Cold Water tank problem

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My late-1950s house has a typical gravity-fed CH+HW system with a back-boiler, first-floor HW cylinder and cold-water tank in the loft. The CW tank is circular and made of some black rubber-type material and must be at least 50 years old if not more.

Originally the CW tank had no proper lid, just an insulation jacket that was spread across the top and wrapped down around the sides. This perished about 5 years ago, so I fitted a new insulated jacket and also a proper circular black plastic(?)-type tank lid. For some reason I simply could not get a lid that was an exact fit - maybe a mismatch between new metric size lid and old imperial size tank, or perhaps the tank has just 'bulged' a bit over the years ?? - anyway I got the best fit that I could.

It is difficult to pin-point the exact cause of the current problem, but it would appear that water is condensing on the underside of the lid and running down towards one particular 'corner' of the tank where there is quite a bit of limescale type stuff on both the underside of the lid and around the lip of the tank edge. The water then drips over the top of the tank side and down the outside, but behind the jacket, before finally dripping off the bottom of the tank and onto the floor, where there is now a small (but possible growing) wet patch. No idea how long this has been happening, but there was no evidence of it about 9 months ago.

Any ideas please as to what may be the cause of the problem and how best to cure it?
 
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it is going to be condensation. normal roof tanks these days are a plastic box essentially and the lid is a groove so any condensation drips back into the tank.
If you don't have your custom lid basically lodging inside the tank but is more of a flat cover, I would suggest running a bead of silicone just within the circumference on the lid but not touching the tank sides. Any condensation running towards the outside will hit this bead and drip back into the tank.
 
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I took another look at the tank today.

There seems to be a 'damp patch' (about 2"-3" diameter) in one spot just above the bottom of the tank. No evidence of any condensation coming down the outside of the tank from further up above that area.

Is it possible that the tank wall has become a bit 'porous' in that area and that water is seeping out through the tank wall? I've pressed it a few times without any obvious sign of water oozing out of any actual hole.
 
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That shouldn't be there. I've no experience with any rubber based tank so I reckon it'll be a complete tank change. probably for the best anyway seeing the age of it, and you'll probably be horrified to see the crud in the bottom.
 
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It won't be that bad Nemo, things are easier these days what with even fold up tanks that'll fit through the hatch. You don't even need a huge tank , we only had big tanks as a reservoir in case the water mains was cut off in the old days, today that is very rare and the input from the ball cock can usually keep up with the output of the tank. Then you have flexible connecting pipes so you can just cut off the old pipe and even push fit the new one on! isolation valves built in too.
 
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I checked again this AM and the damp patch was much smaller. Wiped it dry, left it for a while, then very gently prodded around the area - no obvious sign of any water oozing out from the tank wall. So maybe just condensation after all, who knows? - I shall keeping watching it.
Nevertheless, a replacement in the near future would seem to be prudent :)
 
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I reckon at your earliest convenience, then all doubt is gone mate.
Derek
 

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