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What does a pressure relief valve do?

a couple looking out the window in their kitchen

To successfully heat your home and provide hot water, a boiler requires a balanced level of water and gas at all times. So, what’s keeping your boiler pressure in perfect balance and stopping the pressure from rising to dangerously high levels? That’s where a pressure relief valve (PRV) comes in.

Read on to find out exactly how a PRV works, how to identify faults and what to do if you’ve come across a leakage.

What is a boiler pressure relief valve ?

A boiler pressure relief valve is a safety device which works by allowing pressurised fluid to leave the system safely. This prevents a dangerous build-up of pressure which could lead to your boiler shutting down or, in extreme cases, rupturing completely.

The valve is usually located inside of the boiler casing at the bottom, If the level of pressure becomes too high, the PRV will open, giving the pressurised fluid a new route to escape from; thus, lowering the pressure. Once the pressure is reduced, the safety valve will close again.

In simple terms, a spring-loaded relief valve works when excess pressure pushes upon a resistant spring, opening a valve and creating a space for the steam or hot water to escape. As the pressure is released, the spring then pushes back down and closes the valve.

What are the benefits of an combi boiler?

A combi boiler (or combination boiler) is a common domestic boiler that produces both your hot water and heating. If you’ve noticed that your combi boiler is losing pressure, you may have a leak as a result of high pressure, a faulty valve or too much water was added to the system during top up with the filling loop.

If the PRV is the cause of a leaking boiler, you would expect to see the leak develop from a pipe which goes from the boiler to the outside, it will end on the outside wall or towards the ground on a long piece of pipe. There are a number of potential causes of a leaking pressure relief valve:


If the water within the heating system is dirty with central heating sludge or debris, this can cause blockages. A flush of the PRV and water within the system will need to be performed by a Gas Safe registered (GSR) engineer to ensure the blockage does not return.


The valve itself may have a build-up of limescale causing the seal to become blocked. This will need to be thoroughly cleaned and descaled by a professional.

It’s doing its job

It’s worth remembering that the job of a PRV is to release fluid if the pressure becomes too high, so some excess liquid should be expected at times. If the leak is constant or excessive, however, this means your boiler is consistently reaching high pressure and could be a sign of a deeper problem. Make sure to get this checked by a heating engineer, or within your annual boiler servicing.

How to replace boiler pressure relief valve

Get in touch with a Vaillant heating engineer. It isn’t safe or recommended to try this yourself, so you’ll need a qualified professional to take a look and replace the safety valve for you. In some cases, a leaky valve can be a sign of a wider issue, which is something an engineer will be able to check whilst replacing the PRV.

It is recommended that you receive a boiler maintenance check every 12 months, however keeping a close eye on your pressure can help catch faults before they cause costly damage.


  • A boiler pressure relief valve works to regulate the pressure within a boiler by diverting pressurised fluid out of the system
  • There are several types of boiler relief valve, though the most popular is a spring-loaded type
  • Leaking PRVs can be caused by limescale or blockages on the PRV seal or deeper heating system issues which need to be checked by a qualified GSR engineer.

If you need to replace your boiler pressure relief valve, contact your local Vaillant installer