What does a boiler pressure relief valve do?
A boiler pressure relief valve protects the sealed heating system from too much pressure. It is a spring-loaded valve on the boiler's water circuit that stays shut in normal use and opens when the pressure reaches its set point, which is around 3 bar on most domestic gas boilers. Check your boiler's installation manual for its exact setting.
When it opens, it dumps water until the pressure falls, then closes again. Vaillant calls it the expansion relief valve in the ecoTEC plus operating instructions, and other makers may call it a safety valve or PRV. Whatever the name, it is the last line of defence if the pressure controls, the expansion vessel or the person filling the system get it wrong.
Normal pressure is far below its setting. A healthy system reads around 1 to 1.5 bar cold and rises a little when hot, as our guide to normal boiler pressure explains. If the relief valve is opening, something has pushed the pressure well above that.
Should a relief valve ever open?
In a healthy system, the relief valve should almost never open. A single discharge after someone overfilled the system is not a fault in itself, as long as the valve closes again and the pipe dries up once the pressure is back in range. Regular discharges, or a pipe that stays wet, mean the cause needs finding. Each discharge also lets out water that later has to be replaced with fresh mains water, so the gauge will creep down between heating cycles.
A white or green streak on the wall below the end of the pipe is a good clue that the valve has been discharging for a while, even if the pipe is dry when you look.
Where does the pressure relief valve pipe go?
The pressure relief valve pipe usually runs from underneath the boiler, through the nearest outside wall, and ends a short way down the wall with its end pointing down or turned back towards the brickwork. It is normally copper, so it can cope with hot water. The idea is that if the valve opens, hot water or steam is released somewhere it cannot hurt anyone.
Installation manuals set out where the pipe may end. Some allow alternatives such as a connection to a drain, so on some homes there is no pipe outside at all. If you cannot find it, follow the pipework under the boiler or check the installation manual.
People often confuse it with the condensate pipe. Use this table to tell them apart.
| Pipe | Usually made of | When water comes out | Is it normal? |
|---|---|---|---|
| Pressure relief discharge pipe | Copper, short run to the outside wall | Only when the relief valve opens | No. Any water means the pressure is too high or the valve is worn |
| Condensate pipe | Plastic, often white or grey | Whenever the boiler is running | Yes. A condensing boiler drains water all the time it runs |
| Overflow from a loft tank | Plastic, high up on the wall | When a tank float valve fails | No, but it is a tank fault, not a boiler fault |
If the plastic pipe is the one causing trouble, for example freezing in winter, see our guide to a frozen condensate pipe.
Why is my pressure relief valve dripping?
A pressure relief valve drips for one of two reasons: the system pressure is reaching the valve's set point, or the valve is worn or has debris on its seat and no longer closes fully. Check the gauge first, because it tells you which one you are dealing with.
If the pressure is high
- The system has been overfilled. Topping up too far, especially with the heating hot, leaves no room for expansion.
- The filling loop is open or passing. A valve left open, or one that no longer shuts fully, lets mains water creep in and push the pressure up over days. Our filling loop guide shows how to check it is closed.
- The expansion vessel has lost its charge. The pressure then spikes every time the heating runs. See our guide to the expansion vessel.
- On a combi, an internal leak between the hot water and heating sides. A fault in the plate heat exchanger can let mains water into the heating circuit. An engineer can confirm this.
To bring high pressure down safely, follow our guide to boiler pressure that is too high.
If the pressure is normal
If the gauge reads a normal 1 to 1.5 bar cold and the pipe still drips, the valve itself is probably at fault. Once a relief valve has opened, grit or limescale can lodge on its seat and stop it closing fully. Worn valves can also start to weep with age. A valve that has had to open repeatedly because of a failed vessel is especially likely to go.
Pressure relief valve leaking: at the valve or from the pipe?
A pressure relief valve leaking from its outside pipe is discharging, as above. A relief valve leaking at its own joints, inside or under the boiler, shows as drips or damp underneath the boiler rather than outside. That is a leak on the boiler itself, and our guide to a boiler leaking water covers the other places that water can come from.
What you notice and what it means
| What you notice | Likely cause | Next step |
|---|---|---|
| Pipe drips, gauge near 3 bar | System overfilled or filling loop open | Close the loop and reduce the pressure |
| Pipe drips only when the heating runs | Expansion vessel lost its charge | Gas Safe registered engineer |
| Pressure creeps up over days with the loop shut | Passing filling loop or combi heat exchanger fault | Gas Safe registered engineer |
| Pipe drips, gauge normal | Worn relief valve or debris on its seat | Gas Safe registered engineer to replace it |
| Drips under the boiler, pipe outside dry | Leak at a joint on or near the valve | Gas Safe registered engineer |
Checks you can do yourself
- Read the gauge with the system cold, then again after the heating has run for an hour.
- Make sure both filling loop valves are fully closed, and on an external loop, that the hose is disconnected.
- Look at the end of the discharge pipe and the wall or ground below it for water or white limescale marks.
- Check whether it drips all the time or only while the heating is on.
Never cap, block, tape over or shorten the relief valve discharge pipe to stop the drips. The valve must be able to release water freely, and the water can be hot enough to scald. Keep children and pets away from the end of the pipe while it is discharging.
Can I replace a pressure relief valve myself?
No. The relief valve is part of the boiler's water circuit, usually inside or directly underneath the case, and replacing it means isolating and draining the boiler and refilling it afterwards. That is a job for a Gas Safe registered engineer. It is also a safety device, so it has to be the correct part for your boiler and be tested once fitted.
Replacement typically costs around £100 to £200 in the UK according to published repair price guides, varying with region, access and the make of boiler. Expect the engineer to check the expansion vessel at the same time, because a new valve on a system with a flat vessel will soon start dripping again.
When to call a Gas Safe registered engineer
Call an engineer if the pipe keeps dripping after you have closed the filling loop and brought the pressure back to the normal range, if the pressure keeps climbing on its own, or if the pipe drips with the gauge reading normal. Tell them your cold and hot readings and whether the drips happen only with the heating on. That points them to the vessel, the valve or the filling loop before they arrive.