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.
This apartment comes with an instantaneous water heater.
hot-tempered person; irritable person
Colloquial metaphorical use, likening a person who quickly gets angry to a water heater that instantly produces hot water.
He gets angry in a flash, just like an instantaneous water heater.