The problem lies in the radiators. Heat pumps can not "lift" the temperature more than 30-40 degrees Celsius without getting inefficient. So at -5°C outside, yo

The problem lies in the radiators. Heat pumps can not "lift" the temperature more than 30-40 degrees Celsius without getting inefficient. So at -5°C outside, you can only warm water up to 35°C. At this temperature, your old radiator doesn't transfer enough energy from the 35°C warm water to the 20°C room. You would need to either heavily insulate your house or replace the radiators.