Which temperature variables are needed to calculate the percent outdoor air when air volumes are unknown?

• A. Return temperature
• B. Mixing temperature
• C. Outdoor temperature
• D. Return, Mixing, and Outdoor temperatures

Answer: (D)

The same idea applies to air mixing: the mixed-air temperature is a weighted average of the outdoor-air temperature and the return-air temperature, with the weights set by how much outdoor air is in the mix. If you don’t know the actual flow rates, you can still find the outdoor-air percentage by measuring the mixed-air temperature in addition to the outdoor and return temperatures and solving the linear mix equation: T_mix = x*T_outdoor + (1 - x)*T_return. Solving for x gives x = (T_mix - T_return) / (T_outdoor - T_return). So you need the return temperature, the outdoor temperature, and the mixed (resulting) temperature to determine the outdoor-air fraction. Having only two of these temperatures leaves the problem underdetermined, so you can’t uniquely determine the percent outdoor air.