Worcester Greenstar 24i Junior - Hot then Cold cycles??

[racerdude]

Hi,
Recently over the past few weeks our boiler keeps going from hot water to cold water. However sometimes it takes a few minutes to go cold then hot.
Sometimes it works for 30minutes. And sometimes it goes cold instantly and doesnt go warm again for a while.
During this process sometimes the boiler makes clicking/banging noises.
We have had a few plumbers around, they dont seem to know whats wrong?
Does anyone know what needs replacing?
Central heating seems to work fine.
Also the pressure was very low 0ish. Then I turned some vavle to increase it to 1.5bar.

 

[ChrisR]

Does anyone know what needs replacing?

Yes, Sir, you need new plumbers!
Having low water would make the beast hiss and bang, but the HW going hot/cold/hot is nearly always because the plate heat exchanger, aka the secondary or domestic heat exchanger, is blocked on the boiler water side.
It needs to be removed and cleaned or replaced. It could be a temp sensor or something else, but about 95% of the time it's that plate thing.

 

[racerdude]

Sometimes the water is excessively hot?
How much do you reckon it will cost to fix?
Do you reckon its any other faults?

 

[ChrisR]

It would be hot. The boiler can't shift the heat soit overheats, then has to cut off.
Actually your plate's a pig to get out so over an hour, and O ring kits etc.

It can be done in situ with chemicals but not many would try, - messy without the right bits and pieces..