Mixed Air Temperature Calculator

How to use this mixed air temperature calculator

What this calculator does

This calculator estimates a two-source mixed temperature in two closely related ways. The composition method treats two gases as a complete 100% mixture and calculates the weighted equilibrium temperature from each gas temperature and percentage. The HVAC method treats outside air and return air as two air streams and calculates their flow-weighted mixed temperature. These are sensible-temperature mixing calculations: they are useful for quick thermal checks, but they do not by themselves determine humidity, enthalpy, latent load, fan heat, heat-recovery performance, or full HVAC equipment capacity.

When to use it

Use the composition method when you know the relative fraction of two gas portions and want the temperature of their idealized mixture. Use the HVAC method when checking an air-handling mixing box, comparing outdoor-air ventilation fractions, or estimating the mixed temperature entering downstream heating or cooling equipment. It is also useful for a classroom or commissioning sanity check because the answer must stay between the two source temperatures when both weights are nonnegative.

How to calculate

1. The calculator opens with a complete demonstration: 20 °C at 40% mixed with 35 °C at 60%, plus an HVAC example using 30 °C outside air at 60 cfm and 23 °C return air at 1,340 cfm. The matching Excel workbook is immediately available.
2. Choose Temperature unit as °C, °F, or K. Switching the unit converts all temperature entries, not just their labels. Choose Flow-rate unit as cfm or m³/h; both HVAC flow values convert together.
3. Replace the demonstration values. For the composition method, enter Temp. T₁, Comp. P₁, Temp. T₂, and Comp. P₂. P₁ + P₂ must equal 100%. For HVAC, enter Temperature of outside air (Tₒₐ), Temperature of return air (Tᵣₐ), Flow rate of outside air (cfmₒₐ), and Flow rate of return air (cfmᵣₐ). At least one flow must be positive.
4. Read the live results and breakdown tables. Then use Download Excel to export the current validated inputs and calculations as a real .xlsx workbook. Reset clears the demonstration and all data fields; after Reset, Excel export stays unavailable until a complete valid state is entered again.

Input guide

Temperature unit is a required mode control. Celsius, Fahrenheit, and kelvin are supported; changing it converts Temp. T₁, Temp. T₂, outside-air temperature, and return-air temperature. The calculator rejects temperatures below absolute zero for the active scale. A common mistake is to type values from different scales without converting them first. NIST explains the relationships among kelvin, Celsius, and Fahrenheit temperature scales.

Flow-rate unit is also required and applies to both HVAC flow entries. The startup example uses cfm; m³/h is available when working in metric airflow units. The calculation depends on the ratio of the two flows, so converting both together leaves the mixed temperature unchanged.

Temp. T₁ and Temp. T₂ are required decimal temperatures, for example 20 and 35 °C. Comp. P₁ and Comp. P₂ are required percentages from 0 to 100, for example 40% and 60%. Higher weight on the hotter gas pulls the mixed temperature upward; higher weight on the cooler gas pulls it downward. Do not enter 0.40 when you mean 40%: the fields expect percentage points.

Temperature of outside air (Tₒₐ) and Temperature of return air (Tᵣₐ) are required decimal temperatures. The startup values are 30 °C and 23 °C. Flow rate of outside air (cfmₒₐ) and Flow rate of return air (cfmᵣₐ) are required nonnegative flows; the startup values are 60 and 1,340 cfm. Commas are accepted only as conventional three-digit grouping, so 1,340 is valid while decimal-comma input such as 1,5 is rejected rather than silently changed.

Output guide

Mixed air temperature (T) is the composition-weighted result. Composition total is P₁ + P₂ and must be 100% for a valid two-gas mixture. Gas 1 contribution and Gas 2 contribution show each temperature multiplied by its fractional composition; their sum equals T. The composition table repeats the same information by component using the columns Temperature, Share, and Weighted contribution.

Mixed air temperature (Tₛ) is the HVAC flow-weighted result. Total flow is outside flow plus return flow. Outside-air share and Return-air share show each stream as a percentage of total flow and therefore add to 100% when total flow is positive. The HVAC table shows Air stream, Temperature, Flow, Flow share, and Weighted contribution. A zero flow for one stream is valid and makes the result equal the other stream's temperature; two zero flows are invalid because the weighted average would divide by zero.

Worked example

For the startup composition example, 20 °C × 40% contributes 8.00 °C and 35 °C × 60% contributes 21.00 °C, giving 29.00 °C. For HVAC, total flow is 60 + 1,340 = 1,400 cfm. The flow-weighted calculation is (60 × 30 + 1,340 × 23) ÷ 1,400 = 23.30 °C. The outside-air share is 4.29% and the return-air share is 95.71%, which is why the HVAC result lies much closer to 23 °C than to 30 °C.

Learn more

For HVAC context, the U.S. Department of Energy's Building Science Education program explains how whole-house ventilation systems supply and exhaust outdoor air. For deeper engineering background, the ASHRAE Handbook covers fundamental HVAC&R principles and system design topics.