Abstract: A device for treating water that circulates in a closed conduit system, said device comprising a combination of a degasifier, on the one hand, and a corrosion inhibitor, on the other hand. The degasifier includes a tubular member (10) which is horizontal in its use position and which is intended to be connected in series with a conduit (13, 14) forming part of the conduit system. The tubular member is surrounded between its upstream and downstream ends (11, 12) by a housing (17) which includes an openable upper gas outlet (16). The tubular member communicates with the surrounding housing space through the medium of at least one aperture (15) provided therein. The corrosion inhibitor includes a sacrificial anode (20) which is positioned in the housing space outside the tubular member (10). The housing (17) also includes an openable bottom drain or outlet (28).