Abstract: The apparatus for heating a fluid comprises a heating enclosure supplied in its lower part with fluid to be heated, a combustion chamber comprising a burner at one of its ends and extending on the side opposite the burner inside the enclosure, a fume circulation tube disposed inside the enclosure to constitute a contra-flow heat exchanger, which tube is hermetically connected to the outlet of the combustion chamber and opens out in a fume evacuation device located in the lower part of the enclosure in order to evacuate the fumes to outside the heating apparatus, and means for taking heated fluid in the upper part of the enclosure. The heat exchanger tube is constituted by an originally flexible, tight, ringed metal tube, disposed spirally substantially in the lower third of the enclosure, and so that the different turns of the ringed tube are tangentially in contact with one another in order to form a compact exchanger.