Silver soldering a previously soft soldered boiler

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Less of an issue in your case where you were able to take the small boiler apart and ensure all traces of soft solder were removed.

It is not so easy on a larger assembled boiler where someone has tried to caulk a leak with soft solder and then tried a silver solder repair especially if it's in the firebox. The soft solder may well have flowed into the joint and no amount of scrubbing will be able to remove it so when you start to silver solder the lead boils and the gasses weaken the new silver solder joint.

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Removing all traces of the soft solder is the key, I cannot fault your procedure. Residual traces of the soft solder would have been visible after the pickle bath.

Whilst its not something I would do on a complete boiler.....if the silver solder has clearly taken to both surfaces (and it is obvious when it hasnt) then the joint should be sound. I have removed soft solder from joints and re-soldered with silver solder, albeit on brass with no ill effect. If youve cleaned away the soft solder and used quality flux, rod and given it enough heat then all should be well. If it passes test at twice wp then you know its taken properly. As stated above, any soft solder would have turned a nasty, almost black colour after pickling and would stand out. Hope this helps.

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