Outdoor fraction from mixed-air temperatures calculator

Outdoor fraction of mixed airflow is calculated as (T_mix−T_return)/(T_outdoor−T_return). Use the stated system boundary and matching input units.

Units

f_{\text{OA}} = \frac{T_{\text{mix}} - T_{\text{return}}}{T_{\text{outdoor}} - T_{\text{return}}}

Calculator

Enter the values you know and leave one box empty. Calculate finds the empty one, the result or any input.

Given

f_{\text{OA}} = \frac{61 - 75}{40 - 75} = 0.4

Symbol	Meaning	Unit
$f_{\text{OA}}$	outdoor fraction of mixed airflow	—
$T_{\text{mix}}$	well-mixed supply-box dry bulb	°F
$T_{\text{return}}$	return-air dry bulb	°F
$T_{\text{outdoor}}$	outdoor-air dry bulb	°F

Symbol	Meaning	Unit
$f_{\text{OA}}$	outdoor fraction of mixed airflow	—
$T_{\text{mix}}$	well-mixed supply-box dry bulb	°C
$T_{\text{return}}$	return-air dry bulb	°C
$T_{\text{outdoor}}$	outdoor-air dry bulb	°C

Results are estimates for study and screening. Check the inputs, units and assumptions before you use them for design, billing or contract decisions.