Inputs
Example Data Table
Example values help verify your setup. Use the calculator for exact outputs.
| Scenario | Outdoor | Return | Mix Inputs | Expected Trend |
|---|---|---|---|---|
| Hot and humid summer | 35 degC, 60% RH | 24 degC, 50% RH | Outdoor 30%, Return 70% | Mixed temperature rises; humidity ratio increases. |
| Cool and dry morning | 12 degC, 40% RH | 22 degC, 50% RH | Outdoor 20%, Return 80% | Mixed temperature drops; humidity ratio decreases. |
| High altitude site | 20 degC, 50% RH | 24 degC, 45% RH | Flows 1500 / 3500, Altitude 1500 m | Lower pressure slightly changes RH and dew point. |
Formula Used
The calculator mixes properties using flow (or percentage) weighting. Temperature and humidity ratio are combined first, then RH, enthalpy, and dew point are derived.
- Mixed temperature: Tmix = (Wo*To + Wr*Tr) / (Wo + Wr)
- Humidity ratio: w = 0.621945*pv / (P - pv)
- Vapor pressure: pv = (RH/100)*psat(T)
- Mixed humidity ratio: wmix = (Wo*wo + Wr*wr) / (Wo + Wr)
- Enthalpy: h = 1.006T + w(2501 + 1.86T) (T in degC, h in kJ/kg)
- Mixed RH: RH = 100*pv/psat(Tmix) using pv from wmix
Notes: W can be flow rate or percentage. Keep units consistent.
How to Use This Calculator
- Select Metric or Imperial units at the top.
- Choose Use Flows or Use Percentage.
- Enter outdoor and return temperatures and RH values.
- Provide flows (both) or an outdoor percentage.
- Set pressure as standard, altitude-based, or custom.
- Click Calculate Mixed Air to see results above.
- Use CSV/PDF buttons to export the calculated report.
Professional Guide
1) Mixed air in ventilation design
Mixed air is the condition entering the coil when outdoor ventilation combines with return air. This state influences coil selection, supply setpoints, and humidity control. Designers evaluate mixed air at peak summer and winter points to confirm sensible and latent capacity.
2) Why weighting method matters
Temperature and moisture must be averaged using the same weighting basis. If you use flow rates, keep outdoor and return flows in consistent units. If you use outdoor percentage, the calculator treats the percentage as the mixing weight. Consistent weighting prevents underestimating humidity ratio and dew point.
3) Interpreting humidity ratio and dew point
Humidity ratio (w, kg/kg) is the most stable moisture metric for mixing. Relative humidity changes with temperature, so mixed RH can look misleading. Dew point is often more actionable: if mixed dew point approaches coil surface temperature, condensate will form and drainage must be planned.
4) Enthalpy for coil load checks
Moist air enthalpy combines sensible and latent energy. Use mixed-air enthalpy to estimate coil load using Q ≈ m·(hin − hout). For example, raising outdoor fraction during hot-humid weather increases mixed enthalpy, pushing the cooling coil toward higher latent removal demand.
5) Typical ranges and sanity checks
Common indoor return conditions for comfort spaces are around 22–25 degC and 40–60% RH. Outdoor summer design can exceed 32–40 degC with high moisture. Mixed temperature should fall between outdoor and return values, and wmix should lie between wo and wr.
6) Outdoor air fraction and energy impact
Outdoor air fractions of 10–30% are common, but requirements depend on occupancy and codes. Increasing outdoor fraction improves dilution but raises heating and cooling energy. Use the calculator to compare mixed conditions at 10%, 20%, and 30% and see the change in coil entering air.
7) Pressure, altitude, and performance
Barometric pressure affects vapor pressure relationships and therefore humidity ratio and RH. At higher altitudes, lower pressure can slightly change calculated mixed RH and dew point for the same temperature and moisture content. Selecting altitude or custom pressure provides better alignment with site commissioning measurements.
8) Field workflow for reliable inputs
Use calibrated sensors for temperature and RH and log steady values for several minutes. Measure flows with balanced dampers, airflow stations, or traverse methods. Enter outdoor and return conditions first, then confirm the mixed outputs are physically reasonable. Export results to document assumptions.
FAQs
1) What does “mixed air” mean?
Mixed air is the combined air stream created when outdoor ventilation air blends with return air before the coil or fan. It represents the entering condition for many air-handling calculations.
2) Should I mix relative humidity directly?
No. Relative humidity is temperature dependent. Mix humidity ratio (w) using weights, then compute mixed RH at the mixed temperature. This approach avoids large errors at different temperatures.
3) Which weights should I use: flow or percentage?
Use flow rates when you know outdoor and return airflow values. Use percentage when ventilation is specified as a fraction of total supply. Keep both weights based on the same total for valid results.
4) Why include pressure or altitude?
Pressure affects moisture calculations and therefore humidity ratio, RH, and dew point. Altitude mode estimates pressure for your site. Custom pressure is best when you have measured barometric pressure.
5) How can I use mixed enthalpy in sizing?
Mixed enthalpy helps estimate total coil load using the enthalpy difference across the coil multiplied by mass flow. It captures both sensible and latent cooling effects in one value.
6) What if my mixed temperature is outside the input range?
That usually indicates inconsistent weights or incorrect units. Ensure outdoor and return weights sum logically, and that both flows use the same unit system. Recheck sensor placement and readings.
7) Is this calculator suitable for humidification or dehumidification checks?
Yes, for preliminary checks. Use mixed dew point and humidity ratio to assess condensation risk and humidification needs. For final design, confirm with your detailed psychrometric and equipment performance data.