Mixed air temperature calculator
Enter the outdoor (OA) and return air (RA) states and airflows to get the mixed air (MA) dry bulb, RH, enthalpy and other properties, mixed by dry-air mass. It also shows how far the textbook volume-weighted temperature is off.
How mixing works
- Mixing by dry-air mass
- In adiabatic mixing, dry-air mass, moisture and energy are conserved, so with x the outdoor share of dry-air mass, WM = x WOA + (1 − x) WRA and hM = x hOA + (1 − x) hRA. Airflows (m³/h) are divided by each stream’s specific volume to get dry-air mass (kg/h).
- On the chart: a point on the line between OA and RA
- On the psychrometric chart the mix point MA lies on the straight line joining OA and RA, and its distances to the two points are in inverse proportion to their dry-air masses. With 30 % outdoor air it sits about 30 % of the way from RA toward OA.
- The simple temperature rule
- Textbooks often use tM ≈ (QOA tOA + QRA tRA) / (QOA + QRA), splitting temperature by airflow ratio. The error is usually under 0.1 K but grows with large temperature (winter outdoor air) or humidity differences. This calculator shows the mass-based value and the difference.
Mixing examples by OA fraction (summer, 101.325 kPa)
Outdoor air 32 °C at 60 %, return air 26 °C at 50 %, 10,000 m³/h in total, changing only the outdoor airflow share. Computed at build time with the same engine as the calculator above; the last column is the difference from the simple temperature rule (mass-based minus simple).
| OA share by volume % | OA fraction by dry-air mass % | Dry bulb °C | Relative humidity % | Enthalpy kJ/kg(DA) | vs simple rule K |
|---|---|---|---|---|---|
| 10 | 9.7 | 26.59 | 51.6 | 55.39 | −0.010 |
| 20 | 19.5 | 27.18 | 53.0 | 57.88 | −0.017 |
| 30 | 29.3 | 27.78 | 54.3 | 60.38 | −0.023 |
| 50 | 49.2 | 28.97 | 56.5 | 65.44 | −0.027 |
Frequently asked questions
How do you calculate mixed air temperature?
Divide each airflow by its specific volume to get dry-air mass, mix humidity ratio and enthalpy by the outdoor mass share x (WM, hM), then get the dry bulb from tM = (hM − 2501 WM) / (1.006 + 1.86 WM). For example, 32 °C at 60 % outdoor air and 26 °C at 50 % return air with 30 % outdoor air give about 27.8 °C at 54 %, enthalpy about 60.4 kJ/kg dry air.
What is the difference between airflow ratio and mass ratio?
At the same m³/h, warm humid air has a larger specific volume and so less dry-air mass. In the example above a 30 % outdoor airflow share is about 29.3 % by mass. Energy and moisture are conserved by mass, so mixing by mass is the exact method; this calculator shows both shares.
Can mixing cause condensation or fog?
The saturation curve bends upward, so the straight line between cold, humid outdoor air and warm, humid return air can cross above it. For example, mixing outdoor air at −10 °C and 80 % half and half with return air at 24 °C and 70 % puts the straight-line mix point (about 6 °C) above saturation, and the excess moisture becomes fog or condensation. This calculator does not report such a supersaturated state; it shows a fog warning with the straight-line dry bulb, so consider preheating the outdoor air or changing the ratio.
Equations and sources
- Dry-air mass: ṁ = Q / v (m³/h ÷ m³/kg(DA) = kg/h), x = ṁOA / (ṁOA + ṁRA)
- Adiabatic mixing: WM = x WOA + (1 − x) WRA, hM = x hOA + (1 − x) hRA
- Mixed dry bulb: tM = (hM − 2501 WM) / (1.006 + 1.86 WM) (other properties from the same state-point equations)
Sources
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: Adiabatic Mixing of Two Moist Airstreams
Last updated 2026-11-09.