Domestic Hot Water Preparation With Heat Pumps in MFH: Single-Pass vs. Multi-Pass Charging

Abstract

An investigation was conducted to examine various charging strategies for domestic hot water (DHW) storage tanks in multi-family homes (MFH) with DHW circulation, using heat pumps. A system consisting of a brine-to-water heat pump, a stainless-steel storage tank, and all hydraulic components was installed and measured on a test bench under dynamic operation. The charging strategies tested included different variants of single-pass charging where the storage tank is charged in a single-pass from top to bottom and multi-pass charging with a stepwise temperature increase introduced at the middle of the storage tank. The results show that multi-pass charging requires a significantly lower heat pump temperature time-averaged over return and supply as well as a lower time-average supply temperature compared to single-pass charging to achieve the same level of comfort. This lower temperature is accompanied by a clear efficiency advantage: while the system's performance factor (PF) for the various single-pass charging variants was around 2.0, multi-pass charging achieved a PF of 2.4. For all charging variants, DHW circulation with return temperatures of at least 55°C as required by national standards led to a loss of system efficiency, as it initiated an early recharge of the entire storage tank volume to over 60°C.