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 water heating system having a system inlet pipe (1.002), a hot water delivery pipe (1.003) and a hot water return pipe (1.006) connected to a building hot water distribution network (1.024); the water heating system including one or more water heaters (1.001), the or each water heater having a heater inlet (1.022) and a heater outlet (1.023), a hot water return pipe (1.006) connected between the system inlet and delivery outlet via the building hot water distribution network to form a close loop hot water supply-return circuit; a pump (1.005) connected to circulate water through the hot water supply-return circuit whereby the pump can circulate water through one or more of the water heaters; a valve means (1.007) [a first non-return valve] adapted to prevent inlet water (water delivered to the system inlet) from flowing into the hot water return pipe or the building hot water distribution network.

A water heater system including a boiler having a heat exchanger and an indirect water heater having a heat exchanger, and a controller configured to activate a pump such that the water flows between the boiler heat exchanger and the indirect water heater heat exchanger, and to control a heat source to provide heat to the boiler at a firing rate. The water heater system measuring temperatures of the water at the boiler water inlet and at the boiler water outlet, calculating an amount of heat transfer from the boiler heat exchanger to the indirect water heater exchanger based on the measured temperatures, and adjusting the firing rate based on the calculated amount of heat transfer to determine a heat transfer capacity of the indirect water heater exchanger.