Identify the pipe before anything else
People use "overflow pipe" for four different pipes that mean four different things. Height, material and water temperature tell you which one you have.
| Where it comes out | What it is | Urgency |
|---|
| White plastic, roughly 22mm, at first floor or ground level near a bathroom or cloakroom | Toilet cistern overflow. The float valve is not shutting off. | Low. Wasteful, not dangerous. |
| White plastic at eaves or roof level | The cold water storage cistern or the feed-and-expansion tank in the loft. | Low to medium. Check the loft — a loft overflow that stops running may mean the tank is now spilling inside instead. |
| Copper, around 15mm, from near the boiler, ending on the wall or turned down near the ground | The boiler pressure relief valve discharge. | Medium. The system is shedding water every time it fires. |
| Larger metal pipe, water hot, from a property with an unvented hot water cylinder | The cylinder's temperature and pressure relief discharge. | High. Treat as urgent. |
The fastest test is temperature, carefully and from the side: cold and clear points at a cistern or a tank, hot points at a cylinder or a boiler valve, and hot is the one that matters.
The cistern or tank drip: yours to fix
A toilet cistern or loft tank overflows because the float valve is not closing. The float has filled with water and sunk, the washer or diaphragm has perished, grit is holding the valve off its seat, or the level has simply been set too high. Any of those is a genuine DIY job.
Turn off the service valve on the feed to that cistern — a small in-line valve with a screwdriver slot — flush, and look. Adjusting the level down so it sits below the overflow, or replacing the diaphragm washer, fixes most of them. Parts are ordinary merchant stock.
Leave it and you are paying for water that goes straight outside. If you are metered, a continuously running overflow is visible on the meter with everything else off, which is worth knowing before you dismiss it — see reading the meter to prove a leak. In freezing weather there is a second problem: a steady drip onto a path or a step makes ice, and that is the way this particular fault actually hurts someone.
The boiler relief valve: not yours
A 15mm copper pipe running from the boiler through the wall is the pressure relief valve discharge. Vaillant describes the PRV as a safety device that releases pressurised water out of the system when pressure gets too high, typically above 3 bar, discharging to a pipe that ends outside the house. Occasional discharge is normal; a constant or heavy drip means the system is repeatedly reaching pressures it should not.
What causes it, in rough order of frequency:
- A failed expansion vessel. The vessel has lost its air charge, so the water has nowhere to expand into when the heating fires. Pressure spikes hot, the valve lets water out, and the gauge then reads low cold. That pattern — high when hot, below normal when cold — is close to diagnostic.
- A filling loop left open, letting mains pressure creep into a sealed system.
- Overfilling after a repressurisation.
- A PRV whose seat has picked up debris and will no longer close, so it weeps continuously at normal pressure.
Only the filling loop is something you can deal with yourself: check it is closed and detached. The rest sits behind the boiler casing, and taking a boiler casing off is gas work restricted to Gas Safe registered engineers. Vaillant's own guidance on boiler leaks says the same in plainer terms — do not attempt it yourself.
Meanwhile the system is losing water and being topped up with fresh, oxygenated mains water, which corrodes it from the inside. If you are refilling often, the underlying question is whether the loss is the valve or something else entirely; the boiler pressure hub works through that, and a gauge that drops with nothing visible is the case where detection earns its keep.
The unvented cylinder discharge: the urgent one
If the property has an unvented hot water cylinder — a sealed white cylinder, no tank in the loft, mains-pressure hot water at every tap — it has a tundish and a discharge pipe fed by a pressure relief valve and a temperature relief valve. Under Part G3 of the Building Regulations these are safety devices, the discharge has to be visible at the tundish, and the discharge pipework exists so that a cylinder in trouble can dump safely rather than fail.
Hot water discharging from that pipe is not a plumbing annoyance. It can mean the expansion vessel has failed, or it can mean the cylinder's thermostat and its energy cut-out have both failed and the stored water is overheating. Turn off the heat source to the cylinder — the immersion at its switched spur, and the boiler at the programmer — and get a qualified engineer to it. Do not wait to see whether it settles down.
Three rules that apply to every discharge pipe on this page, and to that one above all:
- Never cap it, plug it or block it. It is the escape route.
- Never extend or reroute it yourself. Length, fall and diameter are all specified for a reason.
- Never assume it is just condensation. Condensate from a boiler runs continuously in cold weather and is a different, smaller white plastic pipe going to a drain, not a relief discharge.
So: dangerous, or not?
The drip is a symptom of a safety device operating. The danger is in the two things behind it — a cylinder overheating, and the slow one, where a fault is left running for months because it looked trivial. A cistern overflow is money. A relief valve discharge is a system quietly emptying and refilling itself. A hot discharge is a today problem.
When this is not our job
Most of what brings people to this page is not a leak detection job and we will tell you so. A float valve is a part and twenty minutes. A PRV or expansion vessel on a gas boiler is a Gas Safe engineer's call, not ours. An unvented cylinder is G3 work.
Where we come in is when the overflow turns out not to be the whole story: the pressure still drops with the discharge pipe dry, the meter still turns with everything off, or there is damp somewhere that no overflow explains. Then it is a trace — pressure test, thermal imaging, tracer gas on a drained circuit — at £150 per hour agreed before we attend, with nothing to pay for the search time if we do not find it. Tell us what the pipe is doing and we will say whether it is worth us coming.
