What is this mains pressure cold water tank?

25 May 2011
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United Kingdom
Excuse ignorance but...I bought my 1930s North London semi last year. There is a gas boiler with a pressurised central heating system and pressurised hot water cylinder, and mains pressure cold water. On the top floor, converted from an attic into a bedroom and bathroom by the previous owners five years ago, is this pictured side space. On the right is the filling loop, pressure gauge and expansion vessel for the CH, all of which works as expected. But what is the large blue tank, which bears no labels or signage and has a single pipe feeding it? It looks like a large expansion tank for the cold water; but why should such a thing be necessary and how does it work? If the house pipework springs a leak and I cut off the mains feed from the road, must the cylinder drain before the leak will stop? FWIW the pressurised DHW cylinder is on the floor below and the boiler on the ground floor below that.

Apologies if obvious. I've dealt with gravity fed CH and DHW systems and with simple mains-pressure systems but not come across a tank like this before.

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If it's insulated, it looks like a horizontal hot water cylinder. They are used when there isn't enough headroom for a vertical one.Could there be other pipes connected that are out of sight?


Odd that you have a hot water cylinder downstairs too, unless originally that didn't have enough capacity, or have sufficient pressure for the upper floor.
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If only a single pipe connected to it, then most likely an accumulator, although it's an odd location for it.

Essentially a air filled bag inside, when taps in the property are off, mains water enters under pressure and compresses the air, then when taps are opened the pressure of the compressed air inside will maintain the flow and pressure of the water supply for a length of time, until the accumulator is empty. Then it will refill once taps are closed.
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until the accumulator is empty.

Does any installer give the customer any information about how many minutes a shower can be used before the accumulator becomes empty and the shower reverts to a low flow rate.

Obviously the time depends on flow rate from the shower and the size of the accumulator and thus will vary from installation to installation.
Thank you everyone for your replies - much appreciated. It's clearly a cold water accumulator. Odd as there really isn't a pressure issue hereabouts.

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