Mixed Air Temperature & Outside Air Fraction Calculator

How it works

T_mix = T_return + %OA × (T_outside − T_return); %OA = (T_mix − T_return) ÷ (T_outside − T_return); OA load = 1.08 × CFM_OA × |T_return − T_outside|

Air-handling units blend recirculated return air with fresh outside air, and the mixed temperature is simply the flow-weighted average of the two streams — which is why the same equation, read backwards, is the standard field method for measuring how much outside air a unit is really bringing in: log the three temperatures on a cold morning when the difference is large and the fraction falls out. The number matters for ventilation compliance, for the heating and cooling load the outside air imposes, and for protecting coils: hydronic coils can freeze when the mixed air drops below about 38 °F, so controls limit the damper or add preheat. In mild weather the same damper becomes an economizer, using cool outside air for free cooling.

Worked example

With 74 °F return air and 20 °F outdoors, 25% outside air produces a 60.5 °F mixed air temperature; on a 10,000 CFM unit that is 2,500 CFM of outside air needing 146,000 BTU/h of heating. Conversely, measuring 60 °F mixed air would indicate 25.9% OA. Below 38 °F mixed air — about 67% OA on this day — a freeze-stat would trip.

Frequently asked questions