In HVAC systems, particularly in air handlers and packaged air conditioners, outside air (OA) and return air (RA) are blended before passing through the cooling or heating coils. The resulting mixed air temperature (MAT) is critical for coil load calculations, economizer operation, and overall system efficiency.
This article explains the concept, provides the calculation formula, and shows examples for different percentages of outside air and return air, and provides a free downloadable mixed air temperature calculator.
Why Mixed Air Temperature Matters
The MAT determines the entering conditions at the cooling or heating coil. For example:
- Cooling Season: A higher MAT increases coil load and energy use.
- Heating Season: A lower MAT increases heating demand.
- Economizers: When outdoor conditions are favorable, more outside air is used to reduce mechanical cooling but they must be setup correctly to avoid wasting energy.
Accurately calculating MAT ensures coils are sized properly, economizer strategies are effective, and energy costs are minimized. This formula is based on dry bulb temperature.
The Formula for Mixed Air Temperature
The MAT is a weighted average based on the proportion of outside air and return air:
MAT = (%OA × OAT) + (%RA × RAT)
Where:
- MAT = Mixed Air Temperature (°F or °C)
- OAT = Outside Air Temperature (°F or °C)
- RAT = Return Air Temperature (°F or °C)
- %OA = Percentage of outside air
- %RA = Percentage of return air (100 – %OA)
Example Mixed Air Temperature Calculations
Example 1: 20% Outside Air
- Outside Air = 95°F
- Return Air = 75°F
- OA = 20%, RA = 80%
MAT = (0.20 × 95) + (0.80 × 75)
MAT = 19 + 60 = 79°F
Result: The coil sees 79°F entering air.
In this case since the outside air is way above the high limit for any economizer, the outside air would be set to the code minimum per ASHRAE 62.1. It makes no sense to waste energy on cooling outdoor air at 95°F when the return air is at 75°F, not considering relative humidity.
Example 2: 50% Outside Air
- Outside Air = 40°F
- Return Air = 72°F
- OA = 50%, RA = 50%
MAT = (0.50 × 40) + (0.50 × 72)
MAT = 20 + 36 = 56°F
Result: The coil sees 56°F entering air.
Example 3: 75% Outside Air (Economizer Mode)
- Outside Air = 70°F
- Return Air = 75°F
- OA = 75%, RA = 25%
MAT = (0.75 × 70) + (0.25 × 75) = 71°F
Result: The coil sees 71°F entering air. This may or may not be a good setup depending on the relative humidity of the outside air. If the outside air carries a higher latent load than the return air, then this could add significantly to the energy consumption as the coil will need to remove excessive moisture in a wet coil situation.
Example 4: 100% Outside Air (100% Economizer Mode)
- Outside Air = 60°F
- Return Air = 75°F
- OA = 100%, RA = 0%
MAT = (1.0 × 60) + (0.0 × 75) = 60°F
Result: In full economizer mode, MAT = Outside Air Temp. This is where the economizer high limit strategy will come into consideration. See our next video on economizer control high limit strategies.
Practical Considerations
Energy Efficiency: Reducing OA percentage during extreme conditions lowers energy cost, while increasing OA during mild weather can reduce mechanical cooling.
Humidity (Enthalpy Method): While temperature-only calculations are common, true load calculations should also consider humidity. Mixed air enthalpy can be calculated similarly using a weighted average.
Sensor Placement: MAT sensors should be located downstream of the mixing box to avoid stratification errors.
Minimum Ventilation: Codes (ASHRAE 62.1, IMC) require minimum outside air for ventilation, typically ranging from 10–30% based on occupancy type and number of occupants.
Latent Heat Considerations
The mixed air temperature calculation shown here is based on a dry-bulb temperature average, which assumes the cooling coil is operating in a dry (sensible-only) condition. In climates or operating conditions where the coil is wet, this method can be misleading because it ignores the additional latent load of dehumidification. In those cases, a more accurate approach is to use differential enthalpy and differential dry-bulb control, which evaluates both sensible and latent energy. This ensures that economizer operation does not increase cooling energy use in humid conditions.
Get a free copy of our Mixed Air Temperature Calculator in the link below.
