## 3 Results and Discussion
### Experimental AHU Installations
Then, it passes through the cooling/heating coil where the mixed air achieves the desired temperature. Finally, this air is supplied to multiple zones. Other major components of the system supply and return fans, which are in charge of modulating the supply and return air volumes; the return/outdoor/exhaust air dampers, which regulate the airflow that passes through them; and the heating/cooling coils valves, which are employed to adjust the water flow in the coils. Faults occurred in  
Figure 4: FLEXLAB facility where the target data set research was conducted. Figure 3: Energy Resource Station (ERS) facility used o obtain the data on the ASHRAE RP-1312: (**a**) Exterior view of the facility. (**b**) ERS floor plan with the zones served by the different AHUs. AHUs are related to sensor metering errors, fan speeds, damper positions, and leakage or malfunctioning of the heating and cooling coils. Typically, an AHU installation can be used for both heating and cooling purposes. In this study, as cooling is the fastest growing use of energy in buildings [54], the cooling season is used for testing the performance of the methodology. Moreover, to transfer the fault knowledge and verify the methodology, both domains must contain representative samples of the fault conditions that are present in both. Additionally, some of the samples in the target dataset can be used to verify if the fault pattern has been successfully transferred from the reference model, thus validating the fault detection and diagnosis. With these constraints, four conditions are evaluated, the normal operation data, denoted by F0, and three typical fault types are selected and denoted from F1 to F3:  
* F1: Outdoor air damper stuck at the fully closed position;
* F2: Heating coil valve leak;
* F3: Cooling coil valve stuck. To provide some insights into the behaviour and similarities of the considered source and target AHUs systems, Figure 6 depicts the difference in some of the variables used as features during two consecutive days which correspond to samples labelled as healthy in the databases. The outdoor temperature comparison is shown in Figure 6a. As can be seen, the difference between the temperatures measured is in the 7-15 \({}^{\circ}\)C range given the different locations of the facilities used to generate the datasets. The temperature profile corresponds to the average behaviour on a summer day where the ambient temperature is higher between 12:00 h and 16:00 h. From Figure 6b, one can see that the temperature achieved by the AHU to be supplied to the building spaces ranges between 22 \({}^{\circ}\)C when the cooling system is off, and 12.5 \({}^{\circ}\)C when the outdoor air is cooled. As it can be seen, the temperature is near the same for both datasets; therefore, the offset in outdoor temperature is compensated by changes in the systems' components or their operation. These changes can be slightly perceived in Figure 6a where despite both AHUs share the same rule-based schedule to turn on the cooling regime at 6:00 h and turn it off at 18:00 h, there is a delay in the reduction of the temperature in the supply air duct. The differences in the control and operation regime of the AHU systems are more evident in Figure 6c-e, where some of the actuator variables are presented. There are changes in the magnitude of the variables which may be caused by the difference in the components' dimensions or specifications. The feature scaling applied to the features will help to palliate these changes. However, there are also changes in the shape of the curves, especially for the outdoor air damper (Figure 6c) and the supply air fan control signal (Figure 6d). Both signals take continuous values between 0 and 1. For the outdoor air damper, 0 means a fully closed damper and 1 means that the sheets are in the fully open position. In the case of the supply air fan, its  
Figure 5: General scheme of an AHU installation including the supply air circuit with the corresponding heating/cooling coils and the mixing box, as well as the return air circuit.