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Home Inspection Leaking Pressure Relief Valve

Updated on December 29, 2017

Hot Water Heating System

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Why Is My Boiler Boiler Pressure Relief Valve Leaking?

During a recent home inspection water was dripping from the extension pipe on the side of the boiler onto the floor. This piping was connected to a leaking Pressure Relief or Safety Valve. This fairly common problem is actually a relatively 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 throughout your house. As the water is heated and cooled by the boiler that water expands 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. This pressure does vary for houses greater than 2 stories but is a generally accepted industry standard fr 2 story single family homes. 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. Heated water expands and when a boiler expansion tank does not allow for sufficient thermal expansion of the water in the system, the pressure of the system increases 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. Expansion tanks are typically designed with a 12psi setpoint. However, when the feed valve to the boiler system is set to a higher pressure 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, subsequent thermal expansion cannot be accommodated by the already compressed expansion tank bladder. Modification to the internal pressure of the expansion tank and/or reduction of the pressure reducing valve setpoint may be required. The boiler expansion tank setpoint can be increased or decreased using the air recharge valve. A standard bicycle pump should be connected to the air recharge valve and air added to the expansion tank until the 12psi setpoint is reached.

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. Boiler expansion tanks typically utilize a rubber diaphragm to maintain internal pressure of the boiler system. On one side of the diaphragm is air and the other side is the hot water in the boiler system. Like all devices on the heating system the internal diaphragm of the expansion tank can rupture or have a slow leak resulting in little to know ability to accommodate hot water expansion.

Leaking pressure safety valve can be caused by other less common heating system issues. Other potential causes include:

  • Deterioration of the valve. Leaking Pressure Relief Valve can be due to the age and deterioration of the valve itself.
  • Debris can be lodged in the valve assembly resulting in a constant flow leak in the system.
  • Excessive water temperature of the boiler system has the potential to result in higher pressures than the heating system design. Temperatures within a home heating system should be maintained between 160°F and 180°F. Higher temperatures will approach the boiling point causing excessive system pressure.

The exact cause of these issues are not always apparent during a home inspection as a leak can occur during any time of operation. Whenever a leaking boiler pressure safety valve is identified full investigation is required to accurately determine the exact cause. Safety is always a concern when working with home heating systems and all work should always be performed by a qualified plumbing contractor.

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