it looks like on a psychrometric chart

Using FluidTool (practical workflow)

Common pitfalls

Table of Contents

Mixing Outdoor Air & Return Air

Key idea: mix on a dry-air basis

The two values to mix

Mixing formulas (mass-weighted)

What it looks like on a psychrometric chart

Using FluidTool (practical workflow)

Common pitfalls

Table of Contents