While at a home inspection in St. Charles, I noticed water running out of the extension pipe of the water heater’s Temperature Pressure Relief (TPR) valve. TPR valves are design to release water from the water heater if the temperature exceeds a certain temperature or if pressure in the plumbing pipes exceeds a certain level.
As water temperature rises, water expands. As water expands it has to go somewhere. The pipes are already full when the water isn’t heated so what happens to this excess water? In older homes the water would be pushed back into the supply pipes from the city. However, cities don’t like this as there is the chance that this pushed back water is contaminated. For many years, cities have required back flow preventers to be installed at the water meter to stop this excess water from going back into their supply lines.
Therefore, expansion tanks are now installed, typically at the water heater, that absorb the excess water when there is a demand for hot water. In this particular situation, it’s not that the water temperature exceeded a certain level but that the expansion tank is sagging and most likely waterlogged, making it incapable of absorbing the water. The expansion tank needs to be either bled or replaced. Many homeowners see that the TPR valve is leaking and figure it’s faulty when, in fact, it is doing its job. Whenever the TPR valve is leaking, its recommended a plumber be called to diagnose the problem and make the appropriate correction.