Mixed Air Temperature Calculator

🌍 Air Stream Inputs

Outdoor Air Temperature (°F) Design outdoor dry-bulb

Return Air Temperature (°F) Space / return dry-bulb

Outdoor Air CFM Ventilation + economizer OA

Return Air CFM Recirculated return airflow

Outdoor Air Humidity Ratio (gr/lb) From psychrometric chart or weather data

Return Air Humidity Ratio (gr/lb) Space humidity ratio target

CFM Basis Actual CFM is converted to mass with each stream’s specific volume

📊 Mixed Air Results

—
Mixed Air Temp (°F)

—

OA Fraction, by mass (%)

—

Mixed Air CFM

—

Mixed Humidity Ratio (gr/lb)

—

Mixed Enthalpy (BTU/lb)

—

ΔT (MAT vs. Return) (°F)

⚠ Cold Mixed Air: mixed air is below about 45 °F — check preheat and coil freeze protection, or reduce OA.

✔ Economizer Favorable: outdoor air is cooler and lower in enthalpy than return air — if the space needs cooling, extra OA is free cooling.

⚠ Cool but Humid: outdoor air is cooler than return air but higher in enthalpy — a dry-bulb economizer would admit it and add coil load; an enthalpy high-limit would lock it out.

✔ Normal Mixing: outdoor air is warmer than return air — hold OA at the ventilation minimum.

🔥 Flow Diagram

⚡ Enthalpy & Psychrometric Detail

—

OA Enthalpy (BTU/lb)

—

RA Enthalpy (BTU/lb)

—
Mixed Enthalpy (BTU/lb)

Enthalpy: h = 0.240×T + (W ÷ 7000)×(1061 + 0.444×T), with W in gr/lb. W and h mix linearly by dry-air mass; the mixed dry-bulb is solved from the mixed h and W.

📚 OA% Reference Guide

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

📝 About This Calculator

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.

🔢 Formula & Method

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.

❓ Frequently Asked Questions

How is mixed air temperature calculated?

Mixed air temperature is the mass-weighted average of the outdoor and return air streams: approximately MAT = OA fraction times outdoor temp plus return fraction times return temp, where each fraction is that stream's share of the dry-air mass flow. With standard CFM the mass fraction is simply the CFM fraction, so 500 CFM of 95 degree F outdoor air mixed with 2,000 CFM of 75 degree F return air gives about 79 degrees F. With actual CFM at very different temperatures, such as subzero outdoor air, the denser outdoor air carries more than its CFM share and the mix is a few degrees colder; the tool handles both bases.

Why also mix on enthalpy and humidity ratio?

Temperature alone misses the moisture content, which matters for cooling coil load and economizer decisions. This tool mixes the outdoor and return humidity ratios and enthalpies by dry-air mass, which is exact for adiabatic mixing, and solves the mixed temperature from them. Comparing outdoor and return enthalpy, not just dry-bulb, is the basis of an enthalpy economizer, which avoids bringing in cool but very humid outdoor air; the tool flags that case.

When does mixed air need a preheat coil?

In cold weather with significant outdoor air, the mixed air temperature can fall low enough to risk coil freezing or to sit below the desired coil entering temperature. When the mixed temperature drops toward or below about 45 degrees F, a preheat coil or a reduction in outdoor air is typically needed. This tool warns when the calculated mixed temperature is in that range.

What enthalpy formula does this tool use?

It uses the standard moist-air enthalpy relation h = 0.240 times T plus W times (1061 + 0.444 times T), with temperature T in degrees F and humidity ratio W in pounds of moisture per pound of dry air (the tool converts the grains-per-pound input by dividing by 7,000). This is the psychrometric enthalpy equation from ASHRAE Fundamentals, applied to each stream and then mass-weighted for the mixture; the mixed dry-bulb is solved back from the mixed enthalpy and humidity ratio.

🔗 Related Tools

OA / RA % Calculator Enthalpy Calculator Supply Air Temp Ventilation Calculator DOAS Sizing