Mixed air conditions from outdoor and return air streams, with an economizer check.
| OA% | System Type / Condition | Notes | Mode |
|---|---|---|---|
| 10–15% | Minimum ventilation only | Typical; the 62.1 minimum is an OA CFM, not a percentage | Mixed |
| 15–30% | Typical commercial VAV | Standard summer / winter design | Mixed |
| 30–70% | Partial economizer | OA cooler than RA; free cooling assist | Econ |
| 70–100% | Full / near-full economizer | Cool, dry OA; compressor may cycle off | Econ |
| 100% | 100% OA systems (DOAS, most labs) | No recirculation | 100% OA |
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This mixed air temperature calculator blends outdoor and return air streams into the conditions entering a cooling or heating coil. Enter the temperature, airflow, and humidity ratio of each stream, and the tool returns the mixed dry-bulb temperature, the outdoor air fraction, the mixed humidity ratio, and the mixed enthalpy, with a flow diagram and an economizer check.
Mixed air conditions set the entering state for coil sizing and tell you whether free cooling is available or whether preheat is required. By mixing on enthalpy as well as temperature, the tool supports both temperature and enthalpy economizer logic.
| Quantity | Equation |
|---|---|
| Dry-air mass flow | m = CFM (standard basis) or m = CFM ÷ v, v = 0.370486·(T + 459.67)·(1 + 1.607858·W) ÷ p |
| OA fraction (mass) | x = m‑OA ÷ (m‑OA + m‑RA) |
| Mixed humidity | W‑mix = x · W‑OA + (1 − x) · W‑RA |
| Enthalpy | h = 0.240 · T + (W ÷ 7000) · (1061 + 0.444 · T); h‑mix = x · h‑OA + (1 − x) · h‑RA |
| Mixed temp | MAT = (h‑mix − 1061·W‑mix) ÷ (0.240 + 0.444·W‑mix), W in lb/lb |
This is adiabatic mixing as in ASHRAE Fundamentals Ch. 1: humidity ratio and enthalpy are conserved, so they mix linearly by dry-air mass, and the mixed dry-bulb follows from the mixed enthalpy and humidity ratio. Standard CFM (the usual basis for scheduled and 62.1 outdoor airflow) is already proportional to mass. Actual CFM is converted with each stream’s specific volume v; the barometric pressure p cancels, so no elevation is needed. The basis matters in cold weather: at −10 °F, 40% outdoor air by actual CFM is about 44% by mass, and the mixed air is about 3 °F colder than a simple CFM-weighted average. The enthalpy equation uses temperature in °F and humidity ratio W in lb/lb dry air (grains ÷ 7,000). The economizer messages compare outdoor and return dry-bulb and enthalpy; the high-limit type and setpoint your energy code requires are not checked. Saturation checks use sea-level pressure.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.