Fluctuating hot water with Vaillant Ecomax combi 828/2E

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Can anyone help, at wits end with fluctuating DHW temp.
Turn on a HW tap, get variable temp, lukewarm to warm, never 55 deg C as set, HW temp approx 30 deg C.
Looking at boiler (Vaillant combi 828/2E Ecomax) display when HW tap turned on, looks like burner kicks in for a few secs, HW outlet pipe gets hot, gas valve goes off therefore turning burner off, pipe goes cooler, then kicks in again. This cycle goes on, apprx 20secs on, 20secs off, giving DHW problem.
Anyone got any ideas, local plumbers about as much use as chocolate fireguard.
 
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blocked plate exchanger, system full of carp, powerflush will solve both
 
this has been checked twice, once by Vaillant engineer, and no problem with heat exchanger, system fairly clear, boiler fairly new only 2 years old.
 
Did the Vaillant engineer really REMOVE and inspect the plate HE ???

Who else inspected it?

There is a possibility that the diverter valve is faulty although if it really is only two years old that should be the new improved type.

I would still suspect the plate HE !

Tony
 
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When the boiler is faulting, what 'S' codes are displayed when pressing the 'i' button?
 
When HW tap on boiler sequences S13 then S14 as expected, then just before it conks out goes to S16 which according to installation manual means fan over run. However, as expected, manual doesn't explain what this means, and expects you to call out Vaillant engineer for £220 a pop!
 
What is the temp reading on the display when it cuts off?

What are the temp readings on d40 & d41 when it goes off?

Look in d60 and see what number, if any is displayed

You could also look at d35 and tell us what this says when it is on dhw, although this is what the pcb thinks it is rather than actual, but could help.

Did you actually get a proper Vaillant engineer or a Glow Worm numpty from group service?

Call them back and insist on a Vaillant engineer.
 
d40 is 86°C when goes off, d41 is 72°C when goes off, D60 is set at 8 and D35 when DHW on is 1 = hot water.
86°C seems a tad high to me
 
86 is certainly high I think, It is pointing to the plate heat exchnager being blocked on the system side and not being able to transfer all the heat into the dhw side.

This has been suggested by Ben and Tony previously, but from these results I would now say it needs to come off and be at least cleaned with DS40 or replaced.

I still don't think you had a proper Vaillant engineer out though.
 
Heat exchanger came off and was cleaned just before Christmas, all be it by non Vaillant plumber. I wasn't there at the time but was told there wasn't much in there and was cleaned out.
Heat exchanger was taken apart by Vaillant engineer in Dec 2006, but then the problem was attributed to faulty thermostatic mixer on shower which was causing cross coupling. Have checked this with shower mixer this time round and thats not the problem.
Have tried turning down DHW and central heating control so as to try not to get to 86°C to no avail.
 
Well, d60 proves it is overheating, but the reason is obviously what we have to find out.

As a long shot try opening and closing the auto by-pass valve, noting how many turns so you can set it back again.

Has the pump been taken off and cleaned?

I'm still suspicious of the plate HE though, you cannot tell by visual inspection if it is clean thoroughly.

Perhaps get Vaillant to change it under their 5 year warranty, although technically this is just for scale.
 
Right, Vaillant engineer has been. The reason the boiler is overheating is because it's full of gunk. System needs a powerflush and even then might not fix problem. May need new HE's. Question is boiler is only 32 months old and when it was fitted it had a power flush prior to installation. So how has it got so full of gunk in 30 months?
 
How do you tell if it's been flushed properly? I was there when it was done initially and seemed fine to me. Attach machine to sysyem, plug machine in, add chemicals, turn on, takes about 1/2 a day. How can it go wrong? If I get someone in to do another powerflush, how do I know they've done it right, and I'm not just being ripped off again.
 
Attach machine to sysyem, plug machine in, add chemicals, turn on, takes about 1/2 a day. How can it go wrong?

Once those things had been done just what was the guy doing for the rest of the time?

Sitting and reading the paper?

I once saw a plate HE which looked fine when you gazed into the inlet holes but was blocked to the point of almost no flow through it with a calcium/iron compond in the centre.

Tony
 

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