The compressor in a heat pump works harder when there is a larger temperature difference between the outside source temperature and the water temperature needed in your radiators or underfloor heating. The less the compressor needs to work, the less electricity the heat pump uses.
While we can’t control the outdoor source temperature, it’s possible to design heating systems that use low temperature water indoors, meaning the heat pump can use less electricity and still heat your home comfortably.
By using radiators with a larger surface area, or underfloor heating, more heat can be delivered into the room without increasing the water temperature. Running the heating system for longer is another way of delivering more heat into the room with lower temperature water.
If you have radiators with a smaller surface area, then the heat pump will have to run at a higher temperature. This means the compressor is working harder to deliver the same amount of heat as it would with larger radiators, or if it had a longer time to run. When the compressor works harder, it uses more electricity, which makes the system more expensive to run.
The aim of a well-designed system is to reduce the heating water temperature as much as possible. The closer the required temperature is to the source temperature (ie the outside air or ground temperature), the more efficient the heat pump will be, and therefore the lower the running costs.