There is provided an indoor mounting water heater in which combustion exhaust gas is discharged to the outdoors via an exhaust duct, wherein a decrease in draft force caused when the number of burner groups to be burned is reduced by a capacity changeover mechanism to decrease the combustion capacity can be prevented, and the heat efficiency can be improved. An air supply chamber 81 is divided into a plurality of parts 82 and 83 corresponding to a plurality of burner groups 51 and 52. The air sent from an air supply fan 84 is supplied to only a part of the air supply chamber corresponding to a burning burner group.