overflowing hot water cistern

[Softus]

bath and basin taps all single.

It's not those ones then!

sink tap in the kitchen is one of the single lever mixer taps feed from cold mains as is the tap in downstairs wc.

It would be handy if the cold (and/or hot) supply to each of those taps had its own individual isolating valve, then you could test the effect of turning off one at a time.

If not, I would fit valves in order to eliminate them, but they're my odds-on favourite at the moment. If a cartridge has an internal fault then it could allow cold to leach into the hot side, back up the supply, into the cylinder and up the cold feed to the cistern. There are people who believe that this is impossible, but it happens, and I've seen it many times, and it's usually on shower valves with the same type of lever-operated cartridge as your basin and sink monoblocs.

 

[feckem]

Softus you are a b****y hero. Problem all sorted. proplem was the basin mixer. isolated it and will replace today with a new one.

Cheers mate i owe you a pint

Alan

 

[Softus]

I'm glad you sorted it Alan.

While you're replacing the tap, you might consider fitting a full bore single check valve (aka non-return valve) on the hot supply.

Then, if/when the next tap goes faulty, there's a second line of defence against backflow. It isn't guaranteed to work, 'cos once I fitted a check valve that didn't prevent a very slow backflow, but it might work for you.

I emphasis that a full more one is needed. If you can't find one that fit a 22mm ordinary check valve, which won't restrict any more than 15mm pipework.

If you fit two of these to a 22mm valve:

...then it will connect easily and instantly to 15mm pipework, and you can make both the compression joints out in the open where it's easier.