HVAC systems blend fresh outdoor air with recirculated return air. The mixture’s temperature and humidity – found by mass-weighted averaging of humidity ratio and enthalpy – set the load on the coils.
How to use the Mixed Air Calculator
- Air pressure
- Outdoor airflow
- Outdoor temperature
- Outdoor RH (%)
- Return airflow
- Return air temperature
- Return air RH (%)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the air pressure and the airflow, temperature and relative humidity of the outdoor and return air streams.
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 |
|---|---|
| Air pressure | Units: kPa, hPa, psi |
| Outdoor airflow | Units: m³/h, CFM, L/s |
| Outdoor temperature | Units: °C, °F, K |
| Outdoor RH (%) | A number |
| Return airflow | Units: m³/h, CFM, L/s |
| Return air temperature | Units: °C, °F, K |
| Return air RH (%) | A number |
What each result means
- Mixed humidity ratio = ([m1]×[W1] + [m2]×[W2])÷([m1] + [m2])×1000
- Mixed enthalpy = ([m1]×(1.006×[Outdoor temperature] + [W1]×(2501 + 1.86×[Outdoor temperature])) + [m2]×(1.006×[Return air temperature] + [W2]×(2501 + 1.86×[Return air temperature])))÷([m1] + [m2])
- Mixed air temperature = ([Mixed enthalpy] – 2501×[Mixed humidity ratio]÷1000)÷(1.006 + 1.86×[Mixed humidity ratio]÷1000)
- Mixed relative humidity = ([Mixed humidity ratio]÷1000×[Air pressure]÷(0.622 + [Mixed humidity ratio]÷1000))÷(611.2×exp(17.62×[Mixed air temperature]÷(243.12 + [Mixed air temperature])))×100
- Outdoor air fraction = [m1]÷([m1] + [m2])×100
Example values
How the main result changes when Air pressure changes and everything else stays at the example values above:
| Air pressure (kPa) | Mixed air temperature (°C) |
|---|---|
| 50.66 | 25.9663 |
| 75.99 | 25.9644 |
| 101.33 | 25.9634 |
| 126.66 | 25.9628 |
| 151.99 | 25.9624 |
| 202.65 | 25.9619 |
Formula
W = 0.622·p_v/(p − p_v), h = 1.006T + W(2501 + 1.86T) · mixing by mass: W_mix = Σṁ·W/Σṁ, h_mix = Σṁ·h/ΣṁWorked example
Inputs — Air pressure: 101.325 kPa; Outdoor airflow: 500 m³/h; Outdoor temperature: 32 °C; Outdoor RH (%): 60; Return airflow: 1,500 m³/h; Return air temperature: 24 °C; Return air RH (%): 50.
Results — Mixed humidity ratio = 11.3847 g/kg; Mixed enthalpy = 55.1421 kJ/kg; Mixed air temperature = 25.9634 °C; Mixed relative humidity = 54.4294 %; Outdoor air fraction = 24.2526 %.
Key terms
- Return air
- Air drawn back from the conditioned space.
Frequently asked questions
Why mix by mass and not volume?
Air density differs with temperature and humidity.
Is the mixed temperature the simple average?
Close to the flow-weighted average, but not exactly.
What is the humidity ratio?
Mass of water vapour per mass of dry air.
Can mixing cause condensation?
Yes, if the mixed state falls above saturation.
What formula does the Mixed Air Calculator use?
W = 0.622·p_v/(p − p_v), h = 1.006T + W(2501 + 1.86T) · mixing by mass: W_mix = Σṁ·W/Σṁ, h_mix = Σṁ·h/Σṁ.