A water heater (1) includes a housing (8), combustion devices (31, 32), heat exchangers (41A to 42B), a water supply pipe (P1), a hot water supply pipe (P2), a water supply connection member (10), a hot water supply connection member (20), a water quantity control part (50), and a fine bubble generator (9) in fluid connection with either the water supply pipe (P1) or the hot water supply pipe (P2). The fine bubble generator (9) has an inflow port (91H), into which hot water flows from an upstream side (P2U), and an outflow port (92H), out of which hot water containing the fine bubbles is discharged to a downstream side (P2D). The fine bubble generator (9) is located below burner ports (31F, 32F) of the combustion devices.

A method for supplying CO.sub.2 gas to a site comprising a facility (20) that requires CO.sub.2 or a mixture comprising CO.sub.2, such as an abattoir or a greenhouse for cultivating plants, comprises the following steps: a boiler (4) capable of supplying hot water to the site is arranged within the site, the boiler carrying out the process of oxycombustion between a fuel (14) and pure oxygen (1), the oxygen that supplies the boiler being obtained from a liquid oxygen source present on the site; and some or all of the CO.sub.2 contained in the flue gas produced by the boiler is recovered by transferring heat between the flue gas and the liquid oxygen in an exchanger (2).