Mixed Air Temperature Calculator — Free Online Calculator
Calculate the mixed air temperature (MAT) and outdoor air fraction when outdoor air mixes with return air in an air-handling unit. Use this for economizer design, minimum OA verification, AHU commissioning, and pre-cooling load estimation.
Enter airflow and temperatures
From OA damper / min OA calculation
From return duct measurement or AHU schedule
Mixed Air Temperature
Outdoor air fraction
— %
Total supply CFM
— CFM
Pre-cooling sensible load (to 55°F SA)
— BTU/h
Low OA Fraction
Outdoor air fraction is below 10%. Verify this meets your minimum OA requirement per ASHRAE 62.1 or local code.
Pre-cooling load assumes 55°F supply air temperature. Adjust for your actual supply air setpoint.
How the Mixed Air Temperature Calculator Works
Mixed air temperature is a simple energy balance — the enthalpy of the mixed stream equals the sum of the OA and RA enthalpy contributions. For dry-bulb temperature only (ignoring latent heat), this simplifies to a weighted average by CFM.
Economizer high-limit
Economizer controls typically use a 75°F dry-bulb high-limit. When OA exceeds 75°F, the OA damper closes to minimum. When OA is below 75°F and the MAT can meet supply air setpoint, the economizer can provide free cooling without mechanical refrigeration.
ASHRAE 62.1 minimum OA
For commercial buildings, ASHRAE 62.1 requires a minimum OA CFM calculated from occupancy and floor area. Common minimums range from 10–25% of supply airflow. The OA fraction shown here must meet or exceed the 62.1 minimum.
Worked example
A 2,000 CFM AHU with 400 CFM OA on a 95°F summer design day. Return air is at 75°F.
400 CFM OA at 95°F + 1,600 CFM RA at 75°F
Result: The coil enters at 79°F and must deliver 51,840 BTU/h of sensible cooling to reach 55°F supply air. OA fraction of 20% likely meets ASHRAE 62.1 minimum for most occupancy types.
Frequently asked questions
Common questions about mixed air temperature, economizers, and AHU commissioning.
What is mixed air temperature?
Mixed air temperature (MAT) is the temperature of the air entering the cooling or heating coil after outdoor air and return air have been blended in the mixing box. It determines the load on the coil and whether economizer cooling is feasible.
What is the economizer high-limit temperature?
Most economizer controls use a dry-bulb high-limit of 75°F — when the outdoor air temperature exceeds this, the damper closes to the minimum OA position. Some controls use enthalpy instead of dry-bulb, which accounts for humidity.
What outdoor air fraction is required by ASHRAE 62.1?
For commercial buildings, ASHRAE 62.1 calculates the required OA fraction based on occupancy (cfm/person) and floor area (cfm/sqft) for each zone. A system-level multizone calculation (Ez or Ev) is required for VAV systems. Minimum OA fractions typically range from 10–30%.
Why does the mixed air temperature matter for coil sizing?
The cooling coil must reduce air from MAT down to supply air temperature (typically 55°F). A higher MAT requires more cooling capacity. Economizer operation lowers MAT when OA is cool, reducing compressor load.
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