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uk Home & property emergencies overflow pipe running • outside pipe leaking water • constant overflow outside wall • discharge pipe dripping nonstop • water pouring from overflow • overflow pipe won’t stop • outside discharge won’t stop • toilet overflow pipe outside • toilet keeps filling running • loft tank overflow pipe • cistern overflow won’t stop • tundish dripping continuously • unvented cylinder discharge • boiler pressure relief pipe leaking • pipe leaking outside house • mystery pipe dripping outside • hot water cylinder safety discharge • cold water tank overflow

What to do if…
water is running from an outside overflow or discharge pipe and won’t stop

Short answer

Don’t block the pipe. If you can do so safely, turn off the water supply (and switch off boiler/immersion if the water seems hot), then arrange urgent repair.

Do not do these things

  • Do not cap, plug, tape over, or block the outside pipe — it may be a safety discharge.
  • Do not put your hand in the stream (it can be hot enough to scald).
  • Do not ignore a steady flow or repeated restarting — treat it as a fault.
  • Do not keep restarting/resetting the boiler or repeatedly “topping up” pressure while water is discharging.
  • Do not climb into a loft you can’t access safely (poor lighting, slippery hatch, no proper boards).
  • Do not dismantle valves/cylinder parts yourself if you’re not trained — especially on unvented hot water systems.

What to do now

  1. Keep people away and do a quick, safer check: is it likely hot or cold?
    • If you see steam, the pipe is too hot to approach, or it’s clearly coming from the boiler/hot water cylinder area, assume hot/safety discharge and skip to Step 2.
    • Otherwise, you can briefly catch a little in a bucket and feel the outside of the bucket (not the stream). If it feels warm/hot, treat as hot.
  2. If it seems heat-related, make the system safer first (no dismantling).
    • Switch off the boiler at its power switch (or fused spur) and turn heating/hot water off at the programmer if present.
    • If you have an immersion heater, switch it off at its fused spur.
  3. Stop the water supply to buy time.
    • Turn off the main stopcock (usually under the kitchen sink or where the mains enters). Turn clockwise until it stops; don’t force it if seized.
    • If you’re confident it’s just the toilet (it keeps refilling / starts after flushing), you can turn off the toilet isolation valve (small valve on the supply pipe to the cistern) to keep the rest of the home supplied.
  4. Do a quick inside scan to narrow the source (no panels off).
    • Check toilets for constant filling/running.
    • If you have a hot water cylinder cupboard, look for a tundish (a small fitting in the discharge pipe with a visible gap) that’s dripping/running.
    • Note anything obvious (where the pipe exits outside; whether it’s a drip vs steady flow). A photo/video helps.
  5. If you’re in a flat/terrace, reduce knock-on harm.
    • If water might affect a neighbour (shared stack/wall), tell them and/or building management right away.
  6. Get the right urgent help (UK-specific).
    • If you rent: report to your landlord/letting agent’s emergency repairs contact and say you’ve isolated the water.
    • If it’s connected to a boiler/gas heating: ask for a Gas Safe registered engineer.
    • If you have an unvented hot water cylinder (often with a tundish): ask for someone qualified for unvented hot water (G3) work.
    • If you cannot shut the water off, or you suspect the problem is at/near the incoming supply and you can’t isolate it, contact your water company for urgent assistance.
  7. Limit damage while you wait.
    • Put a bucket outside if safe and stable (so it won’t tip/overflow back toward the building).
    • Move valuables away from damp areas and use towels to contain any indoor drips.

What can wait

  • You do not need to diagnose the exact part right now (float valve vs relief valve vs other).
  • You do not need to open up a cistern, remove cylinder covers, or drain tanks in a panic.
  • You do not need to decide “repair vs replace” today — stabilising and getting the correct trade in is enough.

Important reassurance

This is a common “something isn’t shutting off properly” situation. Turning things off in the right order (don’t block; isolate water; switch off heat if needed) usually prevents major damage while you wait for a professional.

Scope note

First steps only. The correct fix depends on what that pipe serves and may involve safety devices on hot water systems that should be handled by a qualified engineer.

Important note

This is general information, not professional advice. If the discharge appears hot or is linked to an unvented hot water system, treat it as urgent and do not obstruct the outlet.

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