Experimental investigation of the dynamic performance of an integrated variable refrigerant flow and domestic hot water system

Integrating domestic hot water (DHW) production with variable refrigerant flow (VRF) systems can improve energy utilization by recovering condenser side heat during cooling operation. However, the transient interaction after DHW branch activation has not been sufficiently investigated in the literature. This study experimentally investigates an integrated VRF-DHW system under four fixed compressor speed conditions. After DHW activation, a transient pressure response persisted for approximately 1–1.5 h, and 42–44% of the combined refrigerant flow passed through the PHX branch. The final tank temperature after 5 h increased from approximately 52 to 72°C as compressor speed increased. Simultaneous DHW operation reduced the cooling capacity by 28.11–32.44%. The reduction was mainly associated with the higher PHX return enthalpy, which increased the mixed evaporator inlet enthalpy and reduced the specific cooling effect. The integrated COP was higher than the cooling-only baseline at a target DHW temperature of 45°C for all cases, but the benefit decreased as the target temperature increased. These results clarify the trade-off between DHW heat recovery and cooling performance and show that higher compressor speed extends the attainable DHW temperature range at the expense of a greater cooling-side penalty.