Mixed Air Calculator

Formula and Method for Mixed Air Temperature

Inside an air handling unit (AHU), return air from the building and outside (ventilation) air pulled in for fresh-air requirements meet in the mixing plenum, upstream of the filters, coils, and supply fan. The temperature of that blended stream — the mixed air temperature (MAT) — is a flow-weighted average of the two inlet temperatures: MAT = (CFM_RA × T_RA + CFM_OA × T_OA) / (CFM_RA + CFM_OA), where CFM_RA and T_RA are the return air flow rate and temperature, and CFM_OA and T_OA are the outside air flow rate and temperature.

Deriving the mixing equation

Mixing two air streams adiabatically (no heat added or removed in the plenum itself) conserves total thermal energy. For an ideal gas, the heat content of a stream is proportional to its mass flow rate, specific heat, and temperature: rate = ṁ·c_p·T. Setting the combined heat rate equal to the sum of the two inlet rates and solving for the resulting temperature gives MAT = (ṁ_RA·T_RA + ṁ_OA·T_OA) / (ṁ_RA + ṁ_OA), assuming c_p is essentially the same for both streams (a good assumption for dry-to-moderately-humid air). Because mass flow ṁ = ρ·Q (density times volumetric flow), and air density varies only a few percent across the temperatures typical of HVAC operation, engineers commonly substitute volumetric flow in CFM directly for mass flow, giving the practical form used by this calculator: MAT = (CFM_RA·T_RA + CFM_OA·T_OA) / (CFM_RA + CFM_OA). The outside air percentage, %OA = CFM_OA / (CFM_RA + CFM_OA) × 100, is simply the weight applied to the outside air term.

Practical notes and when the approximation breaks down

This CFM-weighted average is standard practice for sizing, economizer logic, and freeze-protection setpoints, but it assumes the two streams mix thoroughly and uniformly before reaching the sensor or coil — stratification (cold outside air "layering" beneath warmer return air instead of blending) is a common real-world failure mode, especially at low outside-air percentages or with poorly designed mixing boxes, and can cause a coil-mounted sensor to read a temperature well below the calculated MAT. For precise engineering work at extreme temperature differentials, or when air is very humid, use true mass flow rates and the actual specific heat of each stream rather than the CFM approximation. Also confirm your CFM values are at the AHU's actual operating condition, since fans and dampers are typically balanced at standard air density.

Frequently Asked Questions