Avoiding Explosions -- Meet The TPR Valve
Ok, the TPR valve isn't the most exciting topic... at least until it fails.
TPR (temperature pressure relief) valves (also called a T & P valve) are the safety devices installed on water heaters, and they automatically release water in the event that pressure or temperature in the water tank exceeds safe levels.
TPR valves are generally set to open up if the pressure exceeds 150 psi or 210 degrees F. If the temperature exceeds 210 degrees F, the high pressure in the system will prevent the water from turning to steam. But if the TPR valve malfunctions, pressure will exceed 150 psi and temperature will exceed 212 degrees F. If there is even a slight rupture in the tank, water will instantly expand into steam. One cubic inch of water becomes about 1 cubic foot of steam faster than we can blink an eye. If there's a forty gallon water tank involved, a room can be engulfed in steam immediately.
Oh, and this process can propel a heating tank like a rocket through multiple floors, causing personal injury and extensive property damage. (If you want to see this happen, you may like this classic Mythbusters experiment.)
- it should not be trapped, since standing water may become contaminated and backflow into the potable water
- it should not be directly connected to the drainage system to prevent backflow of potentially contaminating the potable water
- it should not terminate more than 6 inches above the floor (or waste receptor)
- the discharge point should be readily observable, since discharge indicates that something is wrong
- it should not be capped, and should not have a threaded connection at the bottom of the pipe so it can't be capped by mistake
Although most of us will never see a TPR valve become activated, it is an essential safety component on our water heaters. Observing the valves for leakage occasionally is something every homeowner should do. And every homeowner should know how to shut off both the main water supply and the water shut-off at the heater tank.
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