Soldering hot water cylinder

The sister is using ChatGPT pro in her work. She knew about this bit of bodging that I’ve been up to today so she she asked at each stage what it recommended. The replies were unreal. Here’s the first list of options it gave me before I went off and got tec7…

Perfect, that photo helps a lot — thank you.

That pinhole in the middle of the copper boss (just beside the immersion opening) is not on a joint or thread, it’s in the tank wall itself — which means it’s caused by corrosion thinning the copper rather than a loose fitting.

Here’s what that means for repair options:

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⚠️ The Reality
• That area is under pressure and heat, and once a pinhole appears in the cylinder wall, it usually means the metal is paper-thin in that section.
• Trying to solder it won’t hold well because the surrounding copper will bubble or distort as soon as you heat it.
• So while you can seal it, you should treat it as a temporary repair, not a permanent one.

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✅ Best Temporary Fix (and simplest)

Use epoxy putty such as JB WaterWeld or Evo-Stik Plumber’s Putty:
1. Drain a few inches of water so the leak is completely dry.
2. Clean the surface well — remove any green/white corrosion and rough it slightly with sandpaper.
3. Mix and apply the putty over the pinhole, pressing it in firmly and feathering the edges.
4. Let it cure fully (usually 1 hour).
5. Refill slowly and check for leaks.

💡 This can easily hold for months or even a year, depending on how thin the metal is around it.

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🧴 Optional Backup

After the putty cures, you can also brush a thin layer of Fernox LS-X or Boss White jointing compound over the repair for an extra waterproof seal.

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🚫 Avoid
• Soldering: the metal’s too thin, and heat could open the hole wider.
• Filling with internal sealant: not effective for immersion tanks; that’s for central heating systems.

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🧯 Long-term

Eventually, that tank will need replacing — but for now, the epoxy putty method is the easiest and safest fix without touching a blowtorch.

Would you like me to tell you exactly which brand/pack to buy (Woodie’s or Screwfix Ireland) that’ll bond best to copper and handle hot-water pressure?

So that was earlier, I did the repair my way, although adding in scrim tape to the Tec7 as ChatGPT reckoned a mesh would be worth using, here’s the next reply…

Honestly — fair play to him, that’s a very smart bit of DIY thinking, and given the situation, he’s probably given himself the best possible outcome short of replacing the tank.

Here’s what’s actually happening (and why his method makes sense):

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🧩 Inner + Outer Repair Combo

By sealing both inside and outside of the pinhole:
• The inner layer of Tec7 stops direct water pressure from pushing through.
• The outer layer with scrim tape acts like a reinforced patch, spreading the load and giving the Tec7 more surface area to grip.

That scrim tape basically works as a makeshift fibreglass mesh, which is excellent thinking — Tec7 adheres well to the fibres, so once cured it’ll form a strong, flexible shell.

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⚠️ What He Should Still Do
• Leave it at least 24 hours before refilling, to let both layers fully cure.
• When refilling, do it slowly and don’t turn on the immersion for another few hours afterward — Tec7 hardens from the outside in, and heat too soon could soften it.
• Once it’s hot again, keep an eye for the first few days — just a visual check to make sure it’s not weeping under pressure.

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💡 Realistically

If the copper around the hole was still fairly sound (not crumbling or wafer-thin), this repair could hold for months or even longer.
If it was already pitted or greenish-thin, it might eventually seep again — but he’s bought serious time without needing a new cylinder straight away.

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Well, this my first experience with AI helping with diy, and I’m impressed. 30 years ago I the Readers Digest Complete DIY manual and kicked off my homeowners education, and now this. Anyway, as I said I’ll update if it leaks or not.

Thanks Lenar3556 I’ll look into that.