Pressure Relief Valve (PRV) in a Boiler: What It Does, Why It Leaks, and the Signs to Look For in Mid Sussex
- Terry Matthews
- Feb 17
- 7 min read
What’s Inside a Boiler? - Part 2: Pressure Relief Valve
If your boiler pressure keeps dropping, you keep topping it up, or you have water dripping outside from a copper pipe, the PRV is one of the key things to check properly.
It is a safety device designed to protect your boiler and heating system from dangerous pressure levels, but once it has opened, it can sometimes fail to seal again.
In this guide I’ll explain what the PRV does, why it activates, why it can start leaking, what symptoms you’ll notice at home, and what we check during a proper boiler service in RH15, RH16, RH17 and BN6.
Quick answer (read this first)
A PRV is a safety valve that releases water if boiler pressure gets too high.
On most boilers it opens at around 3 bar (it can vary by boiler and system).
If the expansion vessel is flat or undersized, pressure rises fast and the PRV can open.
Once a PRV has opened, it can sometimes fail to reseal and then you get a slow drip outside.
A leaking PRV causes constant pressure loss and frequent topping up.
We test PRVs during safety checks, and if testing reveals it will not seal, it needs replacing.
What is a PRV?
PRV stands for pressure relief valve.
Its job is simple: if the sealed heating system becomes
over pressurised, the PRV opens and releases water quickly, so you do not end up with damage to the boiler, pipe joints, radiators, or other components.
Think of it as the system’s safety escape route.
Where is the PRV located?
On most combi and system boilers, the PRV is inside the boiler casing.
You can also find PRVs externally in certain setups, usually close to:
an external expansion vessel arrangement
filling loop and pressure gauge assemblies
some heat only systems where parts have been upgraded or added over time
You will not always see it easily because it sits in awkward positions on some boiler models.
What pressure does a PRV open at?
On the majority of domestic heating boilers, the PRV is set to open at around 3 bar.
Some systems and appliances can differ, but the key point is this: your boiler is designed to run at much lower pressures day to day, and the PRV only opens when something is wrong or has been pushed too far.
If your gauge is climbing close to the top end when the heating is on, that is a big clue something is not absorbing expansion properly.
Why does boiler pressure rise in the first place?
Pressure rises because heating water expands when it gets hot.
A sealed heating system needs a place for that expansion to go. That is exactly why expansion vessels exist, and why I started this series with them.
Common reasons pressure rises too high include:
1) The expansion vessel has gone flat
This is the most common. If the vessel has lost its air charge, the system has no cushion, so pressure rises quickly.
2) The expansion vessel is too small for the system
Bigger properties, more radiators, larger pipework volume, or system upgrades can mean the built in vessel is not enough. An external vessel is often the right fix.
3) The system has been overfilled
If somebody tops up the pressure too high when cold, it can easily tip into PRV territory once it heats.
4) A filling loop has been left open
If the filling loop is not fully closed, the system can keep creeping up in pressure.
The PRV is not the cause in these situations. It is the thing that reacts to the cause.
What happens when the PRV opens?
When the PRV opens, it releases water rapidly into a discharge pipe.
That discharge pipe normally runs out through the wall and terminates outside so the hot water is sent somewhere safe and visible, rather than flooding inside the property.
Outside, you will usually see a copper pipe that:
points downwards, or
is finished in a way that helps prevent scalding risk and makes discharge easy to spot
If you have ever seen a puddle outside and wondered where it came from, this is often it.
Why PRVs start leaking after they activate
This is the bit that catches people out.
A PRV might sit there for years without ever opening. When it finally opens, it has to seal perfectly again afterwards. If it does not, you get a constant drip and then constant pressure loss.
There are a few reasons it might not reseal:
The internal rubber seat has aged and does not sit back down cleanly
Debris or grit in the system water gets caught on the sealing surface
The valve opens under force and the seat gets damaged
The valve has not been exercised or tested for years, so when it finally moves it does not behave nicely
A good simple analogy is an old tap that has not been turned for ages. The first time you move it, it starts dripping.
The most common complaint we hear when a PRV is leaking
“My system keeps losing pressure.”
That is exactly what happens. The PRV is letting by slowly, and every small drip outside is pressure leaving the system.
If the leak is steady enough, the boiler can lock out on low pressure, and you lose heating or hot water until it is topped up again.
Signs your PRV might be leaking
Look for these:
Boiler pressure drops regularly and needs frequent topping up
Water dripping outside from the copper discharge pipe
Damp patch, staining, or limescale marks under the discharge point
Pressure looks normal when cold, but drops over time even when not using heating much
You fix the expansion vessel issue, but the pressure still slowly falls afterwards
That last one is classic. The root cause gets fixed, but the PRV has already been damaged and will not reseal.
A safe check you can do at home
Only do this if it is safe, accessible, and you are not at risk of burns.
Look at the discharge pipe outside when the heating has been running
Check for drips, wetness, staining, or signs of water flow
If you want to confirm it, the safest option is to place a small container underneath for a short period and see if it collects water
Avoid blocking, capping, sealing, or attaching anything that could restrict discharge. The discharge route is a safety outlet and must stay free.
If you cannot access it safely, do not try. Just take a photo from a distance and we can advise.
Why we test PRVs during a boiler service
PRVs are safety devices. They should be checked as part of a proper service and safety inspection.
That means an engineer should test the function and make sure it opens and reseals correctly. Not testing it is basically skipping a key safety component.
This also explains something that can happen after a service.
“It never lost pressure before you came, now it does”
If a PRV has not been tested for years and it is already tired inside, testing it can reveal that weakness. That does not mean the engineer caused the fault. It means the test exposed a valve that was already on borrowed time.
If it starts letting by after testing, the correct fix is to replace it.
The best way to reduce the chance of this happening is simple: proper servicing every year, so safety parts are exercised routinely rather than left untouched for years.
How we fix PRV related pressure loss properly
A proper fix is not just swapping a part.
We do two things:
1) Deal with the cause of over pressure
Most commonly this is expansion vessel charge, size, or filling loop issues.
2) Replace the PRV if it is passing
If the PRV has been activated and will not reseal, replacement is the right solution. It is generally not an expensive part, but access can be awkward on some boilers, so labour can vary.
Then we retest the system hot and cold to confirm pressure stability.
Is a leaking PRV dangerous?
It is not the same type of danger as a gas leak, but it is not something to ignore.
A leaking PRV can lead to:
repeated low pressure lockouts
water damage or staining outside
ongoing topping up, which introduces fresh oxygenated water and can increase corrosion risk long term
potential for bigger faults if the root cause is still present
And again, never block the discharge. It is there to protect you and the system.
Should you book a service or a repair?
Book a boiler service if:
your annual service is due
you want preventative checks and safety device testing
you have minor pressure movement but no obvious discharge
Book a repair visit if:
the discharge pipe is dripping
pressure keeps dropping and you are topping up regularly
the boiler is locking out on low pressure
pressure swings high when heating is on
If you are unsure, just message us what you are seeing and if possible include a photo of the pressure gauge and the discharge pipe outside.
Quick FAQs
Does every boiler have a PRV?
Most sealed system boilers do, yes. It is a standard safety component in modern domestic systems.
If the PRV opens once, does it always need replacing?
Not always. Some reseal perfectly. But if it starts dripping afterwards, it usually needs replacement.
Can I stop the drip by tightening something?
No. These are not meant to be “tightened” to fix a leak. If it is passing, it is a faulty seal or debris issue and replacement is the reliable fix.
What else can cause pressure loss?
Pressure loss can also be caused by leaks on pipework, radiator valves, hidden joints, or other safety devices. A leaking PRV is common, but it is not the only cause, which is why proper diagnosis matters.
Need help locally with pressure loss?
If you are in RH15, RH16, RH17 or BN6 and your boiler pressure is dropping or you suspect the PRV is leaking, we can get to the root cause and fix it properly.
Next in the series: I can cover the filling loop and pressure gauge next, because they link into this problem all the time.
Great advise, we have spoke with Charlie and booked in with you next Thursday. Diane x