Hybrid control strategies for energy efficiency, thermal comfort and indoor air quality in residential compact HVAC-DHW systems

Improving energy efficiency in buildings while maintaining indoor thermal comfort and air quality is a growing challenge that requires advanced and sustainable HVAC solutions. Multifunctional compact systems are promising technologies, as they integrate ventilation, space heating, cooling, and domestic hot water production within a single compact unit. However, their overall performance depends on the control strategies used to manage the interactions between thermal comfort, indoor air quality, and energy consumption. This study develops a numerical framework that couples a detailed compact system model with a dynamic building energy model to assess and compare alternative control strategies for heating and domestic hot water (DHW) supply. Simulations are performed for a representative winter day using predefined occupancy schedules, domestic hot water draw profiles, comfort temperature ranges, and air renewal requirements. Several control approaches are investigated, including hierarchical multi-criteria control — prioritizing supply airflow rate, supply air temperature, or hot water production — as well as multi-objective optimization. Results indicate that PID-based controllers, used here as a baseline, are well suited for regulating indoor air temperature and ensuring thermal comfort. In addition, strategies relying on supply air temperature control with a fixed ventilation rate also reduce short-term fluctuations in system power demand. However, these approaches do not ensure that indoor relative humidity remains within recommended limits or that energy use is minimized. To improve indoor comfort, based on the monotonic dependence of the system thermal and electrical power on the supply air flow rate and temperature, an optimization problem is formulated as a boundary-search problem. Additionally, a hybrid strategy optimizing the operating mode of the compact system (domestic hot water production vs. space heating) achieves energy savings exceeding 20% in a typical day with high heating needs. Overall, the findings highlight the importance of coordinated control strategies and the use of multicriteria optimization schemes for maximizing the performance of integrated compact systems and support their potential deployment in future high-efficiency buildings.