hot/cold water, sime friendly(not)

[kev25v6]

I have a Sime freindly combi boiler and its still giving me problems with hot water, ch is fine though. The water gets hot then goes cold then hot for a few minutes then cold again. In the last year ive had a new pcb(old one was burnt out), genuine theromcouple(non genuine would not work properly), divertor valve(stopped the hot water going to the rads instead of the taps), a radiator flush to descale/clean the system(water was barely mucky when rinsed out) and a new hw thermister (stopped the pilot light cutting out when you turn off bath taps), the gas pressure is fine and does not drop, the water flow is set to 10l per minute as per the instuction book. The fault finding guide says it could be a 'limit thermostat', am i likely to fix it with putting a new one in, the gas guy im using does not want to just keep replacing parts over and over untill hes basically fitted a new boiler part by part. Any ideas on what else it could be?

 

[Terrywookfit]

Plate heat exchanger blocked?

 

[kev25v6]

would this give no hot water at all or hot then cold all the time?

 

[Terrywookfit]

Slugs of hot then cold!!

 

[ChrisR]

Hot then cold then hot then cold...
Take it (hot water hex) out and clean it, though if you can't manage without removing the gas valve you'll need a Corgi - and not the numpty one whio changed all those parts unnecessarily.

 

[kev25v6]

Ill get someone in to clean the heat exchanger then.Is it a difficult job to clean it out or would most people just replace it with a new one to save having to clean it? Not many gas men round me want to take on work on a Sime boiler. Thinking about it its had a new gas valve on it too some time this last year.

 

[bengasman]

Yes, typical symptoms of blocked plate hex. No point in cleaning or replacing unless you have the entire system cleaned; that is were the blockage comes from.
Powerflush will solve both problems