Valiant turbomax boiler problem help please

[dilalio]

some think that if you post on an internet forum someone will tell you where to buy fairy dust for £1

I've got some fairy liquid, if anyone's interested? Yours for £10.

 

[markymark2406]

Right ladies, Gentleman and those who are gender neutral, this is the thing, I have tried everything you have advised with little success, I now have a boiler engineer saying it’s the secondary heat plate exchanger, we are going to remove this and descale it with ‘Knockout SOS650 Spirits of Salt‘ which should do the trick.
I would like to thank the guys on here for advising me in a professional manner as for the idiots, I will not rise to it

 

[dilalio]

Ok tried the shower on cold only and tried the laps still the same

mate... The above post is where you started to lose your audience. We were all trying to assist you but that post was too brief and nonsensical!

Cartridge based mixer valves (especially shower valves) have a flow cartridge that mixes hot and cold water to the outlet, via a thermostatic valve. These cartridges sometimes have a non return valve that prevents the higher pressure cold water from travelling back up the hot pipe, in fault conditions.
It is not uncommon for a shower valve to fail internally, thus allowing cold water into the hot side of the whole system (known as crossover). By advising you to shut off the cold supply AT YOUR BOILER ONLY this could be proven by then opening any other hot tap... If water continues to flow from a hot only tap, whilst the cold feed to the boiler is isolated, then this proves that cold water is getting into the hot circuit via a mixer i.e shower valve.
Has this test been carried out?

 

[markymark2406]

Yes

 

[dilalio]

Yes