The investigated heat pump system was considered to be able to be reversed by means of a refrigerant change-over, which reverses the flow passage into the two exchangers (Figure 1). In cooling mode, the air exchanger (outside unit) worked as condenser, expelling heat into the outdoor air, while the water exchanger (inside unit) worked as an evaporator, transferring cooling energy to the two-pipe water distribution system. In heating mode, the air exchanger worked as an evaporator, extracting heat from outdoor air, while the water exchanger worked as a condenser, transferring heat into the same distribution system in parallel with the backup boiler. It was assumed that the cold-emission terminal units were adapted for hot emission at low temperatures. The boiler was used when the reversible heat pump could not be operated (below –7 °C) or when its performance was too low due to low outdoor temperatures