Seal from copper to heat exchanger in boiler question.

[Fredxxx]

The boiler is a Vaillant and there is a small leak between the outgoing copper pipe and the heat exchanger.

There is an O-ring of an unknown size that should seal but doesn't quite. The result is a few drips onto the boiler, some of which then finds its way to the PCB!

I was considering using silicone sealant, but am concerned over its acidic properties.

There is no desire to split the joint again, though I would like it to stay flexible to an extent as while movement is minimised there is little strain relief on the joint.

What should I be using?

[John Rumm]

A new O ring by the sounds of it...

If you were going for a slightly less orthodox repair bodge then Fernox LS-X

[Dave Liquorice]

+1 trouble is finding the rights size. Having the imperial and metric O rings kits form Alidle would be useful...

IIRC that is an "acid cure" sealant but should cope with the temperature. You can get non-acid cure sealants but not sure about "high" temperature ones.

[Bod]

Or Silicone. It is heatproof.

[Andrew Gabriel]

You could order the right part from a boiler spares place. The part number will be in the installation/servicing manual.

... after disassembling and cleaning up the parts. Get some descaler to remove the residue from the leak.

[Andrew Gabriel]

12 years ago, Halfords had a comprehensive range of high temp sealants for use in engines. I used one of them for remaking the flue spigot on my boiler. 12 years later it's still fine (it was supplied leaking, which was a common fault with that model of Keston), although the flue temperature doesn't normally exceed 50C.

Halfords no longer stock the range, but you might find them in a more specialist car engine parts place, like Halfords used to be ;-).

However, I'm not so hopeful these will work in this scenario with water pressure behind them.

[Fredxxx]

I have now done that, but the boiler is remote from me, and I'm not sure of the size.

Perhaps I'm not confident that just changing the o-ring will provide a permanent solution.

I'm not sure of the heat exchanger material, so not sure how it might have endured 10 years of constant use and if it is pitted.

I am rather tempted and may well go prepared.

Thanks for all the advice, much appreciated.

[Fredxxx]

That was a consideration of mine, it was the initial release of acetic acid that worried me.