A building is to be heated by radiators using an air-water heat pump. While radiators are not the ideal choice, it's beyond my decision making. Naturally I want the supply temp. to be as high as possible, hoping for 55°C in order to keep the physical size of the radiators to a minimum. As low return temperature supposedly is what every heat pump wants, the return water temperature can be reduced even more by pre-heating tap water through a heat-exchanger before returning to the heat pump. What I wonder is, will this additional reduction in temperature spell doom for my 55°C supply? Is there a maximum temperature difference between the supply and return?