If an excess number of points or much of the Best-Fit Curve strays from the green highlighted area, this is indicative of faulty mixing box dampers. Faulty dampers can allow excess outdoor air intake, driving up cooling or heating energy use, or insufficient intake resulting in poor indoor air quality.

The Health Index is calculated as the proportion of the Best-Fit Curve within the green highlighted area. The Best-Fit Curve estimates the ratio-to-position relationship and is updated daily using all available data points. A higher Health Index indicates healthy operations of mixing box dampers.

The Outdoor Air Ratio chart was generated using inverse greybox model-based algorithms. First, realize that the ratio of outdoor air in the supply air f_OA can be represented by the differences in enthalpy h in Equation A, where OA, RA, and SA represents outdoor air, return air and supply air, respectively. Assuming there is no heating or cooling input to the mixed air, and where relative humidity is not measured, the outdoor air ratio can be approximated using the difference in temperatures T in Equation B. Due to these assumptions, the outdoor air ratio is calculated using only data when the heating and cooling coils valves are completely closed and when the fan is on. The Best-Fit Curve is then fitted to the data points by estimating the parameters (a, b, c) of Equation C and plotting the relationship between outdoor air ratio f_OA and mixing box damper position s_DmpMix.