Mixed Air Temp Calculator

An essential tool for HVAC professionals to determine the resulting air temperature when two air streams are combined.

Chart showing weighted contribution of each air stream to the final temperature.

What is a Mixed Air Temp Calculator?

A mixed air temp calculator is a specialized tool used in the HVAC (Heating, Ventilation, and Air Conditioning) industry. It calculates the final temperature of an air stream that results from combining two separate air streams with different temperatures and flow rates. Most commonly, it’s used to determine the temperature of the air entering a system’s cooling or heating coil after fresh outside air is mixed with return air from inside the building. This calculation is fundamental for energy efficiency, proper equipment sizing, and ensuring indoor air quality.

Anyone involved in HVAC design, installation, or maintenance—from engineers to technicians—uses this calculation. Understanding the mixed air temperature is critical because it directly impacts the load on the heating or cooling system. A miscalculation can lead to oversized or undersized equipment, wasted energy, and poor comfort levels for occupants.

Mixed Air Temperature Formula and Explanation

The calculation for mixed air temperature is a weighted average based on the temperature and proportion of each air stream. The most common formula uses percentages of airflow:

MAT = (%OA × OAT) + (%RA × RAT)

Where %RA is simply 100% minus %OA. This formula provides an accurate estimate of the temperature the system’s coil will “see.” For more on system efficiency, see our guide to HVAC efficiency.

Formula Variables

Variable	Meaning	Unit	Typical Range
MAT	Mixed Air Temperature	°F or °C	-20°F to 120°F (-29°C to 49°C)
%OA	Percentage of Outside Air	%	5% to 100%
OAT	Outside Air Temperature	°F or °C	-20°F to 120°F (-29°C to 49°C)
%RA	Percentage of Return Air	%	0% to 95%
RAT	Return Air Temperature	°F or °C	68°F to 80°F (20°C to 27°C)

Practical Examples

Example 1: Standard Summer Cooling

Imagine a typical summer day where the system is bringing in a minimum amount of fresh air for ventilation.

• Inputs:
  • Outside Air Temperature (OAT): 95°F
  • Return Air Temperature (RAT): 75°F
  • Outside Air Percentage (%OA): 20%
• Calculation:
  • Return Air Percentage (%RA) = 100% – 20% = 80%
  • MAT = (0.20 × 95°F) + (0.80 × 75°F)
  • MAT = 19°F + 60°F
• Result: The mixed air temperature entering the cooling coil is 79°F.

Example 2: Economizer “Free Cooling” Mode

Consider a cool spring day where the outside air is pleasant enough to be used for cooling, reducing the need for mechanical refrigeration.

• Inputs:
  • Outside Air Temperature (OAT): 60°F
  • Return Air Temperature (RAT): 74°F
  • Outside Air Percentage (%OA): 75% (Economizer is mostly open)
• Calculation:
  • Return Air Percentage (%RA) = 100% – 75% = 25%
  • MAT = (0.75 × 60°F) + (0.25 × 74°F)
  • MAT = 45°F + 18.5°F
• Result: The mixed air temperature is 63.5°F, significantly reducing the cooling load. Proper air handler sizing is key to leveraging this effect.

How to Use This Mixed Air Temp Calculator

Using this calculator is a straightforward process:

1. Select Temperature Unit: First, choose whether you are working with Fahrenheit (°F) or Celsius (°C). The labels will update automatically.
2. Enter Return Air Temperature: Input the temperature of the air returning from the conditioned space.
3. Enter Outside Air Temperature: Input the temperature of the fresh air from outdoors.
4. Enter Outside Air Percentage: Input the percentage of the total airflow that is coming from outside. The calculator assumes the rest is return air.
5. Calculate: Click the “Calculate” button. The result will display the final mixed air temperature, along with the weighted contributions of each air stream.
6. Interpret Results: The primary result is your Mixed Air Temperature. The chart provides a visual representation of how much each air stream contributes to this final value.

Key Factors That Affect Mixed Air Temperature

• Damper Position: The physical position of the outside air and return air dampers directly controls the percentages (%OA and %RA).
• Economizer Controls: Automated economizers adjust the damper position based on outdoor temperature and humidity to provide “free cooling,” drastically changing the mixed air temperature.
• Building Load: The heating or cooling load of the building influences the return air temperature (RAT), which is a key part of the equation.
• Ventilation Requirements: Building codes (like ASHRAE 62.1) mandate a minimum amount of outside air for occupants, setting a baseline for %OA.
• Duct Leakage: Leaks in the return or outside air ducts can unintentionally alter the air mixture, leading to a different actual mixed air temperature than calculated.
• Sensor Accuracy and Location: The accuracy of the temperature sensors for OAT and RAT is crucial. Poor placement can lead to false readings and incorrect calculations. Explore our CFM calculator to understand airflow’s role.

Frequently Asked Questions (FAQ)

Why is mixed air temperature important?

It determines the entering air conditions for the heating/cooling coil. This directly affects the system’s capacity, energy consumption, and ability to maintain comfort.

What is the difference between this formula and one that uses CFM?

The formula using CFM (Cubic Feet per Minute) is more direct: MAT = ((CFM_oa * T_oa) + (CFM_ra * T_ra)) / (CFM_total). Our calculator simplifies this by using percentages, which is more common when exact CFM values aren’t known.

Can I use this calculator for heating season?

Yes. The principle is exactly the same. During winter, the OAT will be lower than the RAT, and the calculator will correctly determine the temperature of the cold air mixture the furnace needs to heat up.

What happens if my outside air percentage is 100%?

If %OA is 100, the mixed air temperature will simply be equal to the outside air temperature. This occurs in systems that do not recirculate air or when an economizer is fully open.

Does humidity affect the mixed air temperature?

This calculator uses dry-bulb temperature. While humidity contains energy (latent heat), a simple temperature mix is based on dry-bulb readings. For a more precise energy calculation, you would need to use a psychrometric chart which considers enthalpy.

What is a typical outside air percentage?

For minimum ventilation, it’s often between 10-20%. During economizer mode, it can be anywhere from this minimum up to 100%, depending on conditions.

How does air stratification relate to this?

If the outside and return air streams are not mixed thoroughly, layers of different temperatures (stratification) can form. This can cause the coil to see uneven temperatures, potentially leading to freezing or tripping safety sensors.

Where should temperature sensors be placed?

To get accurate inputs for this calculation, OAT sensors should be in the shade in the fresh air intake duct, and RAT sensors should be in the main return duct before the air streams mix.

Related Tools and Internal Resources

Expand your knowledge of HVAC systems with our other calculators and guides:

• HVAC Efficiency Guide: Learn how to maximize the performance of your systems.
• Air Handler Sizing Tool: Ensure your equipment is properly sized for your needs.
• Ductwork Design Principles: A guide to efficient air distribution.
• Psychrometric Chart Tool: For advanced calculations involving humidity.
• Energy Savings Calculator: Estimate potential savings from HVAC upgrades.
• CFM Calculator: Calculate airflow in your system.