The principle of heat pumps has been known for decades: They recover ambient heat and feed it into a heating circuit. One major specific coefficient indicating their performance and efficiency is fuel use efficiency. It describes the ratio of thermal energy produced to total energy input. Here gas heat pump technology is convincing when compared with conventional heating systems: A gas heat pump needs only roughly 70 per cent primary energy for 100 per cent heat output. By comparison: A conventional condensing gas appliance needs 104 per cent primary energy for the same heat output. Therefore, with gas heat pump technology, the amount of heat generated is much greater than the amount of fossil fuel required by the system.