noun
instantaneous water heater; on-demand water heater
A device that heats water instantly as it flows, without a storage tank. Common in Japanese homes for kitchen or bath.
このアパートには瞬間湯沸かし器が付いています。
This apartment comes with an instantaneous water heater.
noun
hot-tempered person; irritable person
Colloquial metaphorical use, likening a person who quickly gets angry to a water heater that instantly produces hot water.
彼は瞬間湯沸かし器みたいにすぐ怒る。
He gets angry in a flash, just like an instantaneous water heater.