Boiler giving hot water and then freezing, then hot!

[dragon56]

Hi, I'm new to all of this, so I hope you can help me. I have a wocester combi boiler. The thing is that in the last few weeks if you have the hot tap running suddenly it'll go stone cold then go hot and back to cold. No other taps are on and the flow is good. This happens even if you have the tap full on! It makes taking a shower very interesting.
There is no temp. control on the boiler. Only one tap running at a time.
Is there some thing I can do? I've checked the pressure in the boiler and that's fine. I'm not very hands on but will give most things a go if I can.
Any help please?

 

[bellairaphon]

which model?

 

[dragon56]

Sorry! it's a Greenstar 24i junior mark 3 gc number 47-311-86. Hope this helps

 

[bellairaphon]

Possibly scaled up plate heat exchanger.

 

[dragon56]

so that means I need to get a plumber in?

 

[bellairaphon]

Yeah I would. It's not that hard to swap if it is the plate hex. But getting a breakdown engineer. He should be able to diagnos the fault.

Nath

 

[dragon56]

Thank you. I'll do that tomorrow. Thanks for the help.