Outdoor Air Percentage Formula
Outside Air Percent Formula:
Formula to use if you only know Mixed Air Temp, Outdoor Air Temp, and Return Air Temp.
Outdoor Air %={((RAT- MAT))/((RAT- OAT) )} x 100
For Example:
Mixed Air Temp = 55°F
Outside Air temp = 10°F
Return Air Temp = 70°F
Step 1: Outdoor Air %={((70°F- 55°F))/((70°F- 10°F) )} x 100
Step 2: Outdoor Air %={(15°F)/(60°F)} x 100
Step 3: Outdoor Air %={.25} x 100
Step 4: Outdoor Air %=25%
If you know the Enthalpies, you can use a similar formula but plug in Enthalpies.
Outdoor Air %={((MA Enthalpy- RA Enthalpy))/((OA Enthalpy- RA Enthalpy) )} x 100