Water Heater Relief Valve Overflow

water heater
An appliance for supplying hot water for purposes other than space heating or pool heating.
a heater and storage tank to supply heated water
Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both hot water and water heated to steam have many uses.

relief valve
The relief valve (RV) is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire.
Used at the discharge of a positive displacement pump. An adjustable, spring-loaded valve opens when a preset pressure is reached. Used to prevent excessive pressure buildup that could damage the pump or motor.
safety valve: a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level

overflow
The excess or surplus not able to be accommodated by an available space
bubble over: overflow with a certain feeling; "The children bubbled over with joy"; "My boss was bubbling over with anger"
The flowing over of a liquid
(in a bathtub or sink) An outlet for excess water
flow or run over (a limit or brim)
flood: a large flow