I've ignored this small metal plug/pipe for the two years since we bought the house, but now I’m experiencing an issue with my combi boiler (which sits on the other side of the wall, in the kitchen) I want to rule out whether it’s anything to do with that, in case it could be linked to the problem we’re having.

It’s about 1.5 inches long, and seems hollow, with a small slit towards the end. The curious thing is it appears to have been ‘punched in’ on the underneath side - but, since I have no clue what it is, perhaps this is its normal design?

I have a condensate pipe nearby from the boiler (the black pipe you can see in the background) which drains directly into the drain in the ground below), so assuming it’s not to do with that.

Any insight at all would be much appreciated!

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  • 1
    Can you see where it goes? Can you stick a borescope or phone camera behind the boiler to see if it comes out the wall and connects to anything? Or if it goes up/down inside the wall can you see a similar pipe appear in the basement or attic? Blind guess would be it has something to do with the previous boiler and isn't connected to anything now.
    – jay613
    Jan 7 at 21:12
  • Which side of the house is this located and how far up on the wall? How about a picture showing most of the wall so one can get see this in context. Jan 7 at 21:32
  • 4
    With the helpful answers so far, I think I’ve all but confirmed it’s the end of the boiler’s safety release pipe – a hacked ‘stub-out’.
    – marvelman
    Jan 7 at 23:07
  • 20
    Congratulations! It's a boy ;)
    – Matthew
    Jan 8 at 5:52
  • 1
    Please take the tour. Accept or provide an answer rather than reporting your result in a comment.
    – isherwood
    Jan 8 at 14:09

2 Answers 2


All-but-certainly connected to the usual over-pressure/over-temperature relief/safety valve common to boilers and water heaters.

It appears to be configured to turn the boiling-hot water and/or steam back against the building rather than scalding passers by.

  • 1
    Aha, I think you’re right – the pipe appears to come into the wall underneath the boiler and I can see it connects to one of these within the boiler casing: screwfix.com/p/baxi-248056-3bar-safety-valve/797TP?kpid=797TP
    – marvelman
    Jan 7 at 22:59
  • "rather than scalding passers by" that takes all the challenge out of city living! :D
    – FreeMan
    Jan 8 at 14:14
  • 4
    @FreeMan As long as you're not wearing earphones that stop you from hearing cries of gardyloo(!) Jan 8 at 16:55

It looks as if the pipe started its life as an air chamber or stub out.

It probably should have been cut and had an elbow attached with additional pipe to make any water or steam aimed to the ground. However yours was manually "adapted" so it has split.

  • I’d never heard of a stub out, but from the profile it appears you’re right – in combination with the answer from @Ecnerwal above, suggesting it is the pipe for the boiler’s safety release valve, I think you’ve solved the mystery! I’m no professional, obviously, but this “improvised” solution seems like a strange choice, when a quick Google shows there are purpose-built outlets/pipes for this? 🤷‍♂️
    – marvelman
    Jan 7 at 23:02
  • 5
    The pipe is not split - it's been cut and bent.
    – Ecnerwal
    Jan 7 at 23:55

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