Advice on Vaillant EcoTec boiler (maybe generic combi issue)

Hi all,
I'm hoping someone might have had this experience, or be able to point me to a better group to post the question.
I have a Vaillant EcoTec combi. The central heating works fine, but the hot water flow runs hot for a while, then goes cold. It got progressively worse.
At first, upping the pressure in the system helped, but that no longer worked and anyway I wasn't comfortable seeing 2+ bar.
I was told it would likely the the water pressure sensor. I have replaced that, but the problem persists (although not quite as bad).
The only other thing I have noticed is the analogue pressure gauge seems kaput, showing circa 1.5 bar at all times, even with the system drained down.
Any ideas of what I can try?
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On 30/09/2018 11:53, snipped-for-privacy@gmail.com wrote:

A bit more info would help... when it goes cold, does the main burner carry on firing or does it shut down. If it carries on firing and you keep the demand for DHW on, do the rads start to get hot?
Once its stopped heating the DHW, does cycling the DHW on and off make it restart immediately?
--
Cheers,

John.
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On Sunday, September 30, 2018 at 1:50:30 PM UTC+1, John Rumm wrote:

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Thanks guys and apologies for the delayed reply. Some more sleuthing has t urned up this:
- CH pressure is 1.4, for example. Starts and pressure goes up slightly ( as usual) to, say 1.8. Rads get hot. Nothing seems unusual.
- Running DHW on it's own, the boiler senses the demand, fires, heats for about 2-3 seconds, then shuts down. If the DHW is running continuously, it might do this cycle again after a minute or so.
- Running DHW when the CH is on does a strange thing. First, the pressure drops to say 1.0 bar. Not so unusual, perhaps. But then the DHW 'fires' in the sense of the control panel seeing the demand and going through it's usual routing, but then it will lock the whole boiler out with a F.20 error . Naturally, at this point the CH will stop flowing/heating.
- A plumber friend suggested it could be the diverter valve (expensive) or suggested I try the temperature sensors first as they are cheaper.
Anything else you can think I can try to nail it down more accurately? I'v e got a load of cleanser running around in it at the moment, but that hasn' t improved anything.
And it's getting chilly these days! TIA.
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On 09/10/2018 13:22, snipped-for-privacy@gmail.com wrote:

Yup that sounds normal.

That could be the sensing of DHW flow has lost sensitivity, but your later bits make that sound less likely.

That is a bit odd - especially the pressure drop. (although I have not studied a Vaillant combi in action closely enough to tell if its normal)

F.20 is listed[1] as:
F.20 Safety switch-off: Temperature limiter
and gives possible reasons:
"Incorrect earth connection between cable harness and product, flow or return NTC defective (loose connection), black discharge via ignition cable, ignition plug or ignition electrode."
Based on that I would look to see if there is a NTC connected to sense the temperature of the DHW. If that were short circuit or low resistance for some reason the boiler would detect the DHW as being at a dangerously high temperature - that could explain the shutdown.
[1] https://www.vaillant.co.uk/downloads/ecotec-exclusive/ecotec-exclusive-green-iq-1/ecotec-exclusive-greeniq-combi-installation-manual-918018.pdf

Yup sounds like a good plan.
I suppose a hole between primary and secondary sides of the DHW plate heat exchanger might also be possible - but that would only really make sense if the mains water pressure was also very low (otherwise the primary pressure would tend to rise to the incoming main pressure).

Can't see the cleaner helping in this case alas, although it probably won't hurt.
If the diagram on Page 47 of the above doc is correct, it looks like the DHW sensor is a clip on pipe style device, and connects back to pin 1 and 16 of connector X2. Disconnecting that and measuring the resistance shown at the terminals might be a good move. You could also add some heat to the sensor with a hair drier and see if the resistance falls as expected.
--
Cheers,

John.
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On 09/10/2018 18:25, John Rumm wrote:

Just realised I have a photo of a 824:
http://wiki.diyfaq.org.uk/images/3/33/VaillantEcotec824HE.png
I had forgotten how well those were packaged, and hence how easy they are to work on. The connections to the DHW pipe stat looks nice and easy to get at.
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John.
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On 30/09/2018 11:53, snipped-for-privacy@gmail.com wrote:

Diverter valve?
My Valliant Ecotec (non-condensing) combi started doing the same thing after a few years. New diverter valve sorted it until it started doing it again some years later. As it's only used to heat the shop since the flat was de-registered from domestic dwelling to commercial storage I didn't bother getting it fixed as there's no real demand for hot water anymore, not until last year when the heating side packed up and I took out a £300 insurance policy under which Valliant replaced the pump head, diverter valve, primary (C/H) heat exchanger(the actual cause of the heating failure - due to blockage), DHW heat exchangers, burner box insulation and a few other bits.
If I change the fan it'll be virtually a new boiler. :)
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Thanks again John (and others),
That one you posted the photo of looks almost identical to mine.
So, I've taken out the temperature sensors on both the DHW pipe and the heat exchanger. Both give about 11K Ohms at cold and that drops pretty quickly to 500 Ohms or less when I get the hairdryer on them.
Makes me think that they might be functioning ok, though I don't have any data on what the actual readings should be. The fact that both exhibit the same characteristics makes me think they are not kaput.
So I'm struggling now. There does seem to be some corrosion around the diverter valve (I shall try to post a photo(s)) but it is dry as a bone there now.
Anything suggested I can try?
Thanks a million again for helping.
Photos showing the slight corrosion (looks historic) should be here: https://1drv.ms/f/s !AviGTSqlox7cipQYjLFVW6iS0ErJbg
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On 16/10/2018 02:36, snipped-for-privacy@gmail.com wrote:

How much is a new diverter valve? It's going to pack-up sooner or later you can be sure of that. Switch it out, if it solves the problem you're sorted, if it doesn't well, you've got a spare to switch out in the future when it does pack-up. :)
Think about it, The symptoms have got possessively worse as the diverter valve has been gradually seizing up and opening less as the weeks go by so boiler is shutting down quicker as generated heat can't be dumped to DHW quickly enough. And my own one EcoTec boiler diverter valve packed up within the initial warranty period and failed again about 5 years later.
Can't see a pressure sensor etc gradually failing, it would either work or not work (would be my guess)
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On 16/10/2018 02:36, snipped-for-privacy@gmail.com wrote:

Well that sounds like the expected behaviour for a NTC thermistor. The manual may have expected values.

Yup, that sounds plausible to me.

It looks like something has leaked a fair bit, and its in the area of the valve.

Looking at the symptom you described, (i.e. hot water then going cold, and the lockout code for an overheat detection), it could be you have an obstruction in the primary side of the DHW HX. If you were getting inadequate flow through that, that could explain both the loss of temperature on the DHW, and possibly a higher than expected temperature rise on the primary side since there would be too lower a rate of heat flow to the DHW.
Hence it might be worth taking out the plate heat exchanger, and checking you can flush through its primary side in both directions... (I am guessing the secondary side is ok if you get an adequate flow rate of hot water (even if its not hot!)

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Cheers,

John.
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On Tuesday, October 16, 2018 at 11:47:57 AM UTC+1, John Rumm wrote:

Thanks again John,
What you describe could indeed be the case. It might also explain why the analogue pressure gauge is sitting with a static reading, even when drained down on the CH. Could be just blocked pipe.
The heat exchanger does get hot but I am not sure how quickly it should happen. It would make sense therefore that it is either the HX itself (blocked) or the diverter valve which isn't diverting fully.
Maybe time to call a Vaillant engineer!
Cheers
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On 16/10/2018 14:56, snipped-for-privacy@gmail.com wrote:

Valliant will give you the option of £299 engineer fix including parts except for Heat exchanger or for the same price yo buy a D&G boiler insurance that lasts 6 months and includes all parts INCLUDING heat exchanger(s).
Dunno about the Valiant diverter valves but on my home MAN boiler I can remove the actuator head and when the hot tap is run the plunger pin comes out of the head to operate the valve. I can also manually operate the valve pin with a "prodder" :) which then checks operation of the valve it's self. What used to happen on that one was the water seal around the valve plunger would weep and C/H water would get into the actuator head causing things to corode and jam. Replaced the head twice and last year the weep became a full on leak so replaced both the diverter valve body and actuator head at some extortionate price but it had been re-designed so hopefully it will be more reliable than the original.
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On 16/10/2018 14:56, snipped-for-privacy@gmail.com wrote:

Well to be fair, those small pressure gauges quite often go tits up anyway, so I would not read too much into that.

If the diverter did not divert fully, then you would expect to bleed heat into the rads when calling for DHW - so they would get warm, the DHW would be cooler than expected, but you would be unlikely to get an overheat situation, since you would have adequate heat sinks.

--
Cheers,

John.
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Update:
Nothing much to report on the DHW. It still fires and starts to get warm, then the flame goes out and it runs cold. The heat exchanger is hot to touch. This will repeat a few times.
What's new is that the CH now will throw a F.75 error unless the pressure is really high. For example, over 2.0 bar (which then increases of course as the heat flows). But resetting the error and upping the pressure makes the CH run fine.
Could it be that the new pressure gauge I fitted is kaput? What are the chances of that?
Thanks
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On 23/10/2018 14:59, snipped-for-privacy@gmail.com wrote:

Manuelle he says: "Water pressure sensor and/or pump defective, air in the heating installation, insufficient water in the product; connect external expansion vessel to the return"

Possibly... you should have two pressure gauges though - there is the digital one on the front panel display, and then the little mechanical one you can see with the front cover of the boiler removed (to the right of the pump - you may need to fold the electronics box forward to see it).
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John.
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On Tuesday, October 23, 2018 at 4:26:11 PM UTC+1, John Rumm wrote:

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Hi John,
Yes, there are two pressure gauges. The digital one runs from the pressure sensor that I replaced and the analogue one is quite literally stuck on th e same reading whether there is water in the CH or it is drained down. I a m guessing that there is some much in the little copper pipe that runs to i t.
I might try draining down again and testing the pressure in the expansion v essel. Not sure if that will help, but worth a shot.
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On 26/10/2018 13:17, snipped-for-privacy@gmail.com wrote:

You don't need to drain down completely for that - just let out enough water to release the pressure on the system. Just allow for a bit of under reading since you will have the weight of the head of water pushing on the pressure vessel even if there is no actual pressure in the system as such.
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John.
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