Why is my boiler pressure relief valve leaking?
During a recent inspection, water was leaking from the extension tube on the side of the boiler onto the floor. This line was connected to a leaking safety or pressure relief valve. This fairly common problem is actually a relatively good thing and keeps you and your family safe from potential boiler failure. Before the installation of these devices, it was not uncommon to hear about boiler explosions injuring home occupants.
But why is it leaking now? To answer this question, it is important to understand how your heating system works. Your heating system distributes hot water throughout your home. As the boiler heats and cools the water, the water expands and increases pressure. To limit the pressure of the heating system, the hot water heating system includes various safety devices to ensure that your home is safe.
The first and most important device on your boiler is the safety or pressure relief valve. The standard operating pressure of a home boiler system is 12 psi. If for any reason the boiler pressure rises beyond the safe operating pressure (the pressure relief valve set point), the valve will open. This overpressure condition is likely caused by a faulty expansion tank. When a boiler system does not allow for sufficient thermal expansion of the water in the system, the system pressure rises beyond the pressure relief valve set point. Pressure relief valve leakage due to this condition is normally cyclical as the temperature of the boiler heats up and cools down. There are two potential causes for this circumstance.
A typical cause is that the cold pressure of the boiler water system is higher than the expansion tank set point, which is normally designed with a 12 psi set point. However, when the feed valve to the boiler system is higher than this set point, the expansion tank bladder is compressed before the boiler starts operating. Once the boiler is turned on and the water is heated, the already compressed bladder cannot accommodate further thermal expansion. It may be necessary to modify the internal pressure of the expansion tank and/or reduce the set point of the pressure reducing valve.
A second potential cause is an actual leak or failure of the expansion tank bladder. In this scenario, the expansion tank no longer allows thermal expansion of the water in the system, since the entire tank is full of water.
Other causes of pressure relief valve leaks can be due to the age and deterioration of the valve itself. In that case, it is recommended to replace the valve. The work should always be done by a qualified plumbing contractor.