Pressure Relief Valve Problems

What is the purpose of the pressure relief valve?

The Pressure Relief Valve (PRV) or “safety release valve” is there to stop the system from over pressurising. The PRV helps to control the pressures in the boiler so that the pressure level never gets to a dangerous level. The valve will automatically open and close as the pressure gets too high and returns to a normal pressure (safe range)

Pressure relief valve opening / pressure gauge continuously climbing

Fault	Fix
Undersized expansion vessel	upgrade vessel size
Valve faulty or seat contaminated with sludge/limescale/rust	replace valve
Plate to plate heat exchanger faulty (fractured)	replace plate exchanger
Filling loop faulty / passing pressure	replace / cap off
Expansion vessel diaphragm has perished or is damaged. As the boiler heats up cold water the water will expand. The expansion vessel provides space for the expansion of the water to ease pressure. If the pressure is too high it could indicate an issue with the expansion vessel rather than the PRV itself.	replace vessel
Expansion vessel has lost air charge	re-charge vessel
Pump head failing (not at full capacity)	replace pump head
Boiler control thermostat faulty (causing the overheat stat to trip)	replace thermostat
Faulty gas valve (over gassing)	check burner pressures
Water flooding	turn off water supply
Loud whistling noises	replace valve

The pressure relief valve is the ONLY place that steam should be visible on a boiler. If steam is coming out of the boiler somewhere else there could be a leak.

The pressure relief valve should only let out steam when the boiler pressure levels are too high (typically over 30psi). The valve will kick in attempting to reduce the pressure when there is too much pressure in the boiler system, this can be indicated by a loud noise from the boiler.

Read the pressure relief valves pressure from the pressure gauge. Normally this will be around 12 – 15psi. If the level is a lot higher than this then the pressure relief valve could be activated venting the excess pressure from the system. You can bleed the radiators in order to reduce the pressure in the system if required.

If you find that the pressure levels are correct it could indicate that the pressure relief valve needs replacing (e.g. the pressure on the gauge reads 12 – 15 psi). Sludge and sediment can build up on the valve seat over time stopping the valve from sealing correctly.

Replacing the pressure relief valve

This can only be addressed by a Gas Safe registered heating engineer. The boiler will need to be drained to reduce the internal pressure to zero, the valve will be replaced for a new one and checked for correct installation. The system will then be refilled with water and the radiators bled. The PRV is part of the service routine of a standard boiler and will routinely be checked by the heating engineer; a pressure relief valve will be change approximately 5 years.

Other Issues to check

• Noisy Fan
• Noisy Pump
• Noisy Hot Water Flow Switches
• Noisy Solenoid
• Burner Not Igniting
• Boiler Fan Problems
• Boiler Shuts Down
• Explosive Ignition
• Blocked Plate Exchanger