Why is mixed RH not a linear average?

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.

What is the outside air fraction?

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.

Can condensation occur in the mixing box?

Yes. If the mixed dew point exceeds a surface temperature (like cold ductwork), condensation can form even though neither incoming stream is saturated.

What is an economizer?

An economizer uses cool outside air to reduce mechanical cooling. The OA fraction sweep table helps find the optimal ratio for energy savings.

Why correct for altitude?

Higher altitude means lower atmospheric pressure, which changes humidity ratios and saturation behavior. At 1500 m, Patm drops about 16%.

What is enthalpy used for?

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.

Mixed Air Calculator

Calculate mixed air temperature, humidity, enthalpy and dew point when two airstreams combine in HVAC systems, with mixing ratio sweep table.

Temperature Unit

Altitude (m)

Airstream 1 (Outside Air)

Temperature (°C)

Relative Humidity (%)

Airflow (CFM)

Airstream 2 (Return Air)

Temperature (°C)

Relative Humidity (%)

Airflow (CFM)

Mixed Air Temperature

16.5 °C

61.7 °F — flow-weighted average

Mixed Air RH

46.4%

Based on mixed humidity ratio and saturation pressure at Tmix

Mixed Humidity Ratio

5.39 g/kg

Wmix — mass of water vapor per kg dry air

Mixed Enthalpy

30.28 kJ/kg

hmix — useful for coil load calculations

Mixed Dew Point

5.0 °C

Temperature at which condensation begins

Approx. Wet Bulb

12.7 °C

Estimated Twb ≈ T − (T − Tdp)/3

Outside Air Fraction

25.0%

2,000 CFM total mixed air

Temperature Mixing

OA: 0.0°CMix: 16.5°CRA: 22.0°C

OA %	Temp (°C)	RH (%)	W (g/kg)
0%	22.0	40.0	6.56
10%	19.8	42.6	6.09
20%	17.6	45.1	5.62
30%	15.4	47.6	5.16
40%	13.2	50.0	4.69
50%	11.0	52.0	4.22
60%	8.8	53.6	3.75
70%	6.6	54.6	3.28
80%	4.4	54.6	2.82
90%	2.2	53.2	2.35
100%	0.0	50.0	1.88

Planning notes, formulas, and examples

About the Mixed Air Calculator

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.

When This Page Helps

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.

How to Use the Inputs

Select the temperature unit (°C or °F) and enter the site altitude.
Enter the temperature, relative humidity, and airflow (CFM) for airstream 1 (outside air).
Enter the same for airstream 2 (return air).
Use preset buttons for common HVAC mixing scenarios.
Review the mixed air temperature, RH, humidity ratio, enthalpy, dew point, and wet bulb.
Check the OA fraction sweep table to evaluate economizer setpoints.
The temperature mixing bar shows the relative proportions of each airstream.

Formula used

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)

Example Calculation

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.

Tips & Best Practices

Temperature blends roughly by airflow share, but relative humidity must be recomputed from the mixed moisture content.
Check dew point as well as temperature, because a safe-looking dry-bulb value can still lead to condensation on cold surfaces.
Economizer decisions are better when you compare enthalpy, not just outside-air temperature.
Altitude changes the psychrometrics enough that high-elevation sites should not reuse sea-level assumptions blindly.

What Mixed Air Calculations Solve

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.

Moisture Does Not Mix Like Temperature

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.

Practical Design Use

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.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

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.