This system set up is often used to back up an existing heating system. The built-in controller in the outdoor unit can work with a thermostat.
In the case of a wood-fired boiler, the NIBE F2026 is connected to the accumulator tank, which contains a water heater. When the wood-fired boiler is not in use, the heat pump starts automatically, providing an economical heat source. It is controlled by a thermostat in the accumulator tank.
In the case of an oil or gas boiler, the heat pump is connected to the heating circuit just before the boiler, and contributes to heating the house (but not the hot water). It is controlled by a room thermostat.
Both of these installations make use of existing equipment and thus keep installation costs down. However, the energy savings that can be achieved are not as high as with the three other systems described.