The heater has heat reflecting pipe ducting (2) and a burner (3) to produce a fluid heating medium. The medium i.e. exhaust gases, are fed along the pipe so that the temperature of the pipes reduces with an increased spacing from the burner. The heat flow surface of the pipe ducting is increased at the area remote from the burner. The heat reflecting face of the piping at an additional spacing from the burner is enlarged pref. stepwise. The temperature reflection can be compensated so that the reflected heat over the whole piping is constant.
