Why is my boiler pressure relief valve leaking?

During a recent inspection, water was leaking from the extension tube on the side of the furnace onto the floor. This pipe 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. Prior to 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’s 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 the pressure builds. 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 in your boiler is the pressure relief or safety valve. The standard operating pressure for 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 sufficient thermal expansion of the water in the system, the system pressure increases beyond the pressure relief valve set point. Pressure relief valve leakage due to this condition is normally cyclical as the boiler temperature warms and cools. 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 set point of the expansion tank, which is normally designed with a 12 psi set point. However, when the boiler system feed valve is higher than this set point, the expansion tank bladder is compressed before the boiler starts operating. Once the boiler is fired and the water is heated, the already compressed bladder cannot accommodate subsequent thermal expansion. Modification of the internal pressure of the expansion tank and/or reduction of the pressure reducing valve set point may be necessary.

A second potential cause is an actual leak or expansion tank bladder failure. In this scenario, the expansion tank no longer allows for thermal expansion of the water in the system, since the entire tank is filled with water.

Other causes of pressure relief valve leakage can be due to the age and deterioration of the valve itself. In that case, it is recommended to replace the valve. Work should always be done by a qualified plumbing contractor.