A hydro-split air-to-water heat pump system that ‘sorta’ works
An installer is asked to provide a hydro-split air-to-water heat pump to supply several zones of space heating and an indirect domestic water heater. No cooling is required for the project. The hydro-split heat pump consists of an outdoor unit and an indoor unit connected by piping carrying an antifreeze solution. The heat pump is turned on whenever one or more zone thermostats call for heat. It then modulates to maintain a fixed outlet temperature of 110° F. The heat pump’s internal controller boosts its outlet temperature to 130° F when the indirect water heater calls for heat. During this mode, a 3-way diverter valve directs flow from the heat pump through the coil heat exchanger with the indirect water heater.
Although the system operates, the building occupants notice that recovery from a night setback takes a long time. They’ve also noticed that domestic hot water delivery falls off quickly during high-demand periods. The system is piped as shown in Figure 1. Can you think of some changes that would improve the system’s performance?