Table of Contents
Mixed Air Temperature in HVAC
The mixed air temperature (MAT) is the temperature of the air mixture after combining return air from the conditioned space with fresh outdoor air in an HVAC air handling unit. This calculation is critical for determining heating and cooling coil loads, controlling economizer operation, and preventing coil freezing in cold weather. The MAT is controlled by adjusting the outdoor air damper position.
Minimum outdoor air is required by ASHRAE Standard 62.1 for acceptable indoor air quality, typically 15-20% of total supply air for office buildings. During mild weather, the outdoor air percentage can be increased (economizer mode) to provide free cooling, potentially eliminating the need for mechanical refrigeration.
The Formula
Where MAT is mixed air temperature, Treturn is return air temperature, Toutdoor is outdoor air temperature, and %OA is outdoor air fraction (0-1).
Example Calculations
| Return (°F) | Outdoor (°F) | OA % | MAT (°F) |
|---|---|---|---|
| 75 | 95 | 20% | 79.0 |
| 75 | 35 | 20% | 67.0 |
| 75 | 55 | 100% | 55.0 |
| 75 | 10 | 20% | 62.0 |
Frequently Asked Questions
What is the minimum mixed air temperature?
To prevent cooling coil freezing, the mixed air temperature should generally not drop below 40-45 degrees F. If the calculated MAT falls below this, the outdoor air damper should close to minimum position, or preheat coils must be used. Coil freeze-stats typically shut down the unit if air temperature drops below 35 F at the coil.
How does economizer mode work?
When outdoor air is cooler than return air (and outdoor enthalpy is lower), the economizer opens the outdoor air damper beyond minimum to use free cooling. At 100% outdoor air, no mechanical cooling is needed if the outdoor air is at or below the desired supply temperature. The crossover point depends on climate, typically working well when outdoor temperatures are 55-65 F.
What is the difference between dry-bulb and enthalpy economizer?
A dry-bulb economizer compares outdoor and return temperatures only. An enthalpy economizer compares total heat content (sensible + latent). Enthalpy economizers are more energy-efficient in humid climates because they account for the dehumidification load, but they require humidity sensors and more complex controls.