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.

The DHW demand sensor is activating, and the boiler is firing, but the
water just isn't getting hot. The temp shown on the control panel
(which I think is the temp on exit from the boiler, but could be on
exit from the DHW heat exchanger) shows the temp increasing fine -- in
fact it reaches 88degC or so then the boiler switches off until it
drops to 75-ish and it fires up again. (On one occassion the boiler
shut off completely with a "dry fire protection" -- I assume the temp
reached its safety cutoff before the burners switched off.)

Initially it seemed able to maintain a relatively low flow rate (a
half open tap), but not max flow. (At max flow the water would start
to run colder, then the burners would switch off when the circulating
water reached 88degC, which dropped the DHW supply temp even more.)

In the last couple of days it seems to have got even worse, unable to
produce any noticable warmth unless at quite low flow (quater to third
open tap), and unable to maintain it even then (presumably because
quite quickly the burner cycles off as the circulating water
overheats).

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.

Does this sound right, or have I missed something?

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

Given that those all involve giving a plumber several hundred pounds,
and probably several days waiting (esp as it is a bank hol weekend) is
there anything I can do myself to fix/improve the situation?

Should I drain down to get rid of the flushing chems and whatever
sludge I can now (rather than leaving it for the remainder of the 4
weeks), and then refill (and add inhibitor)?

Is the heat exchanger something I can remove and flush out myself?

Cheers

Misha