The European Union’s increasing focus on the energy renovation of buildings has brought greater attention to the share of energy consumption associated with Domestic Hot Water (DHW), especially in highly efficient retrofitted buildings. This paper provides an overview of the main DHW production systems, with a focus on heat pumps (HP), analyzing their efficiency, emissions, and costs across different climatic and building contexts. Centralized and decentralized technologies, both with and without photovoltaic (PV) support, are compared, and hygienic implications related to Legionella proliferation–often counteracted through high supply temperatures–are discussed. Several studies show that the use of HPs, particularly when combined with PV systems and strategies to reduce operating temperatures (e.g., ultrafiltration), can significantly lower energy consumption, emissions, and the number of activation cycles. Decentralized systems are especially attractive solutions in multi-family buildings, offering significant improvements in overall efficiency. It can therefore be concluded that the key to optimizing DHW systems lies in lowering operating temperatures–while maintaining hygienic standards–and adopting intelligent, distributed technologies capable of maximizing integration with renewable energy sources.