One for the boffins, coz I'm stumped!

[johnnyboyoy]

Any chance this is a combi masquerading as a system boiler, serving the heating, coil, and some outlets? Just that the hot water sounds like a diaphragm on its way out on a pressure differential valve. Needs 2 outlets to get the pin to push out enough.

 

[Sid Viscous]

It's deffo a heat only, 100%

 

[Sid Viscous]

Why would faulty or missing check valves on a shower cause a problem if the shower is not in use? As I said in the OP, this happens when any combination of outlets is opened including mixers & non mixers

 

[SJB]

Cold always pressing against the faulty check valve, when the hot pressure drops because an outlet elsewhere gets opened the check valve totally gives out

 

[diamondgas]

If possible turn off the cold mains supply to the unvented cylinder ... If you still get a some supply through the hot tap/s then I'd suspect a faulty TMV/dual mixer valve somewhere is passing? It could account for cold at the hot taps?

 

[Sid Viscous]

Cold always pressing against the faulty check valve, when the hot pressure drops because an outlet elsewhere gets opened the check valve totally gives out

I get how that could happen if the shower were open & in use but not if it's turned off, surely that's the same as if a tap is closed, no water moving anywhere?

 

[gasmanrob]

No one side mixes the water and the other diverts the water to the shower head. The side that blends isn't actually shut off

 

[Sid Viscous]

Got it, nice one! So if I isolate each pair of outlets one at a time, that should get me to the fault? Unless of course, there's no check valves on anything, in which case I'd be p*****g in the wind!

 

[R.Oak]

If possible turn off the cold mains supply to the unvented cylinder ... If you still get a some supply through the hot tap/s then I'd suspect a faulty TMV/dual mixer valve somewhere is passing? It could account for cold at the hot taps?

this.

 

[Sid Viscous]

Isolator on the composite valve is seized solid. I'll have a go at freezing it.
Just another thought, mains pressure is 4.75 bar. If something was passing back, wouldn't that cause the T&P relief to open?