Start at the wrong end and you will get it wrong
Almost everyone diagnoses this pipe from the garden. You see water on a wall or a patio, you look up, and you decide it is "the overflow". That guess is wrong often enough to matter, because four completely different pipes exit a British house near ground level or at eaves height, and one of them is a warning.
The outside end tells you very little. Copper and plastic are both used for relief discharges, cistern overflows and condensate pipes are both white plastic, and the diameters overlap. What identifies the pipe is where it starts, and that takes five minutes indoors with a torch.
This page is about telling them apart. If you already know which pipe you have and you want to know how serious it is, the danger question is answered separately.
The four pipes, by where they begin
| Starts at | It is | Should it ever run? |
|---|
| A white plastic funnel fitted below the valves on a sealed hot water cylinder | The D2 discharge from an unvented cylinder | Almost never. Any discharge means a fault has occurred. |
| Inside the boiler casing, usually 15mm, leaving through the outside wall | The boiler pressure relief valve discharge | Rarely, and briefly. A recurring drip is a fault. |
| A boiler, in smaller plastic pipe, running to a drain, gully or soakaway | The condensate | Yes. Continuously while the boiler runs. Normal. |
| A toilet cistern, a loft storage cistern or a feed-and-expansion tank | An overflow (warning pipe) | No, but it is a float valve fault, not a safety event. |
The tundish is the tell
If the property has an unvented hot water cylinder — a tall sealed cylinder, no tank in the loft, hot water that comes out of every tap at the same pressure as the cold — there is a small white or clear plastic funnel in the pipework near it, usually within 600mm of the valves. That is the tundish. It is an air break, and it is deliberately visible.
It exists so that discharge can be seen. Under Part G of the Building Regulations an unvented cylinder is protected by relief valves discharging through a tundish where the discharge is visible, then out through a D2 pipe to a safe place outside. A cylinder in trouble announces itself at a funnel you can look at, rather than quietly building pressure.
So before you go outside again, look at it:
- Bone dry, no limescale, no staining. The cylinder is not the source. Whatever is dripping outside belongs to another pipe.
- Wet, or water visibly running through it. A relief valve on the cylinder is open. That is your dripping pipe outside, and it is the one that matters.
- Dry now, but scaled, stained or crusted inside. It has discharged repeatedly in the past. Intermittent discharge is still a fault; it is just a fault you have been missing.
There are two valves feeding that tundish and they mean different things. The expansion relief valve opens because pressure got too high. The temperature and pressure relief valve opens because the stored water got too hot — and that one only operates when the cylinder's thermostat and its overheat cut-out have both failed to stop the heat input. If the water leaving the tundish is hot, stop reading and treat it as urgent: switch off the immersion at its spur and the boiler at the programmer, and get a G3-qualified engineer to it.
Why the drip comes after hot water use, not during
The commonest unvented complaint is a discharge that appears twenty minutes after somebody has had a bath, then stops. It looks random. It is not.
Water expands as it heats. On a sealed cylinder that expansion goes into either an internal air bubble or an external expansion vessel. When the bubble is lost or the vessel loses its charge there is nowhere left, so pressure climbs as the cylinder reheats until it exceeds the expansion relief valve setting. The valve opens, sheds a small volume, and shuts — which is exactly what a reheat after a draw-off looks like.
The same mechanism explains a symptom people report separately without connecting it: hot water that spits or hammers on first opening. That is a vessel recharge or replacement, not a leak-detection job.
The distinguishing pattern, then, is timing. Discharge tied to hot water reheating points at the cylinder. Discharge tied to the heating coming on points at the boiler. Discharge with no pattern at all, running day and night, is usually a valve that has picked up debris and will not seat, or an overflow.
The boiler relief valve, which is a different pipe
A sealed heating system has its own relief valve inside the boiler casing, with its own discharge pipe through the outside wall. Vaillant describes it as a safety device releasing pressurised water out of the system when pressure gets too high, typically above 3 bar.
The giveaway is the pressure gauge. If it climbs high while the heating runs and then sits below normal once everything has cooled, water has been shed and the system is short. Repeat that weekly and you are putting fresh oxygenated mains water into the pipework every week, which corrodes it from the inside.
The usual causes are a failed expansion vessel, a filling loop left open, overfilling, or a valve seat that has picked up debris. Only the filling loop is yours to check — the rest is behind the casing, which is Gas Safe territory. If the discharge pipe is dry and the pressure still drops, the loss is elsewhere, which is the case detection is actually for.
Condensate: the one that is supposed to drip
A condensing boiler produces water as a by-product of burning gas efficiently, and that water leaves through the condensate pipe — smaller plastic pipe, usually white or grey, running to a drain, gully or soakaway rather than ending in open air. Three things separate it from everything else here:
- It terminates at a drain. Relief discharges terminate in free air at a safe place.
- It runs constantly while the boiler is firing. Relief discharges should be dry.
- The water is cool and slightly acidic, which is why it stains and etches paving over time.
The only condensate fault worth knowing is freezing. In a cold snap an external run can ice up, the boiler cannot clear its condensate, and it locks out — no heating, no hot water, often a fault code and a gurgling noise. That is a thaw, and a lagging job if it happens every winter. It is not a leak.
What the drip is costing you
A relief valve on a cylinder or boiler discharges mains water you have already paid for — and, in the cylinder's case, water you have already paid to heat. A cistern overflow does the same thing continuously. If you are metered, every litre leaving that pipe is on your bill, and none of it did any work.
You can size it yourself without guessing. Put a measuring jug under the pipe, time how long it takes to collect a known volume, and scale it up. A drip that fills a one-litre jug in ten minutes is six litres an hour, and it does not stop overnight. Then confirm it at the meter: turn everything off, read it, wait an hour using nothing, read it again. Movement with everything off means something is running, and the meter test is the evidence that turns a vague suspicion into a number.
Be honest about scale, though: an intermittent discharge after a bath is pence, while a cistern overflow running unattended for a quarter is not. The reason to fix either is rarely the water — it is that the discharge is the only symptom of a failed safety component, and the component does not repair itself.
Whichever pipe it turns out to be, do not cap, plug or tape it, and do not shorten or re-route it. Length, fall, bend count and diameter on a D2 are specified, and getting them wrong can stop the valve discharging when it needs to.
When this is not a job for us
Most of what brings people to this page is not leak detection, and we would rather say so than attend. A float valve is a part and twenty minutes. A boiler PRV or expansion vessel is a Gas Safe engineer. An unvented cylinder and its relief valves are G3 work. A frozen condensate is a jug of warm water.
What we are for is the case where the pipe outside does not explain everything: the tundish is dry and the pressure still falls, the discharge has stopped and the meter still turns, or there is damp that no overflow accounts for. That is a trace — pressure test first to prove the loss is real, then thermal imaging and tracer gas on a drained circuit — at £150 per hour agreed before we attend, no call-out fee, and nothing to pay for the search time if we do not find it. Where the leak sits under a floor or behind plaster, the visit includes a written report prepared to support a trace-and-access claim; what your policy covers is your insurer's decision, not ours.
If you are unsure which pipe you have, tell us what it does and when. What we charge is published, and we will say on the phone if the answer is a plumber, a Gas Safe engineer, or nobody at all.
