## 3 Results and Discussion
### Experimental AHU Installations  
In order to validate the proposed transfer learning based on FDD methodology, this study is based on AHU's equipment due to its extended presence in HVAC systems. Indeed, AHUs are the key components for maintaining a comfortable indoor environment and healthy air quality. To facilitate the reproducibility of the methodology and comparison of its performance, this study uses two publicly available datasets as its source and target domains. On one hand, the source domain is a real-world AHU faulty operational dataset coming from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) project RP-1312. This dataset was collected by [PERSON] et al. [51] through a series of experiments carried out in the summer of 2007 and the spring and winter of 2008. The research was conducted at Iowa Energy Center's Resource Station (ERS) located in Ankeny, Iowa. The ERS, depicted in Figure 3a, was established to compare energy-efficiency measures and demonstrate HVAC concepts. Several experiments have been conducted in the ERS and details of the test facility were provided by [52]. To perform side-by-side testing, the facility is equipped with three AHUs and the floor plan of the ERS (see Figure 3b) is comprised of three distinct and separate areas. AHU-1 serves the common areas and the remaining AHUs serve two sets of test systems. AHU-A is identical to AHU-B, and each of them serves 4 test rooms with identical construction and exposures yielding to identical thermal loads. A rooms are serviced by AHU-A and the B rooms by AHU-B. In the dataset, one of the AHUs ran under normal conditions while the other ran under several fault circumstances. \begin{table}
\begin{tabular}{c p{142.3 pt} p{142.3 pt}} \hline
**Variable** & **Meaning** & **Description** \\ \hline \(F_{sa}\) & Supply air fan speed control signal & Signal to modify the speed of the supply air fan. It takes values in the interval [0, 1] \\ \(D_{oa}\) & Outdoor air damper control signal & Signal to open or close the metallic sheets of the dampers. It takes values in the interval [0, 1] \\ \(D_{ea}\) & Exhaust air damper control signal & Signal to open or close the metallic sheets of the dampers. It takes values in the interval [0, 1] \\ \(D_{ra}\) & Return air damper control signal & Signal to open or close the metallic sheets of the dampers. It takes values in the interval [0, 1] \\ \(V_{cc}\) & Cooling coil valve control signal & Signal to modify the opening of the water valve of the cooling coil, increasing or decreasing the amount of water that enters the heat exchanger. It takes values in the interval [0, 1] \\ \(V_{hc}\) & Heating coil valve control signal & Signal to modify the opening of the water valve of the heating coil, increasing or decreasing the amount of water that enters the heat exchanger. It takes values in the interval [0, 1] \\ \(O_{c}\) & Occupancy mode indicator & Occupancy signal created from the schedule fixed by the building management team. \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._On the other hand, the target dataset considered in this study comprises experimental data obtained from a single-zone VAV-AHU located at the Lawrence Berkeley National Laboratory (FLEXLAB) in Berkeley, California [53]. FLEXLAB facilities, shown in Figure 4, is one of the most advanced integrated building and grid technologies test beds and since 2014, over a dozen of case studies have been developed to explore energy efficiency measures. The target dataset was collected during the summer of 2017. The collection time interval for both datasets was \(1\) min. In general, each fault was tested for one day, meaning that we have at least 1440 samples for each of the faults considered. To ease the understanding of the operation and possible faults present in an AHU, a typical single-duct variable air volume AHU is shown in Figure 5. The outdoor air is the main input to the AHU system that, after being provided to the AHU, is mixed with the return air from the multiple zones.