Vokera Excell 80 SP - DHW temperature fluctuating (Soft water area, Glasgow, Scotland)

[Jamie McNaught]

Hi All,

I think I've read almost everything on DHW heat fluctuation on this group, but I've not hit a point where I am not sure how to continue.

For last couple of months this combi boiler has not been keeping a flow of hotwater at a constant temperature. When we have showers or try to run a bath the DHW temperature goes hot, then cold, then hot etc. The boiler appears to be cutting out while it waits for the temperature to go down. Central heating is fine.

Late Nov 07 I checked the diaphram, it was fine. Secondary heat exchanger showed a fair amount of sludge buildup (discolouration) on the DHW side. The whole system was treated with Sentinal X400 and after about three/four weeks we have reasonable showering again... but not perfect.

Approx 30/12/07, the system was emptied, refilled and then sprung a leak. Leak was fixed and system refilled. Worryingly the water leaving the system was dark orange (I have emptied a radiators in the past and not added inhibitor when topping the system back up).

However, DHW started fluctuating again within a week - I'd not put in inhibitor as I was planning on emailing this group to ask if there was anything more I should do to improve the showering.

Anyway, in early Jan 08 I tried another treatment of X400, it's been in for about 5 weeks now. Goign to take it out in the next couple of days and refilling with inhibitor.... but the temperature still fluctuates (it does go cold in the short... albeit briefly).

1) Does this sound like a problem with the secondary heat exchanger? Could it still be clogged?

2) Should I try something other than Sentinel X400? It seems to be quite weak, with few warnings for handling and can be kept in for weeks/indefinitely??

3) Should I remove the secondary heat exchanged and check it for sludge, potentially replace it?

4) Should I be worried about the dark orange colour of the water when I am draining the system?

5) Should I give up at this point, admit defeat and call in a professional?

Does anyone have any suggestions?

If you have read this far, thanks so much! Any help would be greatly appreciated!

Regards,

Jamie McNaught

[plummer_a]

...

I had a (different) boiler that had fluctuating temperature of DHW, which had flakes of some sort of iron/rust (it was magnetic) in the primary of the PHE. according to my mobile phone, replacing the PHE cured it (i.e. I haven't had a call from the client to the contrary :-)).