A pipe dripping on an outside wall is an overflow doing its job, and the fault is indoors. White plastic at eaves level is a loft tank; behind a bathroom it is a toilet cistern. Copper low down near the boiler is the pressure relief valve. Plastic drips can wait days. A constant copper drip needs an engineer.
You have spotted a drip from a small pipe poking through the render, maybe with a dark streak down the wall beneath it, and you are not sure whether it matters. Most Cornish homes have three or four of these pipes, each connected to something different inside, so the first job is working out which one is talking to you. This guide sorts them by position and material and separates the ones that can wait until next week from the one that should not.
Read the pipe before you go looking indoors
Stand back and note two things: how high up the wall the pipe comes out, and what it is made of. Between them they narrow the source down before you have opened a cupboard.
| Where it exits | Material | What it is connected to | How urgent |
|---|---|---|---|
| Under the eaves or close to the roofline | White plastic | The cold water storage tank or the small feed and expansion tank in the loft | Fix within days. It wastes water and keeps the wall wet |
| Upstairs, behind or beside a bathroom | White plastic | A toilet cistern, on older installations that overflow through the wall | Fix within days |
| Low down, near where the boiler sits inside | Copper, small bore, open end pointing at the wall | The boiler’s pressure relief valve discharge | An occasional drip after a top-up is common. A constant drip needs an engineer |
| Below a cylinder cupboard, sometimes steaming | Copper, larger bore, fed through a tundish indoors | An unvented hot water cylinder discharge | A brief trickle while heating can be normal. A continuous flow needs an engineer promptly |
If the position leaves you unsure, use elimination. Flush each toilet in turn and watch the pipe for a minute or two, then run a hot tap, then turn the heating on from cold. Whichever action brings the drip on tells you which system it belongs to, and you have not been near the loft hatch.
The white pipe at the eaves: a loft tank
Older properties across Cornwall, including many granite and cob cottages plumbed decades ago, still run on gravity. A large cold water storage tank in the loft feeds the bathroom, and a much smaller feed and expansion tank tops up an open-vented heating system. Each has a float valve and its own overflow out through the wall near the roofline.
When a valve stops closing, the tank fills past the overflow and out it goes. The usual reasons are ordinary wear: a hardened washer, a cracked float that has filled with water and can no longer lift the arm, or grit from the mains holding the valve part open. Cornwall’s soft water means limescale is rarely the culprit.
If the small feed and expansion tank only overflows while the heating is running, tell the engineer, because the heating may be pushing water up into it rather than the float valve failing. Our guide to a cold water tank in the loft that keeps overflowing covers the loft-side checks and how to do them safely.
The white pipe behind the bathroom: a toilet cistern
This is the most common overflow of all, and the easiest to confirm. Lift the cistern lid: if the water sits at or above the top of the internal overflow tube, the fill valve is not closing. You will often hear it too, a faint hiss that never stops, or a thin trickle down the back of the pan between flushes.
Modern toilets usually overflow into the pan rather than through the wall, so a pan that is always wet with nobody using it means the same thing. The fix is small: close the isolation valve, adjust the float down a touch, and if that does not hold, fit a new fill valve. Do not leave it, though. A running cistern on a metered supply shows up on the South West Water bill, and it is one of the first things we ask about when someone rings because their water bill has jumped.
The copper pipe near the boiler: the pressure relief valve
A combi or system boiler runs as a sealed circuit with a pressure gauge on the front. Cold, it should read about 1 to 1.5 bar. If pressure climbs well above that, a pressure relief valve opens and releases water through a small copper pipe that ends outside, low down, near the boiler. A few drips after a top-up are not unusual. A drip that never stops comes down to one of three things.
- The filling loop was left slightly open. Mains water keeps feeding in, pressure creeps up, and the valve lifts. Check the small levers on the filling loop are fully closed.
- The expansion vessel has lost its charge. The vessel absorbs the extra volume of water as it heats. Once its air charge has gone, pressure spikes with each heating cycle and the valve opens each time. A drip that appears only when the heating is running is the classic sign.
- The valve seat has worn. Grit or age stops it sealing, so it weeps even at normal pressure. The valve needs replacing.
Never cap or plug this pipe. It is how the boiler releases pressure safely, and work on the valve, vessel or filling loop belongs to a Gas Safe registered engineer. If the pipe drips and the gauge also keeps falling between top-ups, the system is losing water elsewhere too, and our article on why a boiler keeps losing pressure explains what that usually means.
The larger copper pipe from a cylinder cupboard: an unvented cylinder
A mains-pressure hot water cylinder has its own safety valves and discharge pipe. Inside the cupboard the pipe passes through a small funnel called a tundish, so you can see water passing, then runs outside in copper.
A little water through the tundish while the cylinder heats can be normal, because the water expands. A steady flow, or water hot enough to steam at the outlet, is not. It usually means the expansion vessel needs recharging or a relief valve has failed. Unvented cylinders may only be worked on by someone qualified to do so, so this is a phone call rather than a spanner job. Keep children and pets away from the outlet in the meantime, because the water can scald.
Which ones can wait and which cannot
- A slow drip from a plastic pipe. Book a repair this week. Nothing is about to fail.
- A plastic pipe running freely. Sooner. A valve stuck wide open puts a surprising volume through in a day, and in a cold snap the water ices the path below.
- An occasional drip from the boiler’s copper pipe. Check the gauge and the filling loop, then book an engineer if it continues.
- A constant drip, or hot water, from a copper pipe. Get a qualified engineer out promptly. The fault behind the valve will not fix itself.
Holiday lets deserve a mention. An overflow that starts running in an empty cottage in Fowey or Port Isaac can run for weeks before anyone notices the wet wall. Ask whoever does the changeovers to glance up at the pipes on each visit.
When the drip is hiding a bigger problem
Most of the time, fixing the valve fixes everything. A few signs should make you look harder: a wall that stays damp inside after the overflow has stopped, damp spread wider than one drip could cause, a water bill up by more than a running cistern would explain, or a drip you cannot match to a tank or cistern at all, because the water may be tracking along the pipe from a leaking joint inside the wall.
That is where guessing stops being useful. Our water leak detection survey uses acoustic listening, thermal imaging and tracer gas to trace where the water is really coming from before anything is opened up, and the written report is what your insurer will ask for if the damp turns into a claim. We provide leak detection across Cornwall, and a description of the pipe over the phone is usually enough to tell you who you need.
Frequently asked questions
Is an overflow pipe dripping outside an emergency?
Rarely. A slow drip from a white plastic pipe can wait a few days for a repair, though it wastes water and keeps the wall wet. A constant drip from a copper pipe, or hot water at the outlet, is more pressing and should be looked at by a qualified engineer as soon as you can arrange it.
How do I know which overflow pipe is dripping?
Use the height and the material. A white plastic pipe near the roofline comes from a loft tank, one behind a bathroom from a toilet cistern, and a copper pipe low down near the boiler from the pressure relief valve. Flushing each toilet and running the heating in turn confirms which system brings the drip on.
Why does the copper pipe by my boiler only drip when the heating is on?
That timing usually points to the expansion vessel. It is meant to absorb the extra volume as the water heats, and when it has lost its air charge the pressure spikes on each cycle and the relief valve opens. A Gas Safe engineer can recharge or replace the vessel and check the valve.
Can I cap the overflow pipe to stop the drip?
No. The pipe is a safety outlet. Capping a tank overflow sends the next overfill through your ceiling, and capping a boiler or cylinder discharge removes the only safe route for excess pressure. Fix the valve that is letting the water through instead, and leave the pipe exactly as it is.
Not sure which pipe it is?
Describe the pipe to us and we will tell you honestly whether it is a plumber’s job, a heating engineer’s job or a hidden leak. If it is a hidden leak, we find the source without opening up the house and give you a report your insurer can use.
