Mixed Air Temperature Calculator

Mixed Air Temperature

When two air streams at different temperatures combine, the supply air temperature depends on each stream's flow rate and temperature. HVAC engineers use this to predict supply air conditions before heating or cooling coils.

Formulas

Flow-weighted mixing (HVAC):

$$T_s = \frac{\text{cfm}_{oa} \times T_{oa} + \text{cfm}_{ra} \times T_{ra}}{\text{cfm}_{oa} + \text{cfm}_{ra}}$$

Composition-weighted mixing (two gases):

$$T = \frac{T_1 P_1 + T_2 P_2}{P_1 + P_2}$$

The flow mode is equivalent to $T_{mix} = (\dot{m}_1 T_1 + \dot{m}_2 T_2) / (\dot{m}_1 + \dot{m}_2)$. See also the Mixed Air Calculator for humidity ratio mixing.

Frequently Asked Questions

What is supply air temperature?

Supply air temperature is the dry bulb temperature of air delivered to the conditioned space after mixing outside and return air.

Can I use cfm for mass flow?

When air density is similar in both streams, volumetric flow (cfm) is proportional to mass flow and the weighted average gives the same result.

What if gas percentages do not add to 100%?

The calculator normalizes by the sum of entered percentages, so partial compositions still produce a valid weighted average.

Does this account for humidity?

This tool computes dry bulb temperature only. For enthalpy-based mixing with humidity, use the Mixed Air Calculator.

What is thermal equilibrium?

When two bodies reach the same temperature with no net heat transfer, they are in thermal equilibrium per the zeroth law of thermodynamics.

Report

Mixed Air Temperature Calculator