Hot water immersion cylinder 2 tanks how does it work?

24 Nov 2014
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United Kingdom
Hello, I have an older copper cylinder consisting of 2 tanks - a large one at the base and a smaller one on top.

The top tank was rising and exiting the overflow, and I noticed the ball cock valve was not shutting off completely (drip, drip). I have replace this.

My concern is now, is there another way for it to overflow?

How does this configuration work? The element is in the base tank only. How is the base tank fed cold water - from the top tank or vice versa?

I assume the heated water in the bottom tank expands and goes where?

I noticed in the top tank a second exit pipe, not sure if this goes to the bottom tank or another overflow?

Confused :-$
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The lower hot water tank gets topped up with cold water from the upper tank.

It works in the same way as a conventional system where you would instead have a completely seperate header tank and hot water cylinder.

Normally the only thing likely to cause it to overflow would be a faulty ball valve, as you have already found out.

The water in the hot tank will expand but it isn't a problem as a little water can be displaced back up towards the cold tank. The second "overflow outlet" in the cold tank may be an air vent to prevent an airlock forming in the lower tank and allow steam to blow off if the tank should ever boil.
Thanks, that's much more clear.

Not sure where the top tank exits to connect to the bottom tank though.

One of the overflows was plastic, so I assume that is only for overlow use (less often). The other I think was copper and built into the tank, 2/3rds of the way up the top tank.

There was no outlet at the bottom of the top tank, only 2 outlets and an inlet (valve). Is it possible for the ball cock to shut the water off before it 'overflows' to the bottom tank? Which would leave the bottom tank dry and the element overheating.

I say this because, as it stands, the water isn't going to reach the copper outlet at all.
I would have expected the cold outlet to be near the bottom of the upper tank. There's likely to be a high tide mark in the cold tank which will give you a hint on how to set up the valve float.

TBH I haven't seen one of these tanks for a long while, my parents still had one in their council flat in the early 1980s.

If hot water is flowing as normal from your hot taps you should have no worries about the level of water in the hot tank.
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They are still around and sold.

Usually in cheaper flats.

The fill level should be about 50 mm below the over flow outlet pipe. That gives space for the water to expand into when its heated.

OK that's useful to know, except that the tenant doesn't use the cylinder - only the shower. Hence the overflow.
I just need to be sure that left alone (and full of water) it doesn't leak.
I hope the ball cock change has nailed it. Cheers.

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