What does a pressure relief valve do?

If you’re wondering how a PRV works, or how to fix a broken one, then you’re in the right place. 

What does a pressure relief valve do in a boiler?

PRVs, also sometimes known as safety release valves, are in place to prevent a buildup of pressure. The valve opens to release pressure when the level becomes too high, and closes again when the pressure has dropped to within a safe range. They can be used in all kinds of appliances and industries, including boilers and water heaters as well as clean water, petrochemical and power generation industries.

There are many different types of valves that all work in different ways but are still able to maintain the pressure in a system effectively. Spring-loaded relief valves are the most popular type, however other types that are available include vacuum relief valves, low-lift or full-lift safety valves and temperature-actuated pressure relief valves. Below, we detail how a spring-loaded relief valve works.

The valve itself is comprised of many features, the three most important ones being the spring-loaded valve, a sensor in the form of a piston and a spring. As the pressure builds, the spring is able to control the valve by not allowing the pressure, such as hot water or steam, through. However, as the force applied becomes greater than the force the spring can withhold, the spring moves to release the valve. Once this valve is opened, the water or steam is able to pass through. As the pressure drops, the spring is able to apply enough downward force to reseal the valve.

You can see the cross section of a pressure relief valve below.

The turquoise point at the top of the valve is the adjustment screw, which is lifted by the spring to allow the pressure to escape out of the top of the valve. As the spring moves back down, so does the screw to trap the pressure again.

PRVs are usually made from a range of materials, from brass and aluminium to plastic and stainless steel. The spring that’s used within the valve is typically made from stainless steel. This material is less likely to rust and therefore should continue to work efficiently for longer without the need for a replacement. In some cases, the valve needs to be light and so aluminium is used.

Because PRVs feature in a variety of industries, they're sometimes used with corrosive fluids, instead of hot water or steam. In this case, stainless steel is a requirement as it’s one of the only metals that can cope with being subjected to such substances. 

What could cause a pressure relief valve to leak?

There are many reasons why your boiler’s PRV might leak. It may be that the seal around the valve has eroded or the valve may be blocked with limescale or other forms of debris, such as rust.

Blockages that are caused by dirt and debris within the heating system could prevent the valve from closing properly. You could have your valve thoroughly cleaned or flushed by a suitably qualified heating engineer. This would get rid of any blockages that may be preventing the spring from moving up or down. It would also be advisable to have your heating system cleaned out, otherwise the blockage in the PRV could return.

If you have your PRV replaced with a new one and it continues to leak, you may have a problem with the boiler’s internal expansion vessel. As your boiler heats up cold water within the heating system, the water will expand. The expansion vessel provides space where this excess water can go to help ease the pressure on the system. If your boiler’s pressure is too high, it may be due to an issue with the expansion vessel rather than the PRV working incorrectly. You should call a suitably qualified heating engineer to check this for you.

How to replace a pressure relief valve on a combi boiler

If you think the PRV needs replacing in your combi boiler, a Gas Safe registered engineer can do this for you. It’s likely that you will need to turn off the boiler before they arrive to give it time to cool down. The engineer will then drain the boiler until the pressure gauge reads zero. They will swap the old PRV for a new one and make sure it’s installed correctly. The system can then be refilled with water, and the radiators will need to be bled. The engineer will ensure the new PRV is working before they leave. 

How often should pressure relief valves be replaced?

The PRV should be tested every time you have a boiler service, around every 12 months, and changed every three to five years. If you find that it’s beginning to leak more frequently or is constantly in need of repair, then it’s worth getting it replaced. A suitably qualified heating engineer can take a look at the valve for you and assess whether a replacement is required.