water heater; domestic boiler
Refers to a device that heats water for household use, such as bathing or kitchen taps. Often used in contexts like apartments, homes, and commercial facilities.
We installed a new water heater, so the bath heats up quickly now.
The water heater is acting up and there's no hot water.
給湯器 refers broadly to hot-water supply systems, often built into buildings, while 湯沸かし器 usually means a standalone household appliance for heating water.
electric water heater
Compound of 給湯 (kyūtō, 'hot water supply') + 器 (ki, 'device'). The term is straightforward and modern, with no obscure historical derivation.