Mixed Air Calculator
Enter the flow rate, dry-bulb temperature, and relative humidity for your outside air stream and return air stream. The calculator solves the adiabatic mixing equations to give you the mixed dry-bulb temperature, relative humidity, humidity ratio, dew point, specific enthalpy, specific volume, and mass flow rate of the combined stream. Switch between metric and imperial units at any time.
What is mixed air in HVAC?
Mixed air is the blend of fresh outside air and conditioned return air that enters the air handling unit (AHU) before passing over the cooling or heating coil. Modern air handling systems intentionally combine these two streams rather than conditioning 100% outside air because the return air is already close to the target temperature and humidity, which saves energy. The proportion of outside air (the outside air fraction) must meet minimum ventilation requirements set by standards such as ASHRAE 62.1, while the balance is recirculated return air. The mixed air condition determines the entering conditions at the coil and therefore drives coil sizing, fan selection, and energy consumption.
How the mixed air calculation works
Mixing two air streams is an adiabatic process: no heat is added or lost during the blend, so the combined stream conserves both total energy (enthalpy) and total moisture (humidity ratio). Three simultaneous balances govern the result. First, dry air mass balance: m1 + m2 = m3. Second, moisture balance: m1*omega1 + m2*omega2 = m3*omega3, where omega is the humidity ratio in kg of water per kg of dry air. Third, energy balance: m1*h1 + m2*h2 = m3*h3, where h is specific enthalpy in kJ per kg of dry air. Volumetric flow rates are converted to mass flow rates using the specific volume of each stream at its conditions. The mixed humidity ratio and enthalpy follow directly from the mass-weighted averages, and the mixed dry-bulb temperature is then solved from the enthalpy equation: T = (h - 2499.86 * omega) / (1.006 + 1.86 * omega).
Why enthalpy matters more than dry-bulb temperature alone
A common shortcut treats mixed air temperature as a simple weighted average of the two dry-bulb temperatures: MAT = (%OA x OAT) + (%RA x RAT). This is accurate only when both streams have identical humidity ratios or when the coil operates in a purely sensible (dry) mode. In practice, outside air often carries far more moisture than return air, especially in humid climates. Ignoring that moisture underestimates the total load on the cooling coil - the latent component (dehumidification) is not captured. Using the full enthalpy-based adiabatic method gives the true entering condition: higher enthalpy means a larger coil load, more refrigerant capacity, and more energy use. This calculator uses the full psychrometric method at all times.
Practical HVAC applications
Engineers use the mixed air calculation at multiple design stages. During coil selection, the entering condition from this calculator feeds the coil model to size sensible and latent capacity. During economizer design, the outside air fraction is varied to find the conditions under which free cooling is available (outside enthalpy below return enthalpy). For commissioning and fault detection, field-measured mixed air temperature and humidity are compared to the predicted value; deviations reveal damper leakage, sensor drift, or unexpected infiltration. In energy modelling, the mixed air conditions drive annual load calculations across a weather file. This calculator covers all of those scenarios: sweep the outside air fraction from 0% to 100% using the chart to see how each mixed parameter changes.
ASHRAE comfort and ventilation reference ranges
| Parameter | Recommended range | Notes |
|---|---|---|
| Dry-bulb temperature | 20-26 C (68-79 F) | ASHRAE 55 comfort zone for sedentary occupancy |
| Relative humidity | 30-60% | ASHRAE 55; mold risk rises above 70% |
| Humidity ratio | 7-12 g/kg (48-84 gr/lb) | Typical comfort envelope |
| Dew point | 2-15 C (36-59 F) | Coil condensation begins at dew point |
| Enthalpy | 40-60 kJ/kg (17-26 BTU/lb) | Typical conditioned supply air range |
| Outside air fraction | 15-40% | ASHRAE 62.1 minimum ventilation typical range |
| Specific volume | 0.82-0.88 m3/kg (13-14 ft3/lb) | Sea-level moist air at comfort temperatures |
Key psychrometric thresholds for occupied commercial spaces per ASHRAE 55 (comfort) and ASHRAE 62.1 (ventilation).
Frequently asked questions
What is mixed air temperature and how is it calculated?
Mixed air temperature (MAT) is the dry-bulb temperature of the combined outside air and return air stream entering an air handling unit. A quick approximation is the weighted average: MAT = (fraction_OA x T_OA) + (fraction_RA x T_RA). For accurate HVAC design, however, you should use the full enthalpy-based adiabatic mixing method - this calculator uses mass flow rates, humidity ratios, and specific enthalpies to derive the true mixed conditions including relative humidity and dew point, not just temperature.
Why does the mixed air humidity differ from a simple average of the two RH values?
Relative humidity is not a conserved quantity during mixing - it depends on temperature as well as moisture content. Humidity ratio (g/kg or gr/lb) is the conserved property. When you mix two streams, the resulting humidity ratio is the mass-weighted average of the two humidity ratios. You then compute the saturation pressure at the mixed dry-bulb temperature to find what percentage of saturation that moisture represents, which gives you the correct mixed relative humidity.
What is the outside air fraction and what is a typical value?
The outside air fraction (also called the OA percentage) is the share of the total supply air flow that comes from outdoors rather than from recirculated return air. ASHRAE 62.1 sets minimum ventilation rates based on occupancy and floor area, which translate to an OA fraction typically in the range of 15 to 40 percent for commercial office buildings. Economizer operation can push this to 100 percent when outdoor conditions are favourable. Very low fractions (under 10 percent) can cause indoor air quality problems and code violations.
What is adiabatic mixing and why does it apply here?
Adiabatic mixing means the two air streams exchange heat and moisture only with each other, not with the surrounding ductwork or structure. This is an excellent assumption for the short mixing section upstream of the AHU coil because any heat exchange with the duct walls is negligible compared to the enthalpy of the streams. As a result, the combined stream conserves total dry-air mass, total moisture mass, and total enthalpy, which is what the three simultaneous balance equations express.
How does atmospheric pressure affect the calculation?
Atmospheric pressure affects the partial pressure of water vapour and therefore the humidity ratio, specific volume, and enthalpy of each stream. At higher altitudes (lower pressure), air is less dense, specific volume increases, mass flow rates drop for the same volumetric flow, and the psychrometric properties shift. Sea level is 101.325 kPa; Denver (1600 m) is about 83.5 kPa; Mexico City (2240 m) about 77 kPa. Always input the actual site pressure for accurate results, especially for high-altitude projects.
Can I use this calculator for 100% outside air systems?
Yes. Set the return air flow rate to zero. The calculator will use only the outside air stream and the mixed air outputs will equal the outside air properties. This is the design condition for dedicated outdoor air systems (DOAS) and cleanroom applications where no recirculation is permitted. Conversely, setting outside air flow to zero gives you the 100% recirculation condition.
What is dew point and why does it matter for mixed air?
Dew point is the temperature at which the mixed air stream becomes saturated and moisture begins to condense. It is determined solely by the moisture content (humidity ratio) of the air, not by the dry-bulb temperature. If you cool the mixed air on the coil below its dew point, you get condensation, which removes latent heat and dehumidifies the air. Knowing the mixed air dew point tells you whether the coil will operate in sensible-only or wet mode, which significantly affects coil capacity and energy use.