**1. Introduction**

The building sector contributes to approximately 40% of overall energy demand in industrialized countries, with Heating, Ventilation, and Air Conditioning (HVAC) systems accounting for a large part of this energy consumption [1,2]. Several scientific publications have discussed the application of efficient/innovative technologies [3] and/or optimal control strategies [4] with the aim of reducing the energy demand and related greenhouse gas emissions of HVAC units. However, HVAC systems could be exposed to various abnormal faults during operation as a consequence of a failure of components, lack of correct maintenance, or wrong installation. Each component of HVAC systems can be eventually altered by a fault, where a fault is an anomalous state of the system, i.e., an undesired divergence of at least one of the properties of the HVAC unit with respect to healthy/standard conditions [5]. The faults can involve sensors, devices, equipment, and controllers [5]. A study conducted on more than 55,000 Air Handling Units (AHUs) showed that up to 90% runs with one or multiple faults [6]. Lin et al. [7] underlined that an effective detection of faults in HVAC units could save from 15% to 30% of overall energy

**Citation:** Rosato, A.; Guarino, F.; Sibilio, S.; Entchev, E.; Masullo, M.; Maffei, L. Healthy and Faulty Experimental Performance of a Typical HVAC System under Italian Climatic Conditions: Artificial Neural Network-Based Model and Fault Impact Assessment. *Energies* **2021**, *14*, 5362. https://doi.org/10.3390/ en14175362

Academic Editor: Chi-Ming Lai

Received: 21 July 2021 Accepted: 25 August 2021 Published: 28 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

required by buildings. In addition, Au-Yong et al. [8] highlighted a relevant impact of poor maintenance of HVAC systems on indoor thermo-hygrometric comfort, identifying several maintenance factors significantly correlated with occupants' satisfaction. This means that adopting a proper maintenance strategy is fundamental. Companies generally adopt a reactive maintenance or a preventive maintenance. Repairs are carried out only in the case of failures when a reactive maintenance is adopted; this approach could be expensive and may cause safety issues. In the case of a preventive maintenance, systems are examined and maintained at given periods (whatever their state is); however, this approach requires identifying a proper maintenance schedule in order to not waste component life that is still profitable as well as avoid safety problems.
