"mish" wrote in message
om...
Our combi boiler (Vaillant TURBOmax+ 28) has recently started playing
up. The CH is fine, but the DHW starts to run hot, then alternates
(just about -- nowhere near as hot as it was running a couple of weeks
ago) hot and cold.


SNIP

In summary:

1) burner & main heat exchanger must be fine (otherwise CH wouldn't
work)
2) pump must be fine (otherwise CH wouldn't work)
3) priority diverter valve must be fine (otherwise CH wouldn't work,
or it would shut off with "pump failure/blockage" when in DHW mode)
4) DHW demand flow sensor must be fine (since burners fire when demand
is applied)

The only thing left that I can think of is that the DHW heat exchanger
is not working properly (so the circulating water stays hot and the
DHW stays cold).

The trigger for this problem seems to have been draining the CH system
(to replace 2 rads downstairs) and adding flushing chemicals. The
chemicals were meant to stay in the system for another 2 weeks (4
weeks recommended for existing systems).

My theory is that the combination of drainig/refilling and the
flushing chems has dislodged a load of gunk/corrosion which has
blocked most of the heat exchanger.


Its quite a common failing in your circumstances.

You will need to take out the secondary heat exchanger, clean it out
manually and reverse flush it with a hose
Also check your diverter is clear in the HW port
Does this sound right, or have I missed something?

And if so, how to fix it? New heat exchanger? New boiler? Power flush?

Try the cheap way first