Low DHW temperature from combi boiler

[mish]

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

[John]

SNIP

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

Try the cheap way first

[andrewpreece]

Not sure about what the problem is, but I'm fairly confident that the lcd display shows the boiler loop temperature, not the actual DHW temperature. The DHW temperature is controlled open-loop as it were, I measured the DHW temperature as being about 20C lower than the LCD display indicated temperature. This also means the DHW temperature varies with the temperature of the groundwater, so you need to adjust the front panel temperature knob a few times per year. The display gives you the impression that the DHW temperature is settable to within a degree C if you choose the DHW menu, but it's only indicative, I'd ignore that menu if I were you. Your display temperature of 88C will translate to a DHW temp of about 65C. In fact fully anticlockwise the temperature knob should give you a temperature rise of 30C over the groundwater temperature, fully clockwise that rises to an increase of 47C, at least it does on my Turbomax. I tell you this info in case it helps your diagnostics.

Andy.

[Ed Sirett]

Whilst I agree that the display temp will not be the same as the HW outlet temp. The variation will between the two will depend on the flow rate. The water tempertaure knob sets the _maximum_ DHW temp. This will come into play only if the flow rate is low enough. Also both sides of the secondary heat exchanger need to be clear, as has been noted.

[andrewpreece]

OK, I should qualfy that by saying that I had a cloakroom basin hot tap on full kilter during my experiments! Also I believe that there is a lower limit to flow below which the combi won't operate correctly, so the DHW temperature is never quite going to get up to the boiler loop temperature,

Andy.