Mixed Air Temperature Calculator

Calculate the mixed air temperature (MAT) in an air handling unit by blending outdoor air and return air flow rates and temperatures using the standard flow-weighted mixing equation.

Quick Facts

Mixing formula
MAT = (CFM₁T₁ + CFM₂T₂) / (CFM₁+CFM₂)
A flow-weighted average of outdoor air and return air temperatures.
Freeze protection
Low-limit trip ≈ 35-40°F (2-4°C)
Protects hydronic coils from freezing and bursting.
Economizer control
OA damper modulates to hit target MAT
Uses cool outdoor air for "free cooling" when conditions allow.

Your Results

Calculated
Mixed Air Temperature
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Flow-weighted average of OA and RA
Total Supply Airflow
-
CFM(OA) + CFM(RA)
Outdoor Air Fraction
-
Share of total airflow that is outdoor air
Freeze-Risk Guidance
-
Compares MAT to typical coil low-limit setpoint

Ready

Enter outdoor air and return air conditions, then press Calculate.

Formula and Method for Mixed Air Temperature

In an HVAC air handling unit (AHU), outdoor air (OA) brought in for ventilation and return air (RA) recirculated from the building merge in a mixing plenum before passing over the heating or cooling coil. The mixed air temperature (MAT) is the flow-weighted average of the two streams' temperatures:

MAT = (CFMOA × TOA + CFMRA × TRA) / (CFMOA + CFMRA)

where CFM is the volumetric airflow rate of each stream (cubic feet per minute) and T is each stream's dry-bulb temperature. Because this is a linear weighted average, it produces a consistent result whether temperatures are entered in Fahrenheit or Celsius — no conversion to an absolute scale like Kelvin is required, unlike ratio-based thermodynamic formulas such as Carnot efficiency.

Deriving the mixing equation from an energy balance

The mixing equation follows from conservation of energy applied to the adiabatic mixing of two air streams. The sensible heat carried by each stream is approximately Q = ṁ × cp × T, where ṁ is mass flow rate and cp is the specific heat of air. Because outdoor air and return air have nearly identical density and specific heat at typical building conditions, mass flow rate is proportional to volumetric flow rate (CFM), and the cp terms cancel out of the balance ṁOAcpTOA + ṁRAcpTRA = (ṁOA + ṁRA)cpTMAT. Solving for TMAT leaves the CFM-weighted average shown above. This simplification is standard in HVAC design practice (per ASHRAE Fundamentals) and is accurate to a fraction of a degree across typical comfort-conditioning temperature ranges.

Freeze protection and economizer control

MAT is not just an intermediate number — it drives two real control decisions in an AHU. First, many units include a low-limit thermostat ("freezestat") that trips when MAT drops too low, commonly around 35-40°F (roughly 2-4°C), because cold, humid mixed air can freeze the water inside a hydronic coil and split the tubes; the controller responds by closing the outdoor air damper and/or opening the heating valve. Second, in "economizer" mode, the building automation system modulates the outdoor air damper position specifically to hit a target MAT close to the supply-air setpoint, so the AHU can use cool outdoor air for free cooling instead of running mechanical refrigeration.

Frequently Asked Questions

What is mixed air temperature (MAT) in an HVAC system?
MAT is the temperature of the air produced when outdoor air (OA) brought in for ventilation combines with return air (RA) recirculated from the building, before that combined airstream passes over the air handling unit's heating or cooling coil. It is calculated as the airflow-weighted average of the OA and RA temperatures.
What is the formula for calculating mixed air temperature?
MAT = (CFMOA × TOA + CFMRA × TRA) / (CFMOA + CFMRA). This weights each stream's temperature by its share of the total airflow, so a small outdoor-air fraction has a small effect on MAT even when the outdoor temperature is extreme.
Why do air handling units need freeze protection based on mixed air temperature?
When cold outdoor air mixes with return air, the resulting mixed air can still be cold enough (typically below about 35-40°F / 2-4°C) to freeze the water inside a chilled-water or hot-water coil downstream, cracking tubes and causing costly damage. A low-limit sensor (freezestat) monitors MAT and shuts the outdoor air damper or opens the heating valve if it drops too low.
Does humidity or altitude affect the accuracy of the mixed air temperature calculation?
The CFM-weighted formula assumes outdoor and return air have similar density and specific heat, a good approximation for dry-bulb temperature at typical building conditions. Very humid air or high-altitude, low-density applications introduce small errors, so precise coil-load calculations in those cases use mass-flow-weighted enthalpy balances instead of a simple temperature average.

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