Overflow Pipe Dripping or Running Outside Your London Home — Causes & How to Fix It (2026)

Almost everyone spots it the same way: a wet patch on the outside wall, a steady drip onto the path, or the sound of water trickling somewhere it shouldn't be. An overflow pipe running outside is one of the most common calls we get across London, and the good news is that the pipe itself is rarely the problem. It is a warning device, doing exactly what it was designed to do — telling you that a valve inside the house has stopped shutting off. This guide explains what these pipes are, how to work out which one is discharging, which cases are urgent, what you can check yourself, and what a repair costs in London in 2026.

What an Overflow or Discharge Pipe Actually Is

Two different jobs get lumped under "overflow pipe." The first is a warning (overflow) pipe: a length of pipe fitted to any cistern that holds water — a WC cistern, a cold-water storage tank — that carries water safely away if the valve filling the cistern fails to close. It usually terminates somewhere visible outside precisely so that a discharge catches your eye. If it is running, the fill valve isn't sealing.

The second is a safety-relief discharge pipe, and it belongs to pressurised hot-water equipment: an unvented (mains-pressure) hot-water cylinder such as a Megaflo, or a combi or system boiler. These carry a relief valve that opens if pressure or temperature climbs too high, and the water it releases is piped, via an air-break funnel called a tundish, to a termination outside — normally low down on the wall, near the ground or over a gully. This pipe should only ever discharge rarely; if it is running, something has gone wrong with the pressure control or, worse, the temperature control.

Telling the two apart is the whole game, because the fixes and the urgency are completely different. The single most useful test costs nothing: is the water hot or cold? Cold water points to a cistern or a storage tank. Hot or warm water points to a cylinder or a boiler relief valve — the case to take seriously.

The Four Things That Drip Outside — and How to Tell Them Apart

In a typical London home, an outside discharge comes from one of four places. Work down this list and you can usually name the culprit before an engineer arrives:

• A WC cistern. Cold water. Older toilets have a small (typically 22mm plastic) overflow pipe out through the wall behind the cistern; most modern cisterns overflow internally, so instead of an outside pipe you see a constant trickle running down the inside of the pan and hear it never quite stop.
• A cold-water storage tank in the loft. Cold water, and usually the largest, highest discharge — a warning pipe emerging under the eaves or high on the gable, often dripping steadily. Common in period London houses and conversions that still run a traditional gravity-fed (vented) hot-water system with a tank in the loft.
• An unvented hot-water cylinder. Hot or warm water, from a copper pipe low on the wall near a bathroom or airing cupboard. Inside, it runs from a tundish under the cylinder. This is the higher-stakes case.
• A combi or system boiler. Hot or warm water, from a 15mm copper pipe that exits close to the boiler (often just outside the kitchen or utility). This is the boiler's pressure-relief (safety) valve letting go, almost always because the system pressure has climbed too high.

1. A WC Cistern Overflow

This is the most common and the most fixable. A cistern fills through a fill valve (the modern successor to the old brass ball-valve), which is meant to shut off when the float rises to the set level. Two failures make it overflow. Either the fill valve won't close — a perished diaphragm washer, grit or limescale on the seat, or a float set too high — so the cistern overfills until it spills into its overflow. Or the flush valve at the bottom of the cistern is leaking: its seal has perished, water dribbles past it into the pan, the level drops, and the fill valve dutifully keeps topping the cistern up, so it runs continuously even though the fill valve is "working."

The tell between the two: if you lift the lid and the water is sitting above the overflow level, the fill valve is at fault; if the water sits at the correct level but you can see a faint ripple where water is trickling out of the cistern into the pan, the flush valve seal has gone. In London's hard-water boroughs — most of north and east London draws hard water from the Lee Valley and the chalk — these rubber parts scale up and harden within a few years, so replacing rather than fiddling is usually the durable fix. It is a job a confident homeowner can do (see the DIY section below); the awkward ones are concealed cisterns and wall-hung frames, where getting at the valve means removing a panel.

2. A Cold-Water Tank Overflow in the Loft

Plenty of London homes — especially Victorian and Edwardian terraces and mansion-block flats that haven't been converted to a combi or unvented system — still have a cold-water storage tank in the loft feeding the hot cylinder and, often, the bathroom cold taps. It fills through a float-operated valve, and it has a large warning/overflow pipe (usually 22mm) running out through the eaves or the gable wall. When that pipe drips or pours, the float valve has failed to shut off.

The usual reasons are a limescaled valve seat that won't seal, a worn valve washer, or a waterlogged float — the plastic ball has cracked, filled with water and sunk, so it never rises enough to close the valve. Because the tank sits directly above the ceilings, this one is worth acting on reasonably quickly: as long as the warning pipe stays clear the water goes safely outside, but if that pipe is itself partly blocked (a wasps' nest, debris, a kinked end) the tank can overtop and come through the ceiling instead. Replacing a float valve and checking the tank over — lid, insulation, byelaw screening — is a short job, and it is a good moment to confirm the tank still meets the Water Fittings Regulations (a close-fitting lid and screened warning/vent pipes to keep contaminants and insects out).

3. An Unvented Cylinder or Boiler Discharge

This is the case to respect. An unvented cylinder (Megaflo, Telford, Joule and the like) stores hot water at mains pressure, so it must safely absorb the expansion of water as it heats. It does that with an expansion vessel — a sealed chamber with an air cushion — backed up by two relief valves: an expansion (pressure) relief valve and a temperature and pressure relief valve. If pressure or temperature exceeds the safe limit, a valve opens and dumps water through the tundish to the outside discharge pipe.

By far the most common reason for that pipe to discharge is a failed or waterlogged expansion vessel: it has lost its air charge, so it can no longer take up the expansion, pressure spikes every heating cycle, and the expansion relief valve weeps or runs. That is a routine (if G3-qualified) repair — recharge or replace the vessel, check the pressure-reducing valve and the relief valve seats. The far rarer but serious scenario is overheating: if the cylinder thermostat and its high-limit safety cut-out have both failed, the water keeps heating and the temperature/pressure relief valve discharges scalding water to protect the cylinder. Any discharge that is genuinely hot (not just warm) and continuous should be treated as urgent — turn off the immersion and the boiler/heating to the cylinder and call an engineer the same day.

A boiler tells a similar story on a smaller pipe. A combi or system boiler has its own expansion vessel and a pressure-relief (safety) valve piped outside. If the gauge is reading high (well above the normal 1–1.5 bar cold) and the relief pipe is dripping, the boiler's internal expansion vessel has usually failed or the system has simply been over-filled during a top-up. Either way the safety valve is doing its job; the fix is to sort the pressure control, not to cap the pipe. Both unvented cylinders and boilers are jobs for a qualified engineer — the cylinder because it is Building Regulations G3-notifiable work, the boiler because it is a sealed pressurised (and, on the gas side, a Gas Safe) appliance.

Is an Overflowing Pipe an Emergency?

Rank it by two things — temperature and flow. A slow drip of cold water from a WC or loft-tank warning pipe is not dangerous, but it wastes water, soaks brickwork, and in a cold snap freezes into an icy hazard on the path; fix it within a few days. A steady run or a gush of cold water means a valve has failed fully open — deal with it promptly, and with a loft tank check the warning pipe is clear so it can't overtop the ceiling. Hot water discharging from a cylinder or boiler relief pipe is the one to act on same-day: at best it is a failed expansion vessel wasting hot (metered, heated) water; at worst it is an overheating cylinder. If in doubt, isolate the appliance and call.

You can nearly always stop the waste yourself while you wait. For a WC or a fed appliance, close the small isolating valve on its supply pipe. For a loft tank, tie up the float arm to hold the valve shut, or turn off the main stopcock. For an unvented cylinder discharging hot, switch off the immersion and the boiler feed to it. If you can't find or turn the stopcock, our guide on isolating the water in a London flat walks through the alternatives.

What You Can Check Yourself First

Before you call, a five-minute look often names the fault and saves a diagnostic visit:

• Feel the water. Hot or warm = cylinder or boiler (skip to isolating the appliance and calling an engineer). Cold = cistern or tank.
• Check the toilets. Lift each cistern lid. Water above the overflow level = fill valve; correct level but a trickle into the pan = flush valve seal. Close the isolating valve to stop it running.
• Look in the loft (if you have a tank). A dripping warning pipe outside plus a tank filling and not stopping = float valve. Gently lift the float by hand — if the water stops, the float or its washer is the fault.
• Read the boiler gauge. If a small pipe near the boiler is dripping and the gauge reads well above 1.5 bar, the pressure is too high — bleed a radiator to bring it down as a stop-gap and book a check of the expansion vessel.
• Note how fast. A drip buys you days; a steady stream doesn't. Isolate and call sooner for anything running freely, and same-day for anything hot.

What not to do: never cap, block or reduce a warning or relief pipe to "stop the drip." On a cistern or tank you'd simply move the flood indoors; on a cylinder or boiler you'd defeat a safety device, which is dangerous and illegal. The pipe is the messenger — fix the valve it's pointing at.

What Repairs Cost in London (2026)

The common overflow faults are among the cheaper plumbing jobs, usually a single visit with the part on the van.

Prices assume reasonable access and daytime hours; a concealed cistern, a cylinder buried in a tight airing cupboard, or an out-of-hours call-out will sit at the higher end. For the current call-out charge and typical job costs see our pricing page, and if the discharge is hot or the water is running freely, our emergency plumbers cover all 32 boroughs. If you'd rather we diagnose it properly the first time — particularly on an unvented cylinder — ring 0207 046 1363 or WhatsApp 07456 975436 for a fixed quote before we attend.