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

📊 Mixed Air Results

—
Mixed Air Temp (°F)

—

OA Fraction (%)

—

Total Supply CFM

—

Mixed Humidity Ratio (gr/lb)

—

Mixed Enthalpy (BTU/lb)

—

ΔT (MAT vs. Return) (°F)

✔ Economizer Benefit: Outdoor air is cooler than return air — free cooling potential exists.

⚠ Heating Required: Mixed air temperature is below coil leaving air temp — consider preheat coil or OA reduction.

✔ Normal Mixing: Mixed air temperature is within typical cooling coil entering range.

🔥 Flow Diagram

⚡ Enthalpy & Psychrometric Detail

—

OA Enthalpy (BTU/lb)

—

RA Enthalpy (BTU/lb)

—
Mixed Enthalpy (BTU/lb)

Enthalpy: h = 0.240×T + W×(1061 + 0.444×T) / 7000, where W is humidity ratio in gr/lb. Mixed conditions use mass-weighted average of OA and RA streams.

📚 OA% Reference Guide

OA%	System Type / Condition	Notes	Mode
10–15%	Minimum ventilation only	ASHRAE 62.1 minimum; cooling season	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	OA at or below setpoint; compressor off	Econ
100%	100% OA systems (labs, OR)	No recirculation; DOAS / critical spaces	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
OA fraction	OA‑frac = OA CFM ÷ (OA CFM + RA CFM)
Mixed temp	MAT = OA‑frac · T‑OA + RA‑frac · T‑RA
Mixed humidity	W‑mix = OA‑frac · W‑OA + RA‑frac · W‑RA
Enthalpy	h = 0.240 · T + (W ÷ 7000) · (1061 + 0.444 · T)

Mixed conditions are the flow-weighted (adiabatic mixing) average of the two streams, the standard relationship in ASHRAE Fundamentals. The enthalpy equation h = 0.240·T + W·(1061 + 0.444·T) uses temperature in °F and humidity ratio W in lb/lb dry air, where the tool divides the grains-per-pound input by 7,000. Comparing outdoor and return enthalpy is the basis of an enthalpy economizer.

❓ Frequently Asked Questions

How is mixed air temperature calculated?

Mixed air temperature is the flow-weighted average of the outdoor and return air streams: MAT = OA fraction times outdoor temp plus return fraction times return temp, where each fraction is that stream's CFM divided by the total. For example, 500 CFM of 95 degree F outdoor air mixed with 2,000 CFM of 75 degree F return air gives a mixed temperature of about 79 degrees F.

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 mass-weights the outdoor and return humidity ratios and enthalpies the same way it does temperature. 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.

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.

🔗 Related Tools

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

Mixed Air Temperature Calculator ASHRAE