I would be greatful for any help/advice anyone can offer please.

I have a Vaillant Ecomax 824E combi. It was installed in November 1999
and was fine for the first 5 years or so. The system was treated with
Fernox from day one which I kept topped up. We started to have DHW
problems with large temperature swings and various faults (F23 - water
flow restricted - was common) . My solution was to take out the DHW
heat exchanger and flush it with mains water. I would also flush the
main heat exchanger in a similar way. I used to dislodge plenty of
debris but this has reduced of late. I also had the whole system power
flushed (but this did not include the boiler itself) and treated the
system with Sentinel. We continued to have problems which I could
temporarily alleviate by flushing the heat exchangers until finally the
boiler would not even run on CH for very long.

I have now replaced both heat exchangers, simulateously flushing the
system and retreating with Sentinel. CH is now perfect and the fault
messages have ceased. However the DHW is nowhere near as hot as it
once was at the shower. The boiler gets up to about 68 degrees, the
gas reduces until it is down to 60 and modulates between the two. (On
CH it gets up to 80.) I am confident the pump and diverter valve are
OK. So do I have a PCB problem do you think? The waterways must all
be clear now so I am at a bit of a loss. I can get a hot bath by
slowly filling it. It is almost as if the boiler cannot cope with the
demands of the shower yet it used to and the CH is very hot.

A couple of other points. The water mains pressure is very good. I
noted that the new DHW heat exchanger came with a couple of tubes which
fitted into the DHW inlet and outlet - the old one did not have them
but nevertheless I fitted them.

Thank you for reading this - your collective experience is far greater
than mine and I am reluctant to buy a new PCB on a whim.