Plumbing

Unvented Cylinder with cold water storage

Mr Aaron R Digby 06/02/2026 - 8.00 AM

I had an unvented McDonald cylinder installed 2 years ago alongside an ASHP and the installer left it connected to the 50 gallon for water feed & discharge. I contacted an plumber earlier in the week who said they would be able to remove the old storage tank and close the system off (connect the cylinder directly to the mains) with no issues. Yesterday they came to complete the job, and just as they started the saw my old storage tank to pieces I noticed water gushing out of the discharge pipe, as they had plumber the cold mains into the cylinder but put the boiler discharge pipe directly into the old one (routed to outside the house). After this they said my cylinder wasn't actually plumbed correctly to allow for direct mains feed and isn't unvented? They left to get a new tank to 'resolve' the issue and returned with a 4 gallon tank to replace the old one. This morning I noticed a drip coming from the discharge pipe, which has gotten worse throughout the day and is only getting worse as time goes on. They are coming back out tomorrow to check the float valve, but I'm concerned this isn't enough to fix the issue. Was the installation of a 4 gallon tank here correct? It doesn't seem like enough if, as they have said, the system isn't setup to be unvented?

Anonymous user

Ok this sounds like a total sham and you’ve been given a lot of miss information by all engineers so they could get paid and out the door. What ever the case more information and maybe pictures would be needed to provide any further help but if you at least post some pictures of the water tank, all pipework and the hot water cylinder then I am sure we can tell you what needs to be done to prevent any further time wasters

Answered11 February 2026

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Vp propertys

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Walthamstow, London

Doesn't sound correct if it is a unvented system there should be no storage tank. Regards Victor

Answered7 February 2026

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Anonymous user

Only possible reason they may have left a storage tank with is maybe on the cold side, if you have a cold valve in a old toilet something like that or a older valve that requires low or gravity fed pressure they may have left them type of valves on gravity to save further issues. That’s just a possibility though. Always tough to know why with out properly looking, you could isolate the feed to the CWST tank and see if anything turns off.

Answered6 February 2026

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