Start at the outside wall
A short copper or plastic pipe low on an outside wall, near the boiler, is dripping, and below it a stain on the brick or a green streak down the mortar. That is the pressure relief discharge. Nothing else terminates there.
Inside, the gauge has been falling. You top up, the heating runs, the pipe drips, and days later the gauge is low again. That loop is the problem. The valve is the last component in the chain and the only visible part of it, which is why it gets blamed and replaced.
What the valve is for, which is the whole point
Vaillant describe it in one line: a safety device that allows pressurised fluid to leave the system safely, preventing a build-up that could shut the boiler down or, in extreme cases, rupture it. The common type is spring-loaded — pressure pushes a disc off its seat, water escapes into the discharge pipe, and the spring pushes the disc back down.
On a sealed domestic system that valve lifts at 3 bar, while the system is designed to sit at 1 to 1.5 bar cold and around 2 bar hot. It should spend its whole life shut, and opens only when something upstream has gone wrong.
So the valve venting is not the fault. It is the report of a fault, the way a tripped RCD is not an electrical problem. Replacing it without reading the report is how this job comes back.
Two completely different stories, one wet pipe
Every passing relief valve turns out to be one of two things, and they need different work.
| What you observe | System is over-pressurising | Valve will not re-seat |
|---|
| When it drips | During or shortly after a heating cycle, as the water expands. | All the time, hot or cold. |
| Hot gauge reading | Climbs hard, up near or past 3 bar. | Normal. Often it never gets above 2 bar. |
| Cold gauge reading | Low once the system cools, because the excess went outside. | Falls steadily, whatever the boiler is doing. |
| How it started | Gradually, usually as the heating season started. | Abruptly, often after one big discharge, a drain-down or a filter clean. |
| What fixes it | The upstream fault: vessel, filling loop, or wherever the expansion cannot go. | A flush and, if the seat is damaged, the valve. |
The first is a system fault with a working safety valve; the second is a faulty valve. The pipe cannot tell you which. The gauge can.
Story one: the system really is making too much pressure
The expansion vessel has lost its charge
By a distance the most common version, and it has its own page here: see expansion vessel failure or a leak for the mechanism and the test. The vessel is the only place the expansion can go. When its gas charge escapes or its diaphragm splits, the hot pressure spikes past 3 bar and the relief valve dumps the excess outside. The system then sits low when cold, because that water is gone.
Read that sequence again. The vessel failed. The valve worked perfectly. The visible symptom is at the valve.
The filling loop is open, or passing
Worcester Bosch describe the three common filling loops — an external braided hose, a keyed internal loop, and a keyless one with a blue lever — and the instruction is the same for all: close it fully once the gauge reaches its target.
Left open, or worn so that it passes, the mains feeds the sealed circuit continuously. Mains pressure across most of London sits comfortably above 3 bar, so the relief valve has no choice: it opens and keeps opening. Check this first. It costs nothing and takes a minute.
The expansion has been shut out
Same outcome, different cause: an isolating valve left closed on the branch to an external vessel, a connection silted up with magnetite, or a vessel undersized for a system that has had radiators added over the years. The expansion has nowhere to go, the gauge reads exactly as it does with a failed vessel, and none of it is leak detection.
Story two: the valve has passed debris and will not seat again
The other half of the caseload is the valve itself, and the cause is whatever is floating around in the system water. Vaillant are explicit: dirty heating water carrying sludge or debris causes blockages, and leaking relief valves are caused by limescale or blockages on the PRV seal. Their fix is a flush of the valve and the system water by a Gas Safe registered engineer.
The mechanics are ordinary. A relief valve seals across a face a couple of millimetres wide. When it lifts, for any reason including one legitimate over-pressure event, it passes a slug of water at speed. If that water carries magnetite sludge, grit from a drain-down or limescale flakes, a particle can lodge on the seat as the disc comes down. The disc then closes onto a grain rather than the seat, held open by a fraction of a millimetre, which is all a weep needs.
Worse, if the particle is hard and the valve cycles on it, the seat gets scored. After that, correcting the pressure will not close it. The sealing face is no longer a sealing face.
The giveaways: it weeps at normal pressure, hot or cold, and it started sharply rather than gradually — often within days of a drain-down, a new radiator, a power flush or a filter being emptied. Most of London is hard water, which is why limescale belongs on this list here more than it would further north.
The readings that tell you which one you have
No tools. Five observations and a dry hand.
- Cold reading, first thing, before the heating comes on.
- Hot reading, after an hour with every radiator open.
- Discharge pipe, hot. Wipe it dry before the heating starts, check at the end of the cycle.
- Discharge pipe, cold. Wipe it dry, leave the heating off for a few hours, check again.
- Second cold reading, the next morning.
A wide cold-to-hot swing with the pipe wet only when hot is an over-pressurising system: look upstream. A normal swing with the pipe wet while the system is stone cold is a valve that will not seat. If the pipe is bone dry at both ends and the gauge still falls, the valve is not involved at all — that is the one version of this that is a real leak, and boiler losing pressure picks it up.
Why fitting a new valve first is the expensive order of work
The valve is cheap and quick, so it gets changed before anyone measures anything. If the fault was a dead expansion vessel, the new valve holds for about one heating cycle, then lifts exactly as designed and you are back where you started minus the part and the visit. If the fault was debris and nobody flushed, it seals until the next particle finds it — which is why Vaillant pair the valve with a flush. And if a filling loop was left open, the new valve is doing the old one's impossible job: holding back the mains.
Measure, identify the story, fix that, then replace the valve if its seat is damaged. The valve is the last step, not the first.
What is our job, and what plainly is not
We are leak detection and leak repair. On most combis the relief valve is part of a gas appliance, so work on it — the flush Vaillant describe, or a vessel recharge — is for a Gas Safe registered engineer. Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 requires work on a gas fitting to be carried out by a competent, registered person. If your readings point at the vessel or the valve, that is the trade to call, and we will say so on the phone rather than send someone to confirm what your gauge already told you.
One thing nobody should do, whatever the trade: do not cap, plug or blank the discharge pipe to stop the drip. That pipe is the system's only pressure release. Blocking it removes the safety function and leaves the fault in place. If the water is damaging a path or a wall, catch it rather than stop it.
And know what the drip is not. A pipe that runs for a minute and stops after someone repressurised the system is an overfilled system finding its level, not a fault. Bring the cold reading back into the 1 to 1.5 bar band — see how to repressurise a boiler. If the gauge is sitting high rather than low, boiler pressure too high covers how to bring it down safely and how to tell an overfill from a filling loop that is letting the mains in.
When this does become a detection job
Call us when the discharge pipe is dry, the vessel has been checked or replaced, and the system is still losing water with nothing visible. The loss is then real and inside the building: a pinhole under screed, a weeping joint in a wall, a failed underfloor circuit. The circuit is isolated and pressure-tested to prove and size the loss, thermal imaging follows the pipe runs, and where that cannot resolve it the circuit is drained and traced with gas. The floor comes up once, at one point, and our own people reinstate it.
That visit is £150 per hour, agreed before we attend, no call-out fee, and if we do not find the leak you do not pay for the time spent looking. It includes a written report prepared to support a trace-and-access insurance claim; what the policy covers is your insurer's decision, not ours. Detail on boiler and heating leak repair and pricing.
