Why Is My Boiler Boiler Pressure Relief Valve Leaking?

During a recent inspection water was dripping from the extension pipe on the side of the boiler onto the floor. This pipeline was connected to a leaking Pressure Relief or Safety Valve. This fairly common problem is actually a reliably good thing and keeps you and your family safe from potential failure of your boiler. 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 through your house. As the water is heated and cooled by the boiler that water expends and increases in pressure. In order to limit the pressure of the heating system the hot water heating system includes various safety devices to ensure your home is safe.

The first and most important device on your boiler is the Pressure Relief or Safety Valve. The standard operating pressure of a home boiler system is 12 psi. If for any reason the pressure of the boiler increases beyond the safe operating pressure (the setpoint of the Pressure Relief Valve) the valve will open. This over pressurizing condition is likely caused by a failing expansion tank. When a boiler system does not allow for sufficient thermal expansion of the water in the system, the pressure of the system increments beyond the Pressure Relief Valve setpoint. Leaking of the Pressure Relief Valve due to this condition is normally cyclical as the boiler temperature heats and cools. There are two potential causes for this circumstance.

A typical cause is the cold pressure of the boiler water system is higher than the setpoint of the expansion tank which is typically designed with a 12psi setpoint. However, when the feed valve to the boiler system is higher than this setpoint the expansion tank bladder is compressed before the boiler begins operation. Once the boiler is turned on and the water is heated, residual thermal expansion can not be accommodated by the already compressed bladder. Modification to the internal pressure of the expansion tank and / or reduction of the pressure reducing valve setpoint may be required.

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

Other causes of leaking Pressure Relief Valve can be due to the age and deterioration of the valve itself. In that case replacement of the valve is recommended. Work should always be performed by a qualified plumbing contractor.


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