This article highlights the significant impact of small faults in heating, ventilation, and air conditioning (HVAC) systems on energy efficiency and operational costs, using a case study from Northern Sweden. Even minor malfunctions, such as a faulty heat exchanger sensor, can lead to drastic inefficiencies and increased energy waste, with financial consequences for homeowners. This article emphasizes the importance of regular maintenance and monitoring and explores the potential of emerging technologies such as industrial AI, cyber–physical systems (CPSs), and digital twins (DTs) to revolutionize HVAC optimization. By adopting proactive strategies, both energy efficiency and sustainability goals can be achieved.