A boiler for producing hot water for heating and for domestic use, of the type comprising a first hydraulic circuit (1) for heating the water destined for a heating system, said first circuit comprising means (2) for the forced circulation of the water through said first circuit, first means (3, 16) for generating heat and transferring it to the water, valve means (4) for conveying the heated water to a delivery pipe (7) or to second heat exchanger means (25) arranged to transfer at least part of the heat of the heated water to the water for domestic use circulating through a second hydraulic circuit (5), expansion means (17) for automatically adjusting the pressure of the water circulating through the primary circuit as its temperature varies, and means for accumulating and maintaining a predetermined quantity of heated water in the primary circuit within a predetermined temperature range, said accumulating and maintaining means being connected to the primary circuit in such a manner as to make available to the second heat exchanger means (25) said predetermined quantity of heated water at least during an initial stage in the heating of the water for domestic use circulating through the second circuit; wherein the expansion means (17) comprise means (20, 21, 23) for integrating into the expansion means said means for accumulating and maintaining the heated water.