All water heaters have a safety relief valve (often called a T&P Valve). Relief valves prevent a hot water tank from exploding. They are simple devices that save countless lives even though they are mostly ignored. They are not ignored intentionally, they are ignored primarily because people do not understand their purpose.
Mythbusters, a US Television Series, demonstrated the danger of a failed safety relief valve by operating a water heater without a relief valve. They allowed the internal tank pressure to double the rated pressure and shortly thereafter, the tank exploded. The failure created a lethal explosion.
As water is heated its volume expands and this heating is performed in a closed system (a tank) the tanks internal pressure increases. The higher the temperature, the higher the pressure. Normally a thermostat prevents the water from being overheated, but if the tank thermostat fails [and this happens], heating will continue and increase the water pressure far beyond the safe working pressure of the tank.
While tanks need to be physically closed to prevent water from leaking out, tanks need a clever way to release this pressure in the event of a system failure. For this, tanks utilize pressure relief valves.
Pressure relief valves are mechanically simple devices. They use a plunger-like mechanism to create a watertight seal between the interior and exterior of a water heater tank. The plunger is constructed from a metal spring at the end of which is a soft rubber stopper [imagine a toilet plunger except for the wooden handle is a spring]. During regular operation, the spring pushes the rubber stopper against a physical hole in the tank, keeping water and pressure contained within. When internal tank pressure is too high the spring force is overcome and the plunger is moved out of the way. As the spring is pushed out of the way, a hole is formed and the internal pressure is released. Once pressure is released, the danger disappears.
The cycles of sealing and releasing can theoretically continue indefinitely. High system pressure pushes the spring out of the way, low system pressure causes the spring to push back into its place and create a seal. In practice, the valve had a limited life that is proportional to the quality of the domestic water, the temperature of the tank and the system pressure.
Within an adequately manufactured and installed valve, water quality is the largest determining factor of valve service life.
The relief valve spring and rubber components are exposed to minerals within the water system. These minerals can accumulate directly on the spring or gasket and prevent the valve from functioning as intended. A valve with mineral buildup may not open at all or may open but not reseal. A valve that does not open produces an explosion hazard while a valve that does not close can creates water damage to your property.
We cut open a new valve [blue gasket] and a used valve [red gasket] to demonstrate the mineral build-up. The valve on the right caused extensive water damage because the debris which accumulated on the gasket prevented the valve from closing after a minor plumbing pressure spike.
While this valve only caused water damage, things could have been much worse if the debris kept the valve from physically opening during the plumbing pressure spike.
Manufacturers of relief valves recommend that the relief valve be operated regularly to prevent mineral build-up from affecting its operation.
Testing is easy, AMRE Supply made a very clear instructional video that everyone should watch.
In the video, the relief valve drained directly onto the floor whereas good practice would be to have the valve directed into a floor drain.
Otherwise, this video serves as a great tool to keep every homeowner safe.
Know your home, stay safe.
Original reporting: https://medium.com/@okrutny/every-water-heater-can-explode-keep-your-home-safe-e679fe8b0c35
