Hugh Cairns: TPR valve leaks

December 25, 2014 5:37 am

Water heater leaking can be the symptom of a more serious problem. Photo: Contributed – (Photo: Hugh Cairns)

Hugh Cairns: TPR valve leaks

As you can imagine a leaking hot water tank is not good news. Small leaks are a clear sign of a looming larger problem.

When water is heated it expands. This is because when water is heated, its density decreases and as a result its volume expands. In the average 40 gallon hot water tank, increasing the temperature of the water from 90°F to 140°F will expand the water volume by a half-gallon.  When your hot water system is closed (when a tap or valve is not open) the extra volume of the heated water must go somewhere. That’s why plumbing supply systems have a series of pressure reducing valves and back flow preventers designed to stop the water from flowing back to the heated supply which eventually increases the pressure in your hot water tank.  

Excessive thermal water expansion in water heaters can cause potentially dangerous conditions. When high abnormal pressure exists in a hot water tank, internal parts like fittings and connections may fail causing water to leak. If high pressure causes a flue way to collapse – it can lead to the potential release of combustion gases into your home or if there is enough pressure the tank has the potential to rupture or distort which will likely void the tanks’ warranty. Al of these conditions can lead the TPR valve to open.

The TPR valve

Our plumbing code requires that thermal expansion be controlled. In the event that excessive pressure is achieved there has to be a away to relieve that pressure. That’s why hot water tanks have a TPR valve (temperature pressure relief valve). Commonly installed on the top side of the tank, TPR valves open up to protect the system in the event of high pressure. If you ever see water coming out of the valve or its extension pipe it’s time to call the plumber.

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