HVAC equipment accounts for one of the largest uses of energy consumption in buildings. While numerous studies have been directed toward standalone systems such as heat pumps or ventilation systems, addressing these separately has revealed several limitations, particularly in energy recovery as well as coordination. This has created the demand for compact multifunctional systems (MCS) integrating HVAC and DHW functions. However, the lack of standardized Key Performance Indicators (KPIs) to evaluate such integrated solutions remains a serious barrier. Therefore, it appears to be essential to identify an approach to evaluate these systems through a global KPI framework. This paper provides a detailed description of a new MCS combining heating, cooling, ventilation, and domestic hot water production (DHW), including its components. It proposes a KPI framework based on a review of key performance indicators commonly used to evaluate conventional systems that compose the multifunctional unit, including heat pumps, ventilation systems, and DHW modules. These KPIs were selected according to their relevance for assessing: (i) the overall energy performance of the integrated system, (ii) thermal comfort when coupled with the building, and (iii) indoor air quality under real operating conditions. This initial qualitative selection focused on identifying measurable, and functionally significant indicators from scientific literature and professional practice. Therefore, this study will serve as a guideline to evaluate and optimize the performance of such a multifunctional compact system in literature, as well as assisting in improving the existing HVAC and DHW standards.