Constant leaks from pipework around hot water cylinder

20 Jan 2023
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United Kingdom
Hi there,

I moved into a "second hand" new build just over a year ago. It has a large unvented Kingspan hot water cylinder in the airing cupboard.

Last year I had the boiler serviced and my plumber noticed the tundish in the airing cupboard was dripping. He order a replacement valve of some kind but whilst waiting for that to arrive, one of the compression joints in the airing cupboard started to leak. I kept a towel wrapped around it for a few days and the plumber prioritised replacing the valve and re-doing the nearby joint.

That was fine, until a few weeks later when another joint in the pipework had a very slow leak. A droplet of water would collect on the compression nut overnight, and eventually drip down the pipe work or evaporate when the heating came on. The plumber came and repaired this connection like the one previous, and all was well again.

Fast forward a few months and the tundish starts dripping again. Plumber came out and re-charged the built-in expansion vessel that is inside the cylinder. I'm not sure if homeowners are meant to do this but it looked straight-forward and he said some cylinders have instructions on the side showing how to do this.

The tundish stopped dripping, but then today (about a week or two later) I have found a different compression joint that is weeping. Unlike the previous two weeping joints, this one looks dirty and scaled up, like it has perhaps always been leaking, or has leaked in the past.

I contacted the plumber and they suggested contacting Kingspan to query the cylinder. They seem worried it might be a fault with the cylinder and the endless leaks are a symptom of something else. I did this and Kingspan said it's just wear and tear on the connections which need tightening periodically.

Personally I think it is just a shoddy new build install and the contractor didn't do up the connections properly, or perhaps over tightened them, or whatever.

Just looking for a second opinion really.

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Nothing to do with Kingspan, if the couplings are leaking, then it is shoddy workmanship or type of fittings used.

Only cylinder related issue is tundish showing discharge due to expansion issues but that would not happen if cylinder saw proper aftercare
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Thanks for your replies. I've attached two photos. One shows a close up of the leak. The other shows:

1. Highlighted in red - the current leak. You can see the connection looks dirty and stained, like it has leaked before.

2. Highlighted in green - the two connections that were leaking but have since been repaired (and seem okay). Before I had them repaired, these connections were clean and didn't look like they had been leaking previously.

3. Highlighted in purple, you might just be able to see that the tundish has a lot of historic scale around it. This made me think the previous owners had been battling some ongoing problem. However I recently learnt that the air gap in the cylinder needs replenishing periodically, which the old owners might not have known about or might have been slow to do.

My thought is that when the air gap needs replenishing the hot water pressure increases (you can feel this through the downstairs kitchen tap) and some of the joints in the airing cupboard were shoddily done, and start leaking under the extra pressure. Does this sound like a possibly theory?



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That's a motorised valve that's leaking has above nothing whatsoever to do with cylinder.
Whilst compression joints should hold just by being tightened I always use a smear of jointing compound,
Bit belt braces and length of string but much better than been called back.
Agree, the two joints that have already been repaired used a smear of jointing compound and they've been water tight. Just a shame I seem to be working through every joint in the airing cupboard.
I prefer not to use joining compound
If a joint leaks, it is because the olive might not be a quality one or fitting issue. In that case PTFE on the olive will sort the problem
These are manipulative fittings ie at some point will need dismantling. Jointing paste when it goes hard will cause more problems when fitting is remade.
Above is just my thought, not saying do not using jointing paste. If that is what ricks your boat, go for it.
My thought is that when the air gap needs replenishing the hot water pressure increases

You cannot compress water. With no air gap, “water compression” will not be seen when hot tap run.
With air gap in place and taps closed, air gap is at system pressure ie 3.5 bar ( if that is what the combination valve setting is)
Air cushion will also be at this pressure

When you run the hot tap, this elevated air gap pressure will be seen as better flow at the tap.
Depending on the cold feed to the cylinder, enhanced flow will diminish during hot water delivery from that point

Why not tighten all the brass capnuts so the issue is fixed
Next time something leak the bite the bullet and have the system drained. Strip all nuts, clean, paste, tighten. While you at it add an inhibitor to protect and service the cylinder and the boiler.

It's more than likely poor instalation.
Thanks, plumber is coming next week to fix the most recent leak. I think you're right, I probably need to bite the bullet and have all the connections redone. I suspect he won't have time to do it this time around but will suggest it as the next course of action.
That looks to be Range Tribune pre-plumbed cylinder with factory made joints. Expansion gap probably needs recharging if those joints keep leaking.
That looks to be Range Tribune pre-plumbed cylinder with factory made joints. Expansion gap probably needs recharging if those joints keep leaking.
Yes it is.

The gap was replenished after the tundish dripped and then I noticed this leak a couple of weeks later. Same thing previously when the tundish dripped and the plumber replaced a pressure relief valve, then two weeks later a leaking joint was spotted. That's why I mentioned above, are the leaks happening due to a combination of poor install and an increase in pressure whenever the expansion gap needs replenishing? I also notice when the gap needs replenishing and you turn on the water heating, there is an initial "blast" of pressure from the kitchen hot tap when you first turn it on.

How frequently do you think the gap should need replenishing? Is every 6 months normal, or too frequent?
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