Mixing fresh outdoor air with return air is a quick way to estimate the air entering a heating or cooling coil. Building controls use the mixed-air temperature to manage economisers.
How to use the Mixed Air Temperature Calculator
- Method
- Outdoor airflow
- Return airflow
- Outdoor air (%)
- Outdoor air temperature
- Return air temperature
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose whether you know the airflows or the outdoor air percentage.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Airflows, Outdoor air percentage |
| Outdoor airflow | Units: CFM, m³/h, L/s |
| Return airflow | Units: CFM, m³/h, L/s |
| Outdoor air (%) | A number |
| Outdoor air temperature | Units: °C, °F, K |
| Return air temperature | Units: °C, °F, K |
Example values
How the main result changes when Outdoor airflow changes and everything else stays at the example values above:
| Outdoor airflow (CFM) | Mixed air temperature (°C) |
|---|---|
| 200 | 25.2222 |
| 300 | 25.7368 |
| 400 | 26.2 |
| 500 | 26.619 |
| 600 | 27 |
| 800 | 27.6667 |
Formula
T_mix = (V_oa·T_oa + V_ra·T_ra)/(V_oa + V_ra) = x·T_oa + (1 − x)·T_raWorked example
Inputs — Method: Airflows; Outdoor airflow: 400 CFM; Return airflow: 1,600 CFM; Outdoor air temperature: 35 °C; Return air temperature: 24 °C.
Results — Outdoor fraction = 20 %; Mixed air temperature = 26.2 °C.
Key terms
- Outdoor air fraction
- Share of fresh air in the supply.
Frequently asked questions
Why is outdoor air added?
For ventilation and indoor air quality.
What is an economiser?
A control that uses more outdoor air when it is cool enough to save energy.
Is this the same as the psychrometric mixed air?
It ignores humidity and density – fine for quick estimates.
What outdoor air percentage is typical?
10–30% for offices.
What formula does the Mixed Air Temperature Calculator use?
T_mix = (V_oa·T_oa + V_ra·T_ra)/(V_oa + V_ra) = x·T_oa + (1 − x)·T_ra.