noun
hot water tank
A tank for storing or heating water, used in domestic, commercial, or industrial settings.
新しい温水タンクを設置したので、お風呂のお湯がすぐに出ます。
We installed a new hot water tank, so hot water comes out right away in the bath.
給湯器 is a water heater that heats water on demand, while 温水タンク is a storage tank for hot water.
Compound of 温水 (warm water) and タンク (tank, from English).