A heat exchanger (1) for heating water in a domestic boiler or a water heater has a casing (2); and a tubular member (3), which is configured for conveying water, is wound in a helix about a given axis (A) for forming a plurality of adjacent coils (4) housed in the casing (2), wherein the casing (2) comprises an end wall (8), which has a inner face (17) and an outer face (18); and is configured for supporting a thermally insulating device (19) along the inner face (17), and for defining an auxiliary heat exchanger (20) along the outer face (18).
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