Mixed Air Calculator

Q: What is the difference between mixed air and supply air?

Mixed air is the blend of outdoor and return air before it hits the coil. Supply air is what leaves the unit and goes to the rooms. Between the two, the air passes the cooling or heating coil and the fan. Fan motor heat alone usually adds about 1 to 3 °F . So supply air is almost never the same temperature as mixed air.

Q: Why is my mixed air temperature reading wrong?

Almost always because of stratification . Cold outdoor air and warm return air do not blend right away, so a single sensor reads only one layer. Fixes include: Use an averaging sensor that snakes across the whole duct. Take a grid of readings and average them. Add air blenders or move the sensor farther downstream.

Introduction

When an air handler pulls in fresh outdoor air and blends it with return air from the building, the two streams form one new condition called mixed air. Enter the airflow and any two properties for each stream, and this Mixed Air Calculator returns the mixed air dry bulb, wet bulb, dew point, relative humidity, humidity ratio, enthalpy, and specific volume.

It runs in several modes. You can solve for mixed air temperature, required outdoor air flow, outdoor air fraction, return air temperature, or total supply flow. It works in Imperial or SI units, and it adjusts for site elevation, since air at high altitude behaves differently than air at sea level.

The work is shown as well. A step-by-step section lists each equation with your numbers filled in. A psychrometric chart plots all three points and draws the mixing line between them. If the blend drops below saturation, the calculator flags it and tells you how much water will condense.

Use it to size cooling coils, check ventilation rates, plan economizer settings, or troubleshoot a system that is not hitting its target supply temperature.

How to use our Mixed Air Calculator

Enter your outdoor air and return air conditions plus each airflow. The calculator gives you the mixed air temperature, wet bulb, dew point, humidity, enthalpy, specific volume, total flow, and the outdoor air fraction, with a step-by-step solution and a psychrometric chart.

Solve For: Pick what you want to find, like mixed air temperature, required outdoor air flow, or total supply flow. The input boxes change to match your pick.

Unit System: Choose Imperial or SI. Any numbers you already typed are converted for you.

Results Display: Turn this switch on to see results in both Imperial and SI at the same time.

Site Elevation: Type how high your site is above sea level. This sets the air pressure used in every step. Use 0 for sea level.

Use My Location: Click this button to let your browser find your elevation for you.

Target Mixed Air Dry Bulb: Only shows in some modes. Type the mixed air temperature you want to hit. It must fall between your two stream temperatures.

Outdoor Air Fraction: Only shows when you solve for total supply flow. Type the percent of supply air that comes from outside.

Stream 1 Airflow (Outdoor Air): Type how much outdoor air enters the unit, in CFM, L/s, or m³/h.

Stream 1 Properties: Tap any two tiles, such as dry bulb and wet bulb, then type those two values. Two properties fully define the outdoor air state.

Stream 2 Airflow (Return Air): Type how much return air comes back from the space.

Stream 2 Properties: Tap two tiles and type the matching values for the return air state.

Chart Click Mode: Pick Stream 1 or Stream 2, then click a spot on the psychrometric chart to set that state by hand.

Explore Sliders: Drag the six sliders to change outdoor and return air temperature, humidity, and flow, and watch the mixed air result move right away.

Calculate, Reset, and Try Example: Click Calculate to run the numbers, Reset to start over, or Try Example to load a sample job.

What Is Mixed Air in HVAC?

Mixed air is the air that an air handler actually blows through its coils. It is made when fresh outdoor air (OA) joins return air (RA) coming back from the rooms. The two streams meet in the mixing plenum, and the blend that leaves is called the mixed air (MA).

Why Mixed Air Temperature Matters

The mixed air condition sets how hard your cooling or heating coil must work. If you pull in a lot of hot, damp outdoor air, the coil has to remove more heat and more water. If the mix gets too cold in winter, the coil can freeze and burst. Knowing the mixed air dry bulb, wet bulb, and humidity helps you size equipment, pick the right coil, and set economizer and freeze-stat controls.

How Air Mixing Works

Mixing follows a simple rule: heat and moisture are shared based on how much mass of air each stream brings. More airflow means more pull on the final answer. The basic mixed air formula is:

Mixed temperature ≈ (OA CFM × OA temp + RA CFM × RA temp) ÷ Total CFM
Mixed humidity ratio and enthalpy use the same weighted average

The quick CFM version is fine for rough checks. A true answer uses mass flow, found by dividing each airflow by its specific volume. Hot air weighs less per cubic foot, so the mass split is never exactly the same as the volume split.

Outdoor Air Fraction

The outdoor air fraction is the share of the supply air that comes from outside. A 25% OA fraction means one quarter of the supply is fresh air. Codes like ASHRAE 62.1 set minimum outdoor air for good indoor air quality, so this number is both a comfort and a health item.

The Mixing Line on a Psychrometric Chart

Plot the outdoor point and the return point on a psychrometric chart, then draw a straight line between them. The mixed air point always lands on that line. It sits closer to whichever stream has more mass flow. If the line crosses the saturation curve (100% relative humidity), water drops out of the air and fog or condensate forms in the plenum.

Elevation and Air Pressure

Air gets thinner as you go up. Higher elevation means lower barometric pressure, lighter air, and different enthalpy and specific volume values. That is why job sites in the mountains need elevation entered before the numbers can be trusted.

Key Terms

Dry bulb: plain air temperature on a thermometer.
Wet bulb: temperature with evaporation cooling the sensor; it hints at moisture.
Dew point: the temperature where water starts to condense.
Humidity ratio (W): pounds or grains of water per pound of dry air.
Enthalpy (h): total heat in the air, sensible plus latent.
Specific volume (v): cubic feet of air per pound; used to turn CFM into mass flow.

Where Technicians Use It

Field techs use mixed air math to check damper position, prove an economizer is working, troubleshoot a coil that will not hit design, size heating for cold-climate outdoor air, and confirm ventilation rates during test and balance.

Formulas used

Mass flow from volumetric flow and specific volume

\dot m_i = \frac{Q_i}{v_i}

Mixed air humidity ratio (mass-weighted)

W_3 = \frac{\dot m_1 W_1 + \dot m_2 W_2}{\dot m_1 + \dot m_2}

Mixed air enthalpy (mass-weighted)

h_3 = \frac{\dot m_1 h_1 + \dot m_2 h_2}{\dot m_1 + \dot m_2}

Mixed air dry-bulb temperature from enthalpy and humidity ratio (IP)

t_3 = \frac{h_3 - 1061\,W_3}{0.240 + 0.444\,W_3}

Moist air enthalpy (IP)

h = 0.240\,t + W\,(1061 + 0.444\,t)

Humidity ratio from water vapor pressure

W = 0.621945\,\frac{p_w}{p - p_w}

Specific volume of moist air (IP)

v = \frac{0.370486\,(t + 459.67)\,(1 + 1.607858\,W)}{p}

Barometric pressure from site elevation (IP)

p = 14.696\,\left(1 - 6.8754 \times 10^{-6}\,z\right)^{5.2559}

Frequently asked questions

What should the mixed air temperature be?

It depends on the season and the outdoor air percentage. Some common ranges:

Summer cooling: often 78-85 °F, between return air and hot outdoor air.
Economizer mode: around 55 °F, so little or no mechanical cooling is needed.
Winter: keep it above about 40 °F so the coil does not freeze.

There is no single correct number. The mix should land between your outdoor and return air temperatures, and closer to whichever stream has the larger airflow.

How do you calculate outside air percentage using temperatures?

Measure the return air (RA), outdoor air (OA), and mixed air (MA) dry bulb temperatures, then use:

OA % = (RA − MA) ÷ (RA − OA) × 100

Example: RA is 75 °F, OA is 95 °F, MA is 80 °F.

(75 − 80) ÷ (75 − 95) = 5 ÷ 20 = 25% outdoor air.

Why does the temperature method give a bad outdoor air percentage in mild weather?

The math divides by the difference between return and outdoor air. When that gap is small, say 5 °F or less, a one degree sensor error can swing the answer by 20% or more.

On mild days, test outdoor air with a CO2 balance or a direct airflow traverse instead. Enthalpy can also work when the two streams have very different moisture levels.

What is the difference between mixed air and supply air?

Mixed air is the blend of outdoor and return air before it hits the coil. Supply air is what leaves the unit and goes to the rooms.

Between the two, the air passes the cooling or heating coil and the fan. Fan motor heat alone usually adds about 1 to 3 °F. So supply air is almost never the same temperature as mixed air.

At what mixed air temperature will a cooling coil freeze?

Water and chilled water coils are at risk once air reaches about 32-35 °F. Most freezestats are set to trip near 35-38 °F and shut the fan down.

In cold climates, watch the mixing plenum closely. Cold outdoor air often stays in a layer instead of blending, so one part of the coil can be near freezing even when the average reading looks fine.

Why is my mixed air temperature reading wrong?

Almost always because of stratification. Cold outdoor air and warm return air do not blend right away, so a single sensor reads only one layer.

Fixes include:

Use an averaging sensor that snakes across the whole duct.
Take a grid of readings and average them.
Add air blenders or move the sensor farther downstream.

What happens if mixed air drops below its dew point?

Water comes out of the air. You get fog in the plenum, wet coil faces, standing water in the drain pan, and rust. Mold can follow.

This happens when very cold outdoor air meets warm, humid return air. The answer is not more mixing. Lower the moisture or raise the mixed air temperature with a preheat coil.

Why is mixed air temperature not just the average of outdoor and return air?

Two reasons.

First, the streams are usually different sizes. A mix of 25% outdoor air sits much closer to the return air temperature than to the outdoor one.

Second, real mixing works on mass, not cubic feet. Hot air weighs less per cubic foot, so 1,000 CFM of 95 °F air carries less mass than 1,000 CFM of 55 °F air. The mass split is never exactly equal to the airflow split.

How much outdoor air does a building need?

ASHRAE Standard 62.1 sets the minimum. For most spaces it adds two parts together:

A rate per person, often 5 cfm per person
A rate per floor area, often 0.06 cfm per square foot

An office with 20 people and 2,000 sq ft needs about (20 × 5) + (2,000 × 0.06) = 220 CFM of outdoor air. Rates vary by space type, so check the standard and your local code.

How do you find the cooling load from the mixed air condition?

Use enthalpy, since it counts both heat and moisture. At sea level:

Total BTU/hr = 4.5 × CFM × (mixed air enthalpy − supply air enthalpy)

Example: 4,000 CFM, mixed air at 30 BTU/lb, supply air at 22 BTU/lb.

4.5 × 4,000 × 8 = 144,000 BTU/hr, or about 12 tons.

The 4.5 factor assumes standard air, so it changes at altitude.

How do you raise mixed air temperature in winter?

Three common moves:

Close the outdoor air damper to the minimum the code allows.
Add a preheat coil ahead of the mixing plenum or on the outdoor air intake.
Improve blending with air blenders so cold layers do not hit the coil.

Never close the outdoor damper all the way just to stay warm. That starves the building of fresh air.

What is a minimum outdoor air damper position?

It is the smallest damper opening that still delivers the code required ventilation air. It is set during test and balance, then locked in the controls.

A common value is 10% to 20% open, but the right number depends on duct size, fan pressure, and the occupancy of the building. Damper percent open is not the same as airflow percent, so it has to be measured, not guessed.

Does mixing two air streams change the total moisture?

No, as long as nothing condenses. Water is just shared between the two streams. The mixed humidity ratio is the mass weighted average of the two.

The exception is when the blend crosses saturation. Then the air cannot hold all the water, and the extra drops out as condensate. In that case the mixed air leaves at 100% relative humidity with less moisture than the simple average predicts.

Why does an economizer use enthalpy instead of temperature?

Because cool outdoor air can still be very wet. Air at 65 °F and 90% humidity holds more total heat than return air at 75 °F and 45% humidity. Bringing it in would raise the coil load, not lower it.

An enthalpy economizer compares the total heat of both streams and only opens the damper when outdoor air truly costs less to cool. In dry climates, a plain dry bulb changeover works fine and costs less.

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