How to Use Mixed Air Calculator
- Enter outside-air fraction — Percentage of OA in the mix.
- Enter outdoor and return dry-bulb temperatures and relative humidities.
- Read results — Mixed dry-bulb T, humidity ratio W, and resulting RH.
Formula & Theory - Mixed Air Calculator
For each stream (OA, RA):
Pws(T) = 611.2 · exp[ 17.62 T / (243.12 + T) ] # Magnus, T in °C
Pw = (RH / 100) · Pws(T)
W = 0.62198 · Pw / (P − Pw)
Mass-weighted mix (fOA = outdoor fraction):
T_mix = fOA · T_OA + (1 − fOA) · T_RA
W_mix = fOA · W_OA + (1 − fOA) · W_RA
RH_mix = (Pw_mix / Pws(T_mix)) · 100
with Pw_mix = W_mix · P / (0.62198 + W_mix)| Symbol | Meaning |
|---|---|
| T | Dry-bulb temperature |
| RH | Relative humidity (%) |
| W | Humidity ratio (kg water / kg dry air) |
| fOA | Outside-air mass fraction |
| P | Total pressure (assumed 101.325 kPa) |
Assumptions and Limits
- Mass-fraction mix ≈ volumetric mix at typical HVAC densities.
- Magnus correlation valid −40 °C to 50 °C.
- No condensation occurs during mixing (typical for HVAC return + OA).
Use Cases for Mixed Air Calculator
- HVAC sizing — Determine mixed-air entering condition for coil load calculations.
- Demand-controlled ventilation — See how OA fraction changes mixed-air RH.
- Education — Verify psychrometric mix-line concepts.
The Mixed Air Calculator packages the full psychrometric mix in one click.