Hot water problem

[coldinchester]

Hi I'm new to this forum and I'm hoping someone out there might be able give me some clues to a hot water problem I've got.
I have an old Sime Friendly Format 80E combi and the central heating works fine but when I turn on the hot water taps or shower the flame come on for about 20 seconds then goes out, then on again after another 20 seconds. This cycle continues, giving intermittent warm/cold water. I realise the boiler old and we should probably get a new one but any ideas of what the problem might be would be greatly appreciated as money is tight.

Many thanks, Will.

 

[hertsboilers]

Secondly heat exchanger for hot water blocked. Need a clean or new heat exchanger.

 

[coldinchester]

Thanks for your message, I'll give that a try.