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Overflow Pipe Dripping Outside
A small plastic pipe through an outside wall is dripping or running, sometimes soaking the brickwork below.
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What's happening?
An overflow is a warning device, not a fault in itself. It is fitted to anything that holds water at a controlled level — a cold water storage tank in the loft, a feed and expansion tank, an older toilet cistern — and it discharges outside so that a failed filling valve announces itself before the tank floods the house. Water coming out of one means a float valve somewhere has stopped shutting off. Which pipe is running tells you which tank: a small-bore pipe high on the wall is usually a loft tank, one at bathroom level is usually a cistern. A separate discharge from an unvented hot water cylinder's tundish is a different and more urgent matter.
What usually causes it?
- Float valve washer perished in a loft cold water tank, so the tank keeps filling past its level
- Float valve arm set too high, or a float that has taken on water and no longer rises
- Grit or scale on the float valve seat holding it fractionally open
- Old-style toilet cistern with an external overflow and a failed inlet valve
- Feed and expansion tank overfilling because the heating system is pushing water back into it
- Excessive mains pressure forcing past a valve that would otherwise close
Is it dangerous?
The escaping water is clean, but it is a signal that a tank in your loft is at or over its safe level with only a pipe stopping it flooding the ceiling — which is why it should not be left running for weeks. The saturated wall underneath is the other cost: water running down brickwork in winter freezes, spalls the face of the brick and eventually shows as damp inside. A feed and expansion tank overflowing is a separate warning that heating system water is being pushed where it should not go, and that needs looking at rather than just topping up.
Can I fix it myself?
Identify the tank before touching anything. In the loft, lift the tank lid and look at the water level against the overflow outlet — if it is up to it, the float valve is the problem. Lifting the float arm by hand should stop the flow; if it does, the valve washer or the level setting is at fault, and if it does not, the valve itself has failed. Turning off the supply to the tank at its own gate valve stops the discharge while you arrange a repair, but the tank then feeds nothing, so on a gravity system you may lose hot water and upstairs cold.
When should you call a plumber?
Call a plumber promptly — this is a same-week job, not one to leave for the season. Call urgently if the overflow is running continuously, if the loft tank shows any sign of overflowing at its rim, or if the discharge is from an unvented hot water cylinder's tundish rather than a tank overflow, because that means a safety valve on a pressurised cylinder has lifted.
What will the plumber actually do?
- Trace the overflow pipe back to the tank or cistern that is actually discharging
- Replace the float valve or its washer and set the shut-off level correctly below the overflow
- Check the tank for scale, debris and a waterlogged float, and confirm the lid and insulation are right
- Check whether the feed and expansion tank is being pushed up by a heating system fault rather than a valve fault
- Confirm the overflow itself is correctly sized and falls away from the building
- Check mains pressure and advise on a pressure-reducing valve where it is forcing valves open
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