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Mixing Air Streams Calculator

Work out mixing air streams instantly with clear inputs, formula shown and shareable results.

About Mixing Air Streams Calculator

When two air streams merge, the mixed temperature is the flow-weighted average of the two. The outdoor air fraction that results is the quantity ventilation standards regulate, and the sensible penalty it imposes on the coil is 1.08 x outdoor CFM x the temperature gap it has to be pulled across.

Formulas

Mixed condition

T mix = (Q1 T1 + Q2 T2) / (Q1 + Q2)

Outdoor air penalty

BTU/h = 1.08 x outdoor CFM x (outdoor T - return T)

Worked example

With the default values below, Mixing Air Streams Calculator returns Mixed air temperature of 79.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Return air flowminimum 0 CFM
800 CFM
Return air temperatureminimum -40 °F
75 °F
Outdoor air flowminimum 0 CFM
200 CFM
Outdoor air temperatureminimum -40 °F
95 °F

Results

Mixed air temperature
79.00 °F
Total mixed airflow
1,000 CFM
Outdoor air fraction
20.0% %
Extra coil load from outdoor air
4,320 BTU/h

Step by step

  1. 1Values used
  2. 2Mixed condition
  3. 3Outdoor air penalty
  4. 4Mixed air temperature= 79.00 °F
  5. 5Total mixed airflow= 1,000 CFM
  6. 6Outdoor air fraction= 20.0 %
  7. 7Extra coil load from outdoor air= 4,320 BTU/h

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Mixing Air Streams figure worked out?

T mix = (Q1 T1 + Q2 T2) / (Q1 + Q2). When two air streams merge, the mixed temperature is the flow-weighted average of the two.

Does the same weighting work for humidity?

Yes. Humidity ratio and enthalpy mix by the identical flow-weighted average, which is why the mixed point lies on the straight line joining the two states on a psychrometric chart.

What values do I need to enter?

This calculator takes 4 inputs: Return air flow, Return air temperature, Outdoor air flow, Outdoor air temperature. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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