Mixed Air Temperature Calculator

Enter the airflow, dry bulb and humidity of the return air and the outside air to get the mixed air temperature, wet bulb and humidity entering the coil.

Units
Humidity reading

Mixed air dry bulb

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Fill in the boxes above to see the answer.

Mixed air wet bulb
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Mixed air relative humidity
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Mixed air enthalpyPer pound (kg) of dry air
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Outside air shareOf the total airflow
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Quick estimateDry bulbs averaged by airflow
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Leave empty for sea level. Thinner air at elevation changes wet bulb and humidity ratio.

What is mixed air temperature?

Mixed air temperature is the temperature of the air after the return air and the outside (ventilation) air blend together, just before it reaches the coil or furnace. On any system that brings in outside air, like a rooftop unit with an economizer or a fresh air intake, the mix is what the equipment actually has to heat or cool.

It matters because the coil sees the mixed air, not the return air. On a hot, humid day, outside air raises both the temperature and the moisture entering the coil, and that adds to the load the equipment has to handle.

How it's calculated

Each stream: humidity ratio W, enthalpy h and specific volume v from ASHRAE's equations
Dry air mass flow m = airflow ÷ v
Mixed W = (m₁W₁ + m₂W₂) ÷ (m₁ + m₂), mixed h the same way
Mixed dry bulb t = (h − 1061 W) ÷ (0.240 + 0.444 W) °F, or (h − 2501 W) ÷ (1.006 + 1.86 W) °C
Quick estimate = (airflow₁ t₁ + airflow₂ t₂) ÷ (airflow₁ + airflow₂)

Sources

If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.

Checked by Paul Williams · Updated

Questions

How do you calculate mixed air temperature?

The quick way averages the two dry bulbs by airflow. The exact way mixes by mass of dry air: it averages humidity ratio and enthalpy, then finds the temperature that matches. For 1,600 CFM of return air at 75°F and 50% RH plus 400 CFM of outside air at 95°F dry bulb and 78°F wet bulb, the quick estimate is 79.0°F and the exact answer is 78.9°F, with a 66.0°F wet bulb.

Is the quick estimate good enough?

Usually, for dry bulb: hot outside air is lighter than cool return air, so the exact answer comes out a little below the airflow average. The estimate can't give you the wet bulb or humidity of the mix, though, and those are what decide the coil's latent load.

Can I enter the outside air as a percentage?

Enter the airflows in any proportion: 20 and 80 works the same as 400 and 1,600 CFM. Only the ratio matters for the mixed condition.

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