Heat transfer unit for a heat exchanger

The invention relates to a heat transfer unit for a heat exchanger comprising a bundle of heat exchanger tubes (15) in which a heating water to be heated flows for a heating system. The heat exchange tubes (15) are made of plastic containing at least one filler (5) of high thermal conductivity wherein the thermal conductivity of the filler is higher than that of the plastic, so that the pipe wall of the heat exchanger tubes (15) having a large heat transfer coefficient.