What Is the Mixed Air Temperature Calculator?
When two airstreams combine — such as return air and outdoor air meeting in the mixing box of an HVAC air handler — the resulting temperature is a flow-weighted average of the two. This calculator finds that mixed air temperature given each stream's mass (or volumetric) flow rate and its temperature.
How to Use It
Enter the flow rate and temperature of each airstream. The flow units must be consistent (e.g. both in kg/s or both in CFM) and both temperatures must use the same scale (°C, °F, or K). The calculator returns the mixed temperature in that same temperature unit.
The Formula Explained
The mixed air temperature is computed as:
$$T_{mix} = \frac{\text{m}_1 \cdot \text{T}_1 + \text{m}_2 \cdot \text{T}_2}{\text{m}_1 + \text{m}_2}$$
Each temperature is weighted by its share of the total flow. The stream with the larger flow rate pulls the result closer to its temperature. This assumes equal specific heat and pressure for both streams, which is an accurate approximation for ordinary HVAC air mixing.
Worked Example
Suppose 100 CFM of return air at 20°C mixes with 300 CFM of outdoor air at 5°C. Then $$T_{mix} = \frac{100 \times 20 + 300 \times 5}{100 + 300} = \frac{2000 + 1500}{400} = \frac{3500}{400} = 8.75\,°C.$$ Because most of the flow is the cooler outdoor air, the mix sits much closer to 5°C than to 20°C.
FAQ
Can I use volumetric flow instead of mass flow? Yes, as long as the air densities are similar (typical HVAC conditions), volumetric flow gives a good approximation.
Does the temperature unit matter? Use the same unit for both inputs; the result is in that same unit. Mixing °C inputs returns °C.
What if total flow is zero? The result is undefined — at least one airstream must have a positive flow rate.
