Mixed Air Temperature & Humidity Calculator
Enter the airflow and measured condition of outdoor and return air to estimate the mixed-air state at an air handler. The calculation weights each stream by dry-air mass, so it accounts for temperature, humidity, density, and site altitude.
Mixed-air dry-bulb temperature (°F)79.87
- Mixed-air relative humidity
- 51.86%
- Mixed-air dew point (°F)
- 60.61
- Mixed-air humidity ratio (grains/lb dry air)
- 79.02
- Mixed-air enthalpy (Btu/lb dry air)
- 23.88
- Mixed airflow (CFM)
- 2,000.03
- Outdoor-air fraction by dry-air mass
- 24.07%
- Outdoor-air total load versus return air (Btu/h)
- 29,822.92
Calculation details
| Quantity | Value |
|---|---|
| Site atmospheric pressure (kPa) | 101.33 |
Assumes steady, adiabatic, complete mixing at one pressure with no fan heat, duct heat transfer, leakage, humidification, or coil process.
Outdoor-air load compares the outdoor and return enthalpies only; it is not a complete equipment-sizing load.
How to use this calculator
- Enter the outdoor airflow and its dry-bulb temperature and relative humidity from the same measurement period.
- Enter return airflow and the return-air dry bulb and relative humidity at the mixing location.
- Enter site altitude so CFM is converted to dry-air mass at the local atmospheric pressure.
- Read mixed-air temperature first, then use humidity, enthalpy, and outdoor-air load to assess the operating condition.
How mixed air is calculated
Outdoor and return air do not mix by temperature alone. Each stream first becomes a psychrometric state: its relative humidity and dry-bulb temperature give water-vapor pressure, humidity ratio, enthalpy, and specific volume. Specific volume converts the entered CFM to dry-air mass flow, which is the basis for the mix.
The calculator uses the ASHRAE saturation-pressure equations and the local standard-atmosphere pressure. For each stream, W = 0.621945 × Pw ÷ (P − Pw), where W is humidity ratio, Pw is vapor pressure, and P is atmospheric pressure. Moist-air enthalpy is h = 1.006T + W(2501 + 1.86T) in kJ per kg of dry air.
The two conserved quantities in ideal adiabatic mixing are dry-air mass and moist-air enthalpy. The mixed state is therefore Wm = (moWo + mrWr) ÷ (mo + mr) and hm = (moho + mrhr) ÷ (mo + mr). It then solves Tm = (hm − 2501Wm) ÷ (1.006 + 1.86Wm). Dew point comes from finding the temperature whose saturation pressure equals the mixed vapor pressure.
Why the dry-air mass fraction matters
A simple CFM-weighted temperature is often close, but it is not exact when the streams have different temperatures, moisture contents, or densities. Warm, moist air occupies more volume per pound of dry air than cool, dry air. At altitude, every CFM also carries less dry air. The outdoor-air fraction shown here is consequently a dry-air-mass fraction, which can differ from the damper or CFM fraction.
The ventilation-load result is the outdoor dry-air mass flow times the outdoor-minus-return enthalpy difference. A positive result means outdoor air adds cooling load compared with return air. A negative value means the outdoor condition provides an enthalpy benefit, but it does not by itself prove that economizer operation is appropriate.
What this calculator leaves out
This is a steady, complete, adiabatic mixing estimate at one pressure. It does not include fan heat, duct heat transfer, leakage, damper calibration, sensor error, stratification, humidification, coils, or changing airflow. If the calculated relative humidity exceeds 100%, the unconstrained mixed state is supersaturated; fog or condensation may form, and a two-phase analysis is needed rather than clipping the result to saturation.
Worked example
For 500 CFM of 95°F outdoor air at 50% relative humidity mixed with 1,500 CFM of 75°F return air at 50% relative humidity at sea level, the calculator gives mixed air near 79.9°F and 51.9% relative humidity. The mixed humidity ratio is about 79 grains per lb of dry air, and the outdoor stream adds roughly 29,800 Btu/h relative to the return stream.
The outdoor dry-air mass fraction is close to, but not exactly, 25%. Its warmer and moister condition gives it a larger specific volume, so its mass share is slightly lower than its 500 ÷ 2,000 CFM share.
Common questions
How do you calculate mixed air temperature from outdoor and return air?
Convert each airstream to humidity ratio, enthalpy, specific volume, and dry-air mass flow. Then mass-weight humidity ratio and enthalpy, and solve the moist-air enthalpy equation for mixed dry-bulb temperature. A temperature-only weighted average misses the effect of moisture and density.
Why is mixed air temperature not always a CFM-weighted average?
CFM is volume, while conservation of energy and moisture follows dry-air mass. Two equal CFM streams can carry different dry-air mass when their temperatures, humidity, or local pressure differ. The difference is usually modest at ordinary HVAC conditions but can matter for design work and economizer analysis.
How does relative humidity change when two airstreams mix?
Relative humidity is not averaged directly. The calculation mixes humidity ratio by dry-air mass, finds the resulting dry-bulb temperature from enthalpy, then divides the mixed vapor pressure by saturation pressure at that temperature. This can produce a relative humidity higher or lower than either entering value.
Can mixing two unsaturated airstreams produce condensation?
Yes. The ideal no-condensation mix can fall above 100% relative humidity for some stream pairs. In that case, water must condense or form fog until the final state reaches saturation; this calculator flags that situation rather than treating the supersaturated result as a final physical state.
How does altitude affect mixed-air calculations?
Altitude changes atmospheric pressure and therefore the dry-air mass in each CFM. It also changes humidity ratio at a given temperature and relative humidity. Temperature mixing may change only a little, but mass fraction, actual mixed CFM, and ventilation load can change more noticeably.