Boiler pressure relief valve: what it does and why it drips

What the valve is for

A sealed heating system runs under pressure, normally around 1 to 1.5 bar when cold and a bit higher when hot. As water heats up it expands. The expansion vessel inside or next to the boiler absorbs that extra volume, so the pressure only rises a little.

The pressure relief valve is the last line of defence. If the pressure climbs to the valve's set point, usually 3 bar on domestic boilers, it opens and dumps water until the pressure drops back. Without it, pipes, joints and the boiler itself would be put under far more strain than they are built for.

You will often hear it called the PRV or the safety valve. It should not be confused with the temperature and pressure relief valve on an unvented hot water cylinder, which protects the cylinder and discharges into a tundish. That is a separate valve on a separate system.

Where to find it and its pipe

The valve itself is inside the boiler casing, usually low down near the pump and pressure gauge. You do not need to see it. What you can see is the discharge pipe: a copper pipe, normally 15mm, that comes out of the bottom of the boiler and runs through the nearest outside wall.

Outside, the pipe usually ends a short way from the wall and turns back towards it, so any hot water that comes out is pointed at the brickwork rather than at someone walking past. Look for a damp patch, a lime stain or a drip at the end of it.

Never cap, block or crimp the relief valve pipe to stop it dripping. Doing that removes the safety protection from the whole system. Water coming out is a symptom, and the fix is inside the boiler.

Why a boiler pressure relief valve drips

There are three common reasons, and the boiler pressure gauge tells you which one you are dealing with.

Why it keeps dripping after the pressure is fixed

A relief valve is a simple spring-loaded seal. Once it has opened, especially more than once, bits of debris from the system water or scale can lodge on the seat so it no longer closes fully. The result is a slow drip even at normal pressure, and a boiler that gradually loses pressure and needs topping up.

That creates a loop. The system loses water through the valve, you top it up, fresh water brings in oxygen and minerals, and more debris is produced. That is why an engineer will fix the cause of the over-pressure and replace the valve in the same visit, rather than doing one or the other.

What you can safely check yourself

You can gather useful information without opening the boiler.

• Read the pressure gauge with the heating off and cold, then again after the heating has been on for half an hour. Note both figures.
• Look at the filling loop under the boiler. Both taps should be fully closed, and if it is a removable braided hose, it should normally be disconnected after use.
• Check the discharge pipe outside at different times: does it drip only when the heating runs, or all the time?
• If the pressure is too high, you can bring it down by bleeding a radiator with the heating off, as our guide on high boiler pressure explains.
• Write down any fault code and how often you have had to top the pressure up.

Can you replace it yourself?

No. The valve is inside the boiler casing, and on a gas boiler that work must be done by a Gas Safe registered engineer. The replacement also has to match the boiler's specification, and the engineer needs to check the expansion vessel charge and the filling loop at the same time, or the new valve will simply start dripping too.

Some boilers use a relief valve that is part of a larger hydraulic block, which changes how the job is done. The engineer will look up the right part for your make and model.

Is a dripping relief valve dangerous?

A drip is not dangerous in itself. It shows the valve is at least able to open. The risk comes from what happens next: the system loses pressure, the boiler cuts out on low pressure, and if the discharge pipe is left dripping for months the wall can stain or, in winter, the drip can freeze into an icy patch below.

What is dangerous is interfering with it. Do not cap the pipe, and do not keep resetting a boiler that is venting water. If the valve is discharging a lot of hot water or steam, turn the heating off and get it looked at promptly.

Getting it fixed

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day, and boiler work is carried out by Gas Safe registered engineers. Our call-out charge is £60.

When you contact us, tell us the boiler make, the pressure readings cold and hot, whether the pipe outside drips all the time or only with the heating on, and how often you top up. With that, the engineer can usually bring the right valve and check the expansion vessel and filling loop in one visit.