Why Is Water Dripping From My Boiler Pipe Outside?
For water dripping from the small discharge pipe that leaves a boiler and terminates outside, particularly where the drip gets worse when the heating is hot or pressure has been unstable.
What this usually means
If the pipe is the boiler's pressure-relief discharge rather than the normal condensate drain, dripping means the heating system has released or is passing water when it should normally be dry. A pressure-relief valve can leak after operating, but high system pressure, an expansion-vessel fault or a filling loop that is letting mains water into the heating circuit can be the underlying cause. The key first step is identifying the pipe correctly, because a condensing boiler's condensate pipe normally carries water during operation while a pressure-relief discharge pipe should not continuously drip.
What may be causing this
The pressure-relief valve has opened because system pressure became too high
Clues: The external pipe drips when the heating warms and the boiler gauge also rises unusually high.
What to do: The high-pressure cause needs diagnosis. Do not simply replace the valve without checking why pressure rose.
The expansion vessel is not controlling pressure properly
Clues: Pressure climbs sharply when hot, the relief pipe becomes wet, then boiler pressure may be low again after the system cools.
What to do: The expansion vessel and pressure-relief valve need professional testing.
The pressure-relief valve is passing water after it has operated
Clues: The pressure is currently normal but the discharge pipe continues to drip.
What to do: A valve that no longer seals correctly may need replacement by a heating engineer.
The filling loop or filling valve is allowing mains water into the system
Clues: Pressure slowly rises even when the heating is cold or after the filling loop has supposedly been closed.
What to do: Check only that user-operated filling controls are in their normal closed position. A passing valve needs repair.
How to check what is happening
A persistent drip is more likely to be pressure-relief discharge and should be investigated.
An expansion or pressure-control problem may be forcing the relief valve to operate.
The relief valve itself may no longer be sealing properly.
Water may be entering the heating circuit through the filling arrangement or another internal path.
Can you fix this yourself?
A homeowner can identify the pipe from the boiler manual, monitor the pressure gauge and confirm user-operated filling-loop controls are closed. Pressure-relief valves, expansion vessels and internal boiler components should be handled by a qualified heating engineer.
- Boiler user manual
- Phone camera
- Notebook for cold/hot pressure readings
What you can safely try
- 1
Identify the pipe before assuming it is a fault
Condensing boilers normally discharge condensate through a plastic drain route. A separate pressure-relief discharge pipe is different. Use the boiler manual if you are unsure.
- 2
Record the cold and hot pressure
Note the pressure with the system cold, then again once the heating is warm. Do not repeatedly top up or drain the boiler while diagnosing the pattern.
- 3
Confirm the filling controls are closed
If your boiler has a user-operated filling loop, confirm it is returned to the fully closed position after previous repressurising.
- 4
Never block the discharge pipe
The pressure-relief route is a safety feature. Capping it to stop the visible drip would remove the safe escape path for excess pressure.
- 5
Have the pressure-control system checked
A heating engineer can test the expansion vessel, relief valve and filling arrangement, then fix the underlying cause.
Call a heating engineer if…
- Boiler pressure approaches the high/red range.
- The discharge becomes a steady flow rather than an occasional drip.
- Pressure repeatedly falls after the system cools.
- You cannot distinguish the pressure-relief pipe from the condensate drain.
- Any repair would require opening the boiler casing or safety valve.
Get the right heating engineer.
We already know this problem maps primarily to a heating engineer. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- The heating system is constantly losing pressure — what should I do?Worcester Bosch
- What is a tundish?Vaillant
Last reviewed: 31 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.