noun
bath heater
A device that heats water for a bathtub, often a gas or electric unit attached to the tub.
風呂釜が故障したので、新しいのを買わなければならない。
The bath heater broke, so I have to buy a new one.
給湯器 is a general water heater for the whole house, while 風呂釜 specifically heats bath water, often as part of the bathtub setup.
Compound of 風呂 (bath) and 釜 (pot/kettle), referring to a vessel for heating bath water.