Mixed Air Temperature and Outdoor Air Fraction Calculator

Concepts behind this calculation

Predict mixed-air temperature and humidity from two actual inlet states, or use the retained equal-density temperature estimate and dry-bulb outdoor-fraction diagnostic.

A temperature-only mixing estimate treats outdoor and return air as streams with equal density and specific heat. Their dry-bulb temperatures then reveal a weighted average or, inversely, an apparent outdoor-air fraction. The inverse calculation becomes sensitive to sensor error when the inlet temperatures are close together.

Actual moist-air mixing uses each inlet volume and state to obtain dry-air mass flow. Humidity ratio and enthalpy are conserved on that dry-air basis, and the mixed state determines its own actual volume. Relative humidity is not an additive mixing property. A balance that would become supersaturated needs a separate condensation or ice model rather than a clipped humidity result.

How to use this calculator

1. Choose Mixing method for the available measurements.
2. For temperature methods, enter Outdoor inlet temperature and Return inlet temperature with Outdoor-air fraction or Measured mixed temperature.
3. For actual-state mixing, establish Actual inlet states and mixing reference and Actual-flow and single-phase mixing basis, then enter Common total absolute pressure and both actual inlet airflows, dry bulbs, and relative humidities.
4. Review mixed properties and the stated fraction basis together.

Calculation method and applicability

Sensible estimate: Tmix = x Toa + (1-x) Tra. Actual-state method: mda,i = Qi / vi; Wmix and hmix are dry-air-mass weighted; Tmix = (hmix - 1061 Wmix)/(0.240 + 0.444 Wmix); mixed actual CFM = total mda per minute times vmix.

x = outdoor fraction for the temperature-only estimate; T = dry-bulb temperature (°F). In actual-state mixing, mda = dry-air mass flow (lb dry air/min), v = specific volume (ft³/lb dry air), W = humidity ratio (lb water/lb dry air) and h = enthalpy (BTU/lb dry air).

Input definitions and units

Mixing method

Choose the equal-density sensible estimate, a three-temperature diagnostic, or single-gas-phase mixing of two actual moist-air flows at a common pressure. Choices: Sensible mixed temperature (equal density and cp); Diagnose outdoor fraction from three dry bulbs; Mix two actual moist-air streams by dry-air mass.

Outdoor inlet temperature (°F)

Measure near the actual outdoor stream entering the mixing section. Applies to Mixing method: Sensible mixed temperature (equal density and cp) or Diagnose outdoor fraction from three dry bulbs.

Return inlet temperature (°F)

Use a representative air temperature before mixing or added fan/coil heat. Applies to Mixing method: Sensible mixed temperature (equal density and cp) or Diagnose outdoor fraction from three dry bulbs.

Outdoor-air fraction (%)

This sensible-volume fraction assumes equal air density and specific heat. Applies to Mixing method: Sensible mixed temperature (equal density and cp).

Measured mixed temperature (°F)

Use a representative mixed-stream measurement. The tool requires at least 1°F separation between the two inlet temperatures. Applies to Mixing method: Diagnose outdoor fraction from three dry bulbs.

Actual inlet states and mixing reference

Identify actual measurement/design states, each airflow reference, common absolute pressure and mixing-section basis. The tool does not retrieve weather, authenticate field data or determine a ventilation/code target. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Actual-flow and single-phase mixing basis

Confirm each airflow is actual CFM at its own entered state, both share a common total absolute pressure, and steady adiabatic ordinary moist-air mixing applies. SCFM and temperature-only mass fractions are different inputs. Choices: Not established; Actual inlet flows/states and adiabatic mixing basis established. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Common total absolute pressure (psia)

Common barometric absolute pressure for both inlet and mixed states; not duct static/gauge pressure. No altitude lookup or pressure-change model is applied. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Outdoor actual inlet airflow (ACFM at outdoor inlet state)

Actual volume per minute at the outdoor dry-bulb/RH/pressure entered below. Zero is valid if return flow is positive; still supply both inlet states. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Outdoor inlet dry bulb (°F)

Supported dry-bulb range: -40 to 140 F. Below the water triple point, the named saturation relation uses the ice branch. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Outdoor inlet relative humidity (%)

Relative to the named saturation relation at the outdoor inlet, from 0 to 100%. Zero represents dry air; RH is not averaged directly during mixing. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Return actual inlet airflow (ACFM at return inlet state)

Actual volume per minute at the return dry-bulb/RH/pressure entered below. Zero is valid if outdoor flow is positive; still supply both inlet states. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Return inlet dry bulb (°F)

Representative return state before mixing or fan/coil heat. Supported dry-bulb range: -40 to 140 F. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

Return inlet relative humidity (%)

Return-state RH from 0 to 100%. Relative humidity and actual inlet volume fractions do not replace dry-air mass weights. Applies to Mixing method: Mix two actual moist-air streams by dry-air mass.

How to interpret the result

Keep the temperature-derived fraction, actual inlet-volume fraction, and dry-air mass fraction distinct. Unexpected fractions can indicate an unsuitable model or unrepresentative measurements, not necessarily a damper position.

Worked examples

Assumptions and limits

• Temperature-only modes assume equal density and specific heat, no leakage and no fan/coil heat. Humidity, enthalpy and condensation are not calculated in those two modes.
• The inverse temperature diagnostic is withheld below a 1 F inlet separation and rejects inferred fractions outside 0-100%. These are tool guards, not published measurement tolerances or economizer compliance.
• The actual-state method uses ASHRAE 2017 perfect-gas moist-air properties: dry bulb -40 to 140 F, RH 0-100%, positive finite common absolute pressure and two nonnegative actual inlet flows with at least one positive. Both inlet states are required even at zero flow.
• Actual inlet flow is converted to dry-air mass using each inlet specific volume. Humidity ratio and enthalpy mix by that mass; relative humidity is not averaged. The sum of volume rates at different inlet states is not imposed as conserved mixed ACFM.
• Single-gas-phase mixing only. Supersaturated balances are withheld and require a separate liquid-water or ice equilibrium model. RH overshoot only up to 1e-8 percentage points is treated as numerical roundoff at saturation; real supersaturation is not clipped into a reported 100% result.
• The named saturation relation switches ice/water branches at the water triple point. These approximations are not real-gas properties or a full psychrometric process tool. No fan heat, pressure drop, leakage, stratification, coil duty or water-injection/drain energy is modeled.
• Inputs and state references must be established independently. No humidity/enthalpy economizer high-limit control, minimum ventilation rate, equipment sizing, field calibration or current-code compliance is selected.

Sources

• ASHRAE 2017 Fundamentals, IP Chapter 1: PsychrometricsSpecific-volume/enthalpy relations and Equation 44 dry-air, moisture and energy mixing balance. Example 4 chart dry bulb 65.9 F is compared within stated chart-reading tolerance; source Equation 33 converts its entering 40 F / 35 F wet-bulb state to this method's RH input.
• ASHRAE 2017 Fundamentals, SI Chapter 1Hyland-Wexler saturation coefficient basis used consistently with the existing site psychrometric calculations and the triple-point ice/water branch.
• CaptiveAire: Air Mixing CalculatorActual peer controls and method text expose full two-stream mass-weighted mixing. Its broad property selectors/weather/elevation and downstream process features are not copied; no live peer numerical output is claimed.
• AIM Works: Mixed Air Temperature CalculatorActual-CFM mode was executed for 95/75 F, 500/2,000 CFM and 95/65 grains per lb. Its 78.9 F, 19.3% mass, 70.8 grains per lb and 30.02 BTU/lb outputs reproduce at shown precision. Room/economizer flags and an exact mixed-flow claim are not adopted.