How should condensate drain connection be made at top of downpipe?

[Bordonbert]

[ASIDE: I did wonder if this was the best forum for this but figured as it was a boiler related issue that the gas engineers would be dealing with it as a matter of course. If I'm wrong can someone let me know and I'll shift it to a better place.]


I have a condensing boiler installation which is working fine in its main function. It is a Vaillant boiler with a grey short flexible corrugated drain pipe feeding from its underside. This is about 4 inches or so and joins onto a standard white 21.5mm downpipe inside the house. This mates with an unsealed push in rubber reducer as the air gap. Then it travels through the wall to the outside and down to a soakaway well away from the house foundations. It froze up during the winter so we fitted it with an insulated and sleeved system in its outside section. That outside piping all works just as it should.

I noticed that where the 21.5mm pipe entered the rubber bung at its lower end there was a very slight ring of water appearing around the junction. At first I worried that it had been blocked or kinked in some way but there are no problems there. I found the problem at, to me at least, a surprising point. The problem I have is at the top of the indoor 21.5mm pipe where the dedicated short boiler drain pipe fits onto it. This has the downward running Vaillant pipe over the outside of the 21.5mm pipe so it relies on that push on joint being watertight. It obviously isn't.

I'm amazed that the boiler pipe is the female and sits over the end of the other as it travels downwards. It must have been better to mate it to the 21.5mm pipe via a straight coupler solvent welded to the lower pipe only and size it so it could sit inside without restricting the inner area in any way.

I am loathe to solvent weld a not immediately available part like that to the common or garden pipe so I'm looking for the "correct" solution to this. I have considered flaring the end of the 21.5mm pipe so the other sits inside of it. I could also flare the end of a straight coupler as that would need less freedom. How is this normally handled? Is that "inverted" push on fit the usual way? Does it not lend itself to this problem at some stage?

I do know that the boiler pipe, being an awkward plastic type, would need a special two part adhesive if it came to it. One prepares the surface to accept and bond to the second adhesive paste. I have this available from the insulated pipe install which is the same type. Although, maybe the answer is as simple as a gob of Boss White or even a jubilee clip?

 

[ShaunCorbs]

Zip tie ?

 

[Bordonbert]

I did think of that Shaun but I wondered if it would be enough to really pinch it shut. The upper outer pipe end is very thin and the pipe is made to be flexible I grant you but it is still a stiffish plastic. Do you think it would have the grip to prevent any leaks? Maybe with a little gasket sealant of some sort under it?

Mind you, why am i carping about that idea? What have I got to lose by trying it? It's only a tie wrap after all. I'll give that a go and report back. Thanks for the idea.

 

[Bordonbert]

Got it, the penny finally dropped and it's simple - and obvious. A 21.5mm compression straight coupler will do the trick. This accommodates 19-23mm so will take both ends comfortably. The boiler side drain pipe doesn't need to be clamped tightly at all as it only drains vertically down into the coupler with no back pressure. The 21.5mm pipe from there on down will now seal with the lower end of the compression coupler. They don't seem to be too common, (not available at Screwfix for example), but they are "internettable". I'll report if it works or not for future visitors.

 

[ShaunCorbs]

I did think of that Shaun but I wondered if it would be enough to really pinch it shut. The upper outer pipe end is very thin and the pipe is made to be flexible I grant you but it is still a stiffish plastic. Do you think it would have the grip to prevent any leaks? Maybe with a little gasket sealant of some sort under it?

Mind you, why am i carping about that idea? What have I got to lose by trying it? It's only a tie wrap after all. I'll give that a go and report back. Thanks for the idea.

That’s what I use as it’s / the connector is designed for straight pipe

 

[Bordonbert]

Just to round this off with what I found here which has a bit of a humourous twist. I tried to ask Vaillant to tell me how they normally connect their own flexible pipe to the 21.5mm internal drain pipe. Their system for dealing with my request was - - you guessed - - an AI driven "pseudo human" response unit. It could not recognise what I was talking about no matter how I worded it. However it DID recognise the word "leak" and panicked. Panic in an AI response unit is very funny, you ought to try it sometime. The next thing I knew it was arranging a next day visit from an engineer to check things. I wasn't aware of it but we were actually covered under insurance for this so I didn't try too hard to correct its impression.

The guy who turned up was super cool and helpful when he found out it was only a simple drain leak and not gas! He reckoned that the flexible end to their drain pipe had simply softened and stretched over time and was now not a tight enough fit. He solved it with PTFE tape on the 21.5mm pipe. (Coulda, shoulda, woulda...)

In the meantime I had sourced a couple of "Herbie" clips. These are ideal for this sort of work. They have a pair of overlapping serrated tongues which grip to each other as the clip is squeezed together. They form a complete circular grip with no uncovered weak spot to possibly kink and leak. I'm not fitting one yet, I will if I need to, as I want to see how good that simple PTFE tape fix is in the long term.

Thanks for the support and advice Shaun. Your fix would definitely have been workable as it was a really simple problem after all. In fact... If this even can happen, why on Earth don't engineers just fit some sort of generic clip when they do the original installation?