Overflow Pipe Dripping Outside: How to Tell Which Pipe It Is and What Has Failed

What an overflow pipe is for, and why it discharges where you can see it

An overflow pipe that drips outside is not itself the fault. It is a safety device doing exactly what it was installed to do, which is to take water somewhere harmless and, crucially, somewhere visible. Something upstream has failed, the discharge is the symptom, and identifying which pipe is dripping tells you most of what you need to know about how urgent it is.

The visibility is deliberate and it is a regulatory requirement. Under the Water Supply (Water Fittings) Regulations 1999, a storage cistern must have an overflow arrangement with a suitable means of warning of an impending overflow, and the regulations define a warning pipe as an overflow pipe whose outlet is positioned where the discharge of water can readily be seen. That is why it comes out through a wall at eye level over a path and not quietly into a gutter. The regulations also require these pipes to be screened against insects, which is why you will often find a small mesh or a screened outlet on the end.

The same principle runs through Approved Document G3 of the Building Regulations for unvented hot water cylinders, where the discharge from the safety valves passes through an open tundish that must be visible to the occupant, specifically so that a discharge can be diagnosed by looking at it. The whole design philosophy is that these systems should announce their own faults.

So when people ask whether they can just block it off, the answer is no, and it is not a technicality. Removing the warning is how a small fault becomes a collapsed ceiling.

Identify which pipe it is before anything else

Almost all useful diagnosis here comes from three observations: where the pipe emerges, what size and material it is, and whether the discharge is hot or cold. Go and look before you call anyone, because it changes who you need.

What you see outside	Almost certainly	What it is telling you
Plastic pipe, roughly 22mm, high up at eaves or loft level, cold water	Warning pipe from a cold water storage cistern	The float operated valve is not shutting off
Plastic pipe, roughly 22mm, at first floor level near a bathroom, cold	Older WC cistern or a feed and expansion cistern	A float valve fault, or an overheating vented system pushing water over
Small copper or plastic pipe, around 15mm, near a combi boiler, intermittent	Pressure relief valve discharge from a sealed heating system	System pressure or temperature has exceeded the valve setting
Copper pipe, 22mm or larger, discharging hot water, falling steadily	Discharge pipe from an unvented hot water cylinder	A relief valve has lifted. Treat this one seriously
Copper pipe near an outside wall by a boiler, dripping only in cold weather	Condensate pipe, not an overflow	Usually a freezing or blocked condensate run, not a water fault
Pipe at low level with continuous clear flow	Possibly not an overflow at all	Check for a burst supply pipe or a drainage fault before assuming
Pipe over a gutter or downpipe, dry, with staining below	An overflow that has discharged in the past	An intermittent fault that has not resolved itself

One detail worth knowing about toilets. Many WC cisterns installed since the water fittings regulations changed discharge internally into the pan rather than through an external pipe, which means a persistently running toilet in a modern property usually shows itself as water trickling down the back of the pan rather than as a drip outside. If you have an external overflow near a bathroom in an older property, a WC is still a strong candidate, and our guide on fixing a leaking toilet cistern covers the float valve and flush valve faults involved.

Hot or cold, and steady or intermittent

Two further observations narrow it further. If the discharge is warm or hot, you are looking at a heating or hot water system and not a cold water cistern, which moves the job from a plumber's float valve to a diagnosis of a sealed system or an unvented cylinder. If it is intermittent, appearing when the heating runs or when hot water is drawn and stopping afterwards, that timing is the single most useful piece of information you can give an engineer, because it points straight at expansion rather than at a valve that is simply stuck open.

Cistern faults: the float valve is not shutting off

A storage cistern in a loft, or a feed and expansion cistern serving an open vented heating system, fills through a float operated valve and stops when the float rises far enough to close it. When the cistern overflows continuously, that valve is not closing, and there are only a handful of reasons.

• A perished washer or diaphragm. The commonest cause on an older brass valve. It closes but not tightly, so the cistern creeps up over hours and then trickles out of the warning pipe.
• Debris on the seat. Grit from the mains holds the valve fractionally open. On a supply that has recently been worked on, this appears within days.
• A waterlogged or detached float. A plastic ball that has taken on water no longer floats high enough to close the valve at the right level.
• The valve set too high. The valve closes correctly but at a water level above the warning pipe, so the cistern overflows every time it fills fully. Common after a valve replacement.
• The wrong valve pressure rating. High pressure and low pressure float valves have different orifices, and a low pressure valve on a mains fed cistern can be forced open.
• Mains pressure that is genuinely too high. Worth checking, since it also shortens the life of everything else in the house. Our article on whether high water pressure is bad sets out the symptoms.

Why a feed and expansion cistern behaves differently

A feed and expansion cistern that overflows only when the heating has been running is a different fault again. That is usually the system pushing expanded water up the vent pipe and over into the cistern, often because the cistern was filled too high, or because of a pumping over problem where the circulator is forcing water up the open vent. Both are diagnosable, neither is fixed by turning the water off.

Combi boilers: the pressure relief valve has lifted

A sealed heating system is a closed loop of water that expands when it heats. An expansion vessel with a pressurised air cushion absorbs that expansion. A pressure relief valve sits as the last line of defence and opens if the system exceeds its set pressure, dumping water through a small discharge pipe that usually terminates outside.

So a drip from that pipe means one of three things, and the sequence of events tells you which.

Pattern of discharge	Likely cause	Usually confirmed by
Only when the heating is hot, pressure gauge climbing steeply on the run	Failed or flat expansion vessel, or one with the wrong charge pressure	Checking the vessel's air side pressure with the system drained of pressure
Continuous slow weep, gauge stable, no obvious pattern	Debris on the valve seat, or a valve that has been lifted once and not reseated	Inspecting and usually replacing the valve
Pressure creeping up when the system is cold and off	Filling loop passing, or a leaking plate heat exchanger on a combi	Isolating and removing the filling loop and watching the gauge
Discharge shortly after someone topped the system up	Simply overfilled	The gauge reading. Bleeding down to the normal cold fill pressure
Discharge with the system also losing pressure at other times	Two faults, or a valve that is both passing and being lifted	A pressure test on the circuit with the boiler isolated

Why the top up habit makes it worse

A system that repeatedly discharges and is repeatedly refilled through the filling loop is being given a continuous supply of fresh, oxygenated, uninhibited water, which is the recipe for the corrosion and magnetite sludge that produce the next set of faults. BS 7593 treats loss of system water as a trigger for checking and re dosing the inhibitor, and a system topped up a dozen times has almost certainly lost its protection. Our guides on a boiler losing pressure and repressurising a boiler correctly cover this properly.

Unvented hot water cylinders: the one to treat as urgent

An unvented cylinder stores hot water at mains pressure. It has layered safety devices: a thermostat, a thermal cut out, an expansion relief valve and a temperature and pressure relief valve. Under Approved Document G3 the discharge from these passes through a tundish, which must be visible, into a metal discharge pipe terminating in a safe place. The tundish exists so that a householder can see which device has operated.

Look at the tundish first

If you have an unvented cylinder and hot water is discharging, look at the tundish before anything else. Water at the tundish confirms a relief device has lifted rather than a pipe simply leaking, and that distinction is the whole diagnosis.

An expansion relief valve discharging usually means the expansion vessel has failed or lost its charge, which is common and not dangerous in itself. A temperature and pressure relief valve discharging is a different matter, because that valve is set to operate well above normal stored temperature, and it lifting means the primary thermostat and the thermal cut out have both failed. Stored water above its normal setting is a scald risk at every outlet in the building, so stop heating the cylinder and get a qualified engineer to it. Work on unvented systems is controlled under G3 and requires a competent person with the relevant qualification. Our article on a leaking hot water cylinder covers the wider set of cylinder faults.

Is a leaking overflow pipe dangerous?

The honest answer is that it depends entirely on which pipe it is, and the popular framing of the question is slightly wrong. The discharge is not the danger. It is the evidence that a safety device is carrying a load it was only designed to carry briefly.

Source	Immediate risk level	What makes it worse if left
Cold storage cistern warning pipe	Low in itself	Continuous metered water loss, saturated loft timbers, ceiling damage if the warning pipe blocks or the cistern fails
WC or older bathroom overflow	Low	Metered water loss and, in older arrangements, rot behind the cistern
Feed and expansion cistern	Low to moderate	Continual dilution of inhibitor, corrosion, and an underlying pumping over or overheating fault
Heating pressure relief valve	Moderate	Repeated refilling destroys corrosion protection, and the underlying expansion fault does not self correct
Unvented cylinder expansion relief valve	Moderate	Failed vessel, wasted hot water, and progressive stress on the remaining safety devices
Unvented cylinder temperature and pressure relief valve	High. Treat as urgent	It indicates temperature control has failed. Scald risk at every hot outlet
Any discharge running down a wall in freezing weather	Moderate, regardless of source	Ice on a path or step, and saturation of the wall below the outlet

There is also a slower cost that applies to all of them. On a metered supply, a cistern overflowing continuously is water you are paying for, and it can move a bill noticeably without ever causing visible damage, which is why an unexplained bill rise is worth checking against the overflows. Our article on why a London water bill is suddenly high goes through the checks.

Where this diagnosis commonly goes wrong

A dripping overflow looks like a simple fault and is routinely misread, usually in one of these ways.

• Mistaking a condensate pipe for an overflow. A condensing boiler produces condensate continuously while running, and the small plastic pipe carrying it outside drips as a matter of normal operation. What is not normal is it freezing and backing up in cold weather, which stops the boiler.
• Assuming a stopped drip means a fixed fault. Intermittent discharge is the signature of an expansion problem. It stopping because the heating went off is not a repair.
• Replacing the relief valve and nothing else. If the valve lifted because the expansion vessel had failed, a new valve simply lifts again, usually within days.
• Adjusting the float down to below the warning pipe. This stops the symptom and leaves you with a valve that still does not shut off, and a cistern that now overflows whenever demand is low.
• Blocking, capping or extending the pipe into a drain. It removes the warning the regulations require, and on an unvented cylinder a discharge pipe that does not comply with G3 is a safety issue in itself.
• Treating any hot discharge as a cylinder fault. It can equally be a heating relief valve. The pipe size, the route and the presence of a tundish distinguish them.
• Turning the water off and leaving it. On a cistern this works as containment. On an unvented cylinder or a sealed heating system it disables the system without addressing why a safety device operated.

What to do now, in order

First, look at the pipe and work out which one it is, using the size, height, material and whether the water is hot. Second, if the discharge is hot and there is a tundish, stop heating that cylinder and treat it as a job for a qualified unvented engineer. Third, if it is a cold cistern warning pipe, you can isolate the supply to that cistern at its service valve, which stops the discharge and lets you use stored water until an engineer arrives, though it will also leave you without stored cold water once the cistern empties.

Fourth, note the pattern. Whether it is continuous or only when the heating runs, whether the boiler gauge moves, whether anyone has recently topped the system up, and whether it started after any other work. That is the information that shortens the visit.

And fifth, put a bucket or a container under it and see how much comes out in an hour. A pipe that fills a container is a different problem from one producing a drip every few seconds, and it is the fastest way to tell someone on the phone how bad it actually is.