Boiler condensate pipe connection

12 Sep 2014
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United Kingdom
A bit of a smell in the store cupboard with the CH boiler. Had it checked for gas leaks – none. Service engineer says he is 100% sure it's the vent stack, possibly blocked. The boiler in the photo is on the wall separating my semi-detached bungalow from the neighbour's.

The condensate pipe obviously goes into the stack vent the other side of the plaster panel, and the stack vent goes up into and through the loft to ventilator tiles on the ridge of the roof. I understand the boiler, an Intergas Xclusive 24, has a trap inside it, but my question is: shouldn't there be a trap in the pipe, the black one in the photo, before it goes into the stack vent. Otherwise, isn't this black pipe just an opening into the soil pipe which is bound to smell – the whole idea of the stack vent being to take smells to roof level.
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1. Check the Intergas instructions for your boiler.
2. If it says no external trap to be fitted, seal the opening just below the boiler where the black flexible pipe goes into the black rigid pipe.
3. If it says external trap allowed, or doesn't say anything, fit a running trap into the almost horizontal run of rigid black pipe, between the two pipe clips.
That's just a mental/ most weird way of running the condensate pipe. That should be the other way around, the small white pipe going through the wall (21.5mm solvent weld) should be connected to the condensate pipe coming out the boiler and then the black pipe should run through the wall to connect to the stack.

You can then introduce an inline one way valve/trap into the 21.5mm pipe.
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I feel like I'm in Australia with that upside down plumbing! Need a proper stand pipe or similar trap, not a vent directly into the room.
Thanks for the replies. This is the relevant page from the manual:

So it looks as if there should be another trap, an external one, directly under the boiler.

This was the first boiler of its type the gas fitter had installed. I remember him saying he didn't know if something should be open or closed, maybe it was this. The service engineer from the company who did the installation who came to check the boiler and reconnect the gas after the gas emergency man had capped it off said I could seal the opening between the flexible pipe from the boiler and the rigid black pipe. I think doing this there's supposed to be a risk of water building up in the stack if there's a blockage somewhere and ultimately reaching the boiler and possibly doing it irreparable damage – is that right? He also kept saying the smell was because there was a blockage in the stack.

Time to go back to the company who did the installation? The boiler was installed in February – strange no smells all summer?
Nope, the picture is showing you that you need to have an air break in the condensate pipe run, it can be incorporated with a trap, i.e. an appliance standpipe but there are other alternative ways to achieve the same thing....

.... and it certainly shouldn't be dropping to 21.5mm to go through a wall to the outside soil pipe. If anything it should stay @ the same 32mm.

If that waste ppe wasn't a problem then there could have been a trap built into that black waste pipe run using 3 elbows, at the very least.
This was the first boiler of its type the gas fitter had installed
I've never fitted a boiler in my life but it's clear to me it's a lash up, the defect you are experiencing is totally unrelated to the boiler.
Having an open drain in the property doesn't produce smells all the time, it depends which way the wind is blowing and whether the neighbours had curry last night. The idea of the trap is to prevent smells under all normal conditions, not based on luck.
You may have had a blockage but either way the trap needs adding and the waste pipe needs to go from small to large and not back down to small. Definitely don't go against the boiler instructions as technically the flue system ends where the air break is in the condensate system, so you'd be modifying the flue system.
The suggestion about elbows is a good one although ideally it should be accessible for cleaning.
Apologies, didn't pick up on the stack being internal, when I was reading.... doh! That takes care of the possible freezing point for keeping the pipe size the same then. Still, it's a really weird thing to do to drop the pipe size like that. I assume the installer did that as he was using the existing 21.5mm connection into the stack?

Could easily have just kept it 21.5mm with a little piece of 32mm at the start for the air break and popped a tundish into the run. Then just the ad hoc water seal with 3 elbows. Would have looked much more appropriate.
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