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Top Reasons Relief Valves Discharge

Top Reasons Relief Valves Discharge

Though this is a subject we have visited many times it is always good to take a minute and revisit some common reasons your PRV may be discharging when it shouldn’t be.  Below are some of the main issues that have been found while troubleshooting a PRV discharging at odd times:

Waterlogged Steel Compression Tank – Though not as common as it once was, there are still heating systems that utilize a steel compression tank to absorb water that has expanded due to heat.  When a volume of air is displaced from the tank an equal amount of water will replace it.  When this happens there is an immediate increase of pressure in the tank that causes the relief valve to open and discharge the excess pressure.

Pressure Reducing Valve Fails Open – Most systems rely on Pressure Reducing Valves during filling and pressurization to prevent cataclysmic damage to components of the pipeline.  If a reducing valve has been in use for some time, scale and mineral build up on the seat can prevent the valve from seating properly and closing all the way.  This will result in the valve remaining in the open position even after the pipeline pressure has returned to below the set point conditions.  Additionally, there is a manual bypass on most PRV to open the valve that, when left open, will expose the valve to steam pressure which is generally higher than the valve’s rating.

Problematic Expansion Tanks – Whether you are using an older steel tank or a newer diaphragm or bladder style expansion tank, sizing is extremely important.  When an undersized expansion tank is installed in a system the volume of water will be more than it is designed to handle.  This will cause an increase in system pressure; triggering the relief valve to open in order to prevent damage to the pipeline.

Undercharge Diaphragm Expansion Tank - Aside from sizing the tank properly, making sure the expansion tank is set properly is key.  If the pre-charge of pressure in the expansion tank is not set to match the system’s fill pressure then cold water will be allowed to enter the expansion tank before the system water.  This will effectively reduce the capacity of the expansion tank causing the PRV to open.

Diaphragm Tank losing its Air Charge – The material used to make membranes for expansion tanks is permeable and allows for small amounts of air to leak through and enter the system.  When this happens, a pressure drop is noticed in the system and water is added to maintain the required system pressure.  As this begins to happen more water will be directed to the boiler increasing pressure and causing the PRV to open.  If the pressure in the tank gets low enough it can cause the boiler to fill to the point the PRV is spitting on the floor.

Improper Pump and Expansion Tank Location – When a system has both an expansion tank and high head pump on the return line additional pressure from the pump will be added to the system fill pressure.  In many cases these two pressures added together exceed the set point of the relief valve and cause it to remain open.

Tankless Coil Boiler issues – Tankless boilers are often used to produce domestic hot water.  Over time, wear can cause the coils to develop leaks due to aggressive water and corrosion.  If this happens, it will leave the boiler and its relief valve exposed to the cold water line’s pressure which, normally, operates at a higher pressure than the valve’s rating.  This will cause the valve to remain in the open position and discharge constantly.

Systems with High Head Pumps and Pressure Differential Valves – During season change in Spring and Fall months, when many of the zone valves in a system will be closed, pressure differential valves open to help prevent excessive pump head from building within circulators.  When this happens, it will direct that excess head towards the boiler and relief valve.  When the newly added head pressure combined with the system’s static pressure exceeds the set point of the relief valve it will open and discharge.  

To find Xylem’s PRV offering please visit https://mcdonnellmiller.com/valves/series-250-pressure-relief-valves/


For expert help with questions of problems you encounter with any hydronic system please contact your local Bell & Gossett representative.