When Nighttime Setback Becomes Counterproductive: The Influence of Building Heat Loss Coefficient on Air-to-Air Heat Pump Performance

Nighttime setback is widely applied in residential heating, yet its net benefit for inverter-driven air-to-air heat pumps in high-thermal-mass buildings remains insufficiently characterised in Mediterranean climates. This study evaluates continuous operation (R1), timer-based setback (R2), and reactive threshold-based control (R3) for a residential building in Zadar, Croatia, using a validated physics-based model calibrated with high-resolution IoT measurements over a 180-day heating season. Simulations cover building heat loss coefficients of 60–120 W/K and both 3.5 kW and 5 kW heat pumps. Timer-based setback increases electricity consumption by 1.8–4.5% (3.5 kW) and 5.4–6.3% (5 kW) relative to continuous operation, driven by a morning recovery penalty. Reactive control eliminates thermal comfort deficits but offers no energetic benefit. Both effects generalise across heat pump capacity, with penalty magnitude scaling with unit size. A formal energy balance shows that, for the timer-based and reactive setback rules investigated here, the recovery term is set by the thermal state at the start of reheating and is not removed by better knowledge of the same weather. The conclusions apply to high-thermal-mass masonry dwellings of the type investigated; they are not intended for lightweight or low-thermal-mass envelopes, where nocturnal decay and the setback energy balance can differ. These findings provide a quantitative basis for assessing conventional setback rules in this building class.