Pressure Relief Valve Leaking: What to Do and Why It Happens

Do this first, while the water is still running

Turn off whatever is heating the water. On an unvented cylinder that means switching the boiler off at its switch or programmer and switching the immersion off at its wall switch. This matters more than anything else you do in the next ten minutes, because in most cases the valve is lifting on heat expansion. Stop putting heat in and the pressure stops climbing, and a valve that was pouring often slows to a dribble within twenty minutes.

Next, find where the water is going. On an unvented cylinder the relief valves discharge into a tundish, the small open funnel in the pipework near the cylinder, and from there down a pipe that runs outside the building. If the tundish is doing its job the water is going outside and nothing indoors is getting wet, which buys you time. If water is running out of the valve body itself, down the side of the cylinder or pooling on the floor, get a bowl and a towel under it and keep an eye on the ceiling of the room below. If water is escaping into the building rather than reaching the tundish, deal with it as an escape of water first and diagnose the valve afterwards: close the stopcock, protect what is below, and worry about which valve later.

Do not cap, plug or tape the discharge pipe, and do not tighten anything on the valve to try to stop the drip. That pipe and that valve are the safety devices that stop a sealed cylinder becoming dangerous. A leaking relief valve is telling you about a fault. Blocking it removes the warning and leaves the fault in place.

• Switch the boiler off and switch the immersion off at the wall
• Check the tundish and follow the discharge pipe to where it leaves the building
• Put something under any leak that is not reaching the tundish
• Leave the valve and the discharge pipe alone, do not cap either
• To stop it completely, close the mains stopcock, then run a hot tap to release pressure

Water from a relief valve on a hot cylinder can be close to boiling. Do not put a hand under the tundish to feel the flow and do not lean over it to look. Watch from the side. If the discharge is steaming, or the cylinder is banging or rumbling, treat it as urgent.

Work out which valve is actually leaking

People say pressure relief valve for three different components, and the diagnosis changes completely depending on which one is in front of you.

On an unvented cylinder there are normally two. The expansion relief valve, often just called the PRV, sits in the inlet group on the cold feed, usually in the same brass body as the pressure reducing valve and the check valve, and is typically set around 6 bar. The temperature and pressure relief valve screws directly into the cylinder itself with a probe reaching into the stored water, and opens on either high pressure or water temperature around 90 to 95 degrees. Both discharge through the same tundish, which is why a wet tundish on its own does not tell you which one lifted. Reading the discharge itself, hot or cold, a drip or a steady flow, is covered in more detail on our page about a dripping tundish.

The third thing people call a pressure relief valve belongs to the boiler, not the cylinder. Every sealed heating system has one, usually set around 3 bar, discharging through a small copper pipe that ends on an outside wall. If the first sign of trouble was water dripping from a pipe stub outside the house, that is the boiler valve and the cylinder is not involved.

To separate the two cylinder valves, look at which pipe feeding the tundish is wet. Water arriving from the pipe off the inlet group at the cold feed is the expansion relief valve. Water arriving from the pipe leaving the cylinder body higher up is the temperature and pressure valve. If you cannot tell, the pattern of the leak narrows it further anyway.

• Expansion relief valve: inlet group, cold feed side, around 6 bar
• Temperature and pressure relief valve: screwed into the cylinder, probe in the water, around 90 to 95 degrees
• Boiler relief valve: heating circuit only, around 3 bar, discharges to an outside wall

A drip, a trickle or a full flow: what the pattern tells you

The timing of the leak is the most useful evidence you have and it costs nothing to gather. Watch the tundish through a full heating cycle and note when water appears. A tundish dripping in step with the boiler and a tundish dripping steadily around the clock are two different faults with two different repairs, and telling them apart costs nothing but patience.

Water that appears only while the cylinder is heating, runs for a few minutes and stops once the boiler or immersion switches off, is expansion. Water in a sealed cylinder gets larger as it warms, that extra volume has to go somewhere, and it is being dumped through the valve because the expansion vessel is no longer absorbing it. This is by far the most common version of the complaint and it gets worse over weeks rather than failing overnight.

Water that appears when someone runs a tap and stops when they close it points at the cold side instead, usually incoming mains pressure above what the valve is set to hold, or a pressure reducing valve passing too much.

A constant dribble that never stops, whether or not the boiler has run and whether or not anyone has used a tap, is the valve itself. A relief valve seals a soft seat against a machined face. Put a grain of grit or a flake of scale between the two and it cannot seal, and it will weep until the seat is cleaned or the valve replaced. Once a valve has lifted at full pressure and passed debris across its seat, it very often will not reseat properly afterwards, which is why a valve that lifted for a genuine reason frequently keeps dripping after that reason is fixed.

Why a relief valve leaks only while the water is heating

An unvented cylinder is a sealed metal vessel fed directly off the mains. Water does not compress, so when the stored water heats and expands, something has to give. That something is meant to be a cushion of air, held either in a separate expansion vessel plumbed into the cold feed or in an internal air gap built into the top of the cylinder, depending on the make.

In a separate vessel the air sits on one side of a rubber diaphragm and the water on the other. Two things go wrong. The air charge slowly permeates out through the diaphragm over years, leaving the vessel full of water with nothing to compress. Or the diaphragm perforates and water crosses to the air side, which waterlogs it immediately. Either way there is no cushion left, so the moment the cylinder heats, pressure spikes hard and the relief valve lifts to protect the cylinder. Cylinders that use an internal air bubble instead lose it differently: the air gradually dissolves into the water and gets drawn off at the taps until there is not enough of it left. A vented cylinder never does any of this, because the water it heats can expand straight back up the vent pipe into the tank in the loft, and that difference, unvented vs vented cylinder, is the whole reason one of the two has a safety valve that can weep and the other does not.

This shows up as a leaking relief valve rather than as anything else simply because the relief valve is the weakest deliberate point in a sealed system. It is doing exactly what it should. Fitting a new relief valve without addressing the expansion side buys a few weeks of dry tundish and then the same symptom returns.

Anything involving the expansion vessel, the inlet group or the relief valves on an unvented cylinder is work for a qualified engineer. These are sealed pressure systems and the safety devices are the only thing between a fault and a genuinely dangerous failure.

A seized or scaled relief valve that will not reseat

The other half of the diagnosis is the valve itself. A relief valve spends its whole life closed, held shut by a spring, with hard water sitting against it. That is close to ideal conditions for scale to build on the seat and the spindle, and for the valve to lose its ability to close cleanly once it has opened.

Two failure modes matter. In the first the valve seizes closed. It does not leak, it does nothing at all, and nobody finds out until the day the system genuinely needs it. That is why these valves are meant to be exercised at service: lifting the test lever briefly proves the valve can move and flushes the seat. In the second the valve has opened at some point, debris has passed over the seat, and the soft seal is now scored or held off its face. It weeps continuously from then on.

You can sometimes clear a valve that is holding a bit of grit by lifting the test lever and letting it snap shut, which flushes the seat under full flow. Expect a burst of water into the tundish, expect it to be hot if it is the temperature and pressure valve, and understand that if the dribble comes straight back the seat is damaged and the valve needs replacing rather than cleaning.

• Weeps constantly, never stops: seat scored or holding debris
• Weeps only after the system has been at high pressure: it did not reseat after lifting
• Bone dry but stiff on the test lever: seized closed, which is worse than leaking
• White crust on the body or spindle: scale, and the seat will be in the same state

When the mains pressure is the real problem

An unvented cylinder is protected on the cold side by a pressure reducing valve that drops the incoming mains to a working pressure the cylinder is rated for. If that reducing valve fails and starts passing full mains, everything downstream sees more than it was designed for, and a relief valve set at 6 bar can end up lifting on static pressure alone with no heating involved.

London mains pressure varies a great deal by street and by time of day, and it is common to find a supply that sits comfortably during the day running much higher overnight when demand drops. A property that has been fine for years can start lifting its relief valve after the water company works in the street, without anything in the house having changed.

The tell is that the leak has no relationship to heating. If the boiler and immersion have been off all day and water is still reaching the tundish, or the drip speeds up overnight and slows in the morning, the cold side deserves checking before anyone blames the cylinder. A stuck check valve produces the same result, and on most makes the strainer, reducing valve, check valve and expansion relief valve share one body, so the sensible repair is the whole inlet group cartridge rather than chasing individual parts.

If the temperature and pressure valve is the one lifting, switch off

The temperature and pressure relief valve is the last line of defence on an unvented cylinder and the only one of these valves that responds to heat rather than pressure alone. If it is discharging, the stored water has reached somewhere around 90 to 95 degrees, far above the 60 degrees a cylinder should sit at, and something has failed in the controls that were meant to stop the heating long before that point.

The chain of protection runs from the control thermostat, which switches the heat off at the set temperature, to the manual reset thermal cut-out, which trips if the control thermostat fails, to the temperature and pressure valve, which only acts if both have already failed. A discharging temperature and pressure valve therefore means two layers are gone. It is not a valve fault and must not be treated as one.

Switch off the boiler and the immersion, leave the valve and the discharge pipe alone, and do not reset the thermal cut-out. A cut-out that tripped did so for a reason, and resetting it puts the same uncontrolled heat source back on a cylinder that has already proved it cannot regulate itself. Hot water at the cold taps, banging or rumbling from the cylinder, and steam rather than water at the tundish all point the same way. Scalding water at the taps is the same fault seen from the other end, and our page on an overheating hot water cylinder sets out what to switch off and in what order.

A discharging temperature and pressure relief valve is the one symptom on this page that should not wait. Switch the heat off, keep people away from the cylinder and the discharge pipe, and get a qualified engineer to it.

Pressure relief valve dripping outside from the boiler

If the water comes from a small copper pipe ending on an external wall near the boiler, that is the boiler relief valve and the hot water cylinder is not involved. The heating circuit is a separate sealed system with its own expansion arrangement and its own valve, usually set around 3 bar.

The principle is the same: either the heating circuit is going over pressure and the valve is doing its job, or the valve has stopped sealing after a previous lift. The consequence differs, because a boiler valve that weeps is losing system water continuously. It often only drips after the heating has run, so it gets noticed as a random wet patch and ignored for months. A persistent damp stain or a limescale trail below a copper pipe stub on an outside wall means that valve has been passing for some time.

What you can check yourself before calling anyone

Some of this is genuinely householder territory, and there is no sense paying someone to establish what you can establish in ten minutes. What you can do is gather evidence, not carry out repairs.

Watch the timing. Run the hot water through one full heating cycle with someone watching the tundish, and write down when water appears and when it stops. That single observation separates an expansion fault from a pressure fault from a failed seat, and it is the first thing a good engineer will ask about.

Check the cylinder thermostat setting while you are there. If someone has wound it past 60 degrees to get more out of a tired system, the water expands more per cycle and the pressure peaks higher, which is enough on its own to start a marginal system lifting. Then photograph the data plate on the cylinder. It carries the make, model and working pressure, and knowing those before anyone attends means the right cartridge, vessel or valve is on the van instead of being ordered afterwards.

• Time the leak against a full heating cycle and write it down
• Note whether the water reaching the tundish is hot or cold
• Photograph the cylinder data plate and the inlet group
• Check the cylinder thermostat is set around 60 degrees
• Note any recent change: a boiler swap, new plumbing, work on the mains, an empty property

What an engineer does on arrival

The order matters, because testing out of sequence gives misleading numbers. The incoming mains pressure is gauged first, then the pressure downstream of the pressure reducing valve, and both are compared with the cylinder data plate. That settles immediately whether the cold side is the problem.

The expansion side comes next. With the water side isolated and the pressure released through a hot tap, the vessel is checked with a gauge at the schrader valve. A vessel that reads zero, or that spits water when the schrader is depressed, has failed and is replaced. On cylinders using an internal air gap the recharge procedure is carried out instead, isolating and draining down in the correct order so the air space can reform. Then both relief valves are exercised and inspected, the strainer is checked for debris, and any valve that will not reseat cleanly is changed rather than cleaned and hoped over.

Finally the cylinder is run through a full heating cycle with the tundish watched, because a system that behaves on a cold test can still lift the moment it heats a full tank. Nobody should leave until the thing that was leaking has been put back through the conditions that made it leak. Those are the same checks carried out at an annual unvented cylinder service, which is where a marginal vessel or a tired valve usually gets caught a season before it starts discharging. The rate is fixed before we attend and the total depends on how long the job takes, and a straightforward vessel or valve change on an accessible cylinder is not a long job.

All of that is ordinary unvented cylinder repair work: a diagnosis made with a gauge rather than a hunch, one or two known parts, and a retest under heat before anyone leaves the property. It is worth asking whoever attends whether they are a G3 qualified plumber before they start, because the expansion vessel, the inlet group and both relief valves on a sealed cylinder all sit inside the scope of that certification and none of them sit inside the scope of general plumbing.

Why this happens so often in London, and what ignoring it costs

Most of London runs on hard water, and that is why relief valves and expansion vessels here do not last as long as the same parts do elsewhere. Heat water past roughly 60 degrees and dissolved calcium comes out of solution onto whatever surface it touches. On a relief valve that means the seat, the spindle and the spring, all of which need to move freely and seal precisely. Scale on the seat gives the constant dribble. Scale on the spindle gives the opposite fault, a valve seized shut, which produces no symptom at all until the day it matters.

The housing stock is the other factor. Plenty of flats, particularly conversions above shops and in older terraces, run unvented cylinders in cupboards with the tundish and discharge pipework threaded through joinery and out through whichever wall was reachable. In those installations a weeping valve often shows up first as a damp patch in a neighbouring ceiling rather than as anything visible in the flat that owns the cylinder.

A drip does not flood anything and does not stop the hot water working, so it gets filed under jobs for later. The quieter costs are that you are paying to heat water that goes straight outside, that continuous flow across the seat accelerates the scaling until a valve that could have been cleaned has to be replaced, and that the fault underneath carries on developing. A waterlogged vessel takes the whole cylinder and its pipework up to relief pressure and back on every heating cycle, which is a fatigue cycle on every joint. And a valve that has wept for a year is one nobody can be confident will operate correctly when it is genuinely needed.