Analysis of Circuit Connections and Control Strategies for Residential Air-to-Water Heat Pump Systems

This study investigates the impact of secondary circuit hydraulic connection schemes and control strategies on the performance of residential air-to-water heat pump systems providing space heating and domestic hot water (DHW). Five configurations (parallel-connected 200L buffer tank; 200L buffer tank with return bypass; direct connection; 50L buffer on supply; 50L buffer on return) were analyzed with inverter, on-off, and degree-minute heat pump controls. Dynamic simulations using IDA ICE software for a detached house model and moderate climate conditions (Estonia) evaluated Seasonal Performance Factor (SPF), indoor temperature stability, and auxiliary heating. Inverter-controlled systems with larger buffer tanks yielded the highest heating SPF; a 200L buffer with return bypass achieved a SPF of 3.90 with no auxiliary heating. On-off controls performed similarly (SPF 3.86 with bypass). Direct connections resulted in lower SPF values (e.g., 3.55 for inverter) and significantly higher auxiliary heating across all control logics, largely influenced by DHW priority operation. Degree-minute control led to the lowest SPFs and substantial auxiliary heating, attributed to its operational logic, especially after non-heating periods. The defrost cycle reduced overall system SPF by approximately 7.0%. Optimal performance, characterized by high SPF and minimal auxiliary heating, was achieved with an inverter-controlled heat pump using a 200L buffer tank with a heating circuit return bypass, particularly when the system is configured to maintain supply temperatures with auxiliary support.