Vokera Compact 29HE CH problems.

[jayzedkay]

Hi, I have a 3 year old Vokera Compact 29HE that has started acting up. To give you some history it had the odd temperamental operation over the past couple of years or so. only very occasionally, maybe just once or twice and it seemed to be when the boiler had been off for some time. sometimes work abroad and the unit is left off for some months. on return, typically in the winter the heating wouldn't come on. a power-down/reset normally sorted it out and all was good. Each time the LED indicator would flash green every half a second indicating a 'temporary' fault. Because it was very occasional I lived with it. I thought it was something to do with the APS and sitting for a while?

Last week it failed whilst i was at work (i work away in the week) leaving missis with no heating, hot water always works. She called out the home assistance that comes with our gas supplier. Guy came out changed something according to missis and all was good. she said, he said something about it trying to supply hotwater even when it was off? doesn't sound right to me, I have never seen the burner remain on when demand stopped. But it worked and didn't cost, so fine. But it failed again some days after with boiler not starting up for heating and the temp-fault half second green LED flash. Again a reset and it seemed to stabilise, but yesterday it wouldn't operate in heating mode at all. hot water worked fine.

called the home assistance again, but no one could come out on that day as we weren't 'priority'. So i'll see what i can do/diagnose myself and either fix it or aid the guy that eventually comes out?

what the boiler was doing most of the time i managed to get it to start up in heating mode after power-down/resetting etc was shutting down after about 20-30 seconds and the LED would flash every half second for 'temp-fault'. I thought it was actually over-heating or thought it was overheating with an erroneous signal and shutting down?
I noticed that when it had shut down the pump continued to run, no fan, just the pump whirring quietly. pump is on maximum position III.
i checked out the primary NTC and this was about 11-12K ohms when the boiler was cool/sat at room temp. with the hot water running for some minutes until it was scoldingly hot the thermistor value was about 1.4K ohms. Same applied to when it shut down prematurely after running the heating for a few minutes, NTC about 1.4K ohms.
I monitored the High Limit thermostat and it was always close-circuit when the boiler shut down. i concluded the overriding shutdown sensors were o.k?
APS switch appears to work fine you can hear it clicking in/out and it doesn't drop out before the fan stops etc causing a shutdown?
Next i went through 7.10.1 fault finding test 'A' algorithm in the installation and servicing instruction manual. I kept ending up at no control voltage from the PCB (CN8.3 + CN8.5) and replace control board. Intuitively this made some sense; an erroneous signal/malfunction from within the PCB?
I took the plunge and bought a new control board, mistake and an expensive one at that. Boiler operated exactly the same with the new control board. Bummer, I tell myself I have a spare now as consolation and to ease the pain somewhat.

heating started working again last night (not shutting down after short while) and this morning it worked o.k for hot water, as usual. But when i came downstairs the LED was flashing green quickly and i could hear the pump continuing to run. It worked o.k with some washing going on kicking in to supply hot water etc, and the heating came on without problems.

There is something wrong and all i think is that the pump continues to run, which it should not when no demand for CH/DHW is requested and the boiler shuts down under normal operating conditions and this results in the 'temp-fault' code? I don't think i have restricted flow as i don't think the boiler is overheating and shutting down. I think it is an erroneous signal causing it to shutdown/not start. I'm no heating engineer though. I'm happy around the electrics/sensors as it's my background. But hydraulics/valves/actuators etc are not so familiar ground.

Hope I make some sense, I have tried to be as methodical and comprehensive in my info to aid online diagnosis.

Any information on what and how to test what i can to try and solve this somewhat increasing frequent frustrating problem?
Thanks in advance for any info/help given.

 

[Agile]

I know that you have diligently done a lot of checking ( better than most boiler engineers could! )

But did you ever check that the DHW demand switch is not remaining made after the demand has ceased?

All you have described makes me think that it thinks there is a continuing demand for DHW.

You may end up having to check the wiring harness for continuity and that does mean measuring the actual low value resistance.

Also check that all the sockets on each wire on the PCB connecters are fully inserted into the carrier.

Tony

 

[jayzedkay]

Hi, thanks for the reply. i haven't checked the DHW demand swith, i'm not sure what/where it is. i'll go through the documentation i have and locate it etc and see what i come up with.

question for my own better understanding; If the DHW demand signal was present would this not keep the burner on? i've read threads on here where boilers remained on after tap closed etc. this was diagnosed to a faulty demand/flow switch?

PCB connections are good, checked, re-checked etc. first thing i did was re-mate everything i could on the PCB and sensors etc.

frustratingly it's been working fine since this morning. so i'll have to wait till it starts playing up again.

i always thought that a pump was only used for CH circulating the water around the sealed system. Thought DHW was effectively pushed through the heat exchanger under it's mains pressure, with some restriction to aid heating?

i'll report back on my findings.

thanks, j.

 

[Agile]

The burner would stay on but only just until the heat exchanger reaches the set temperature then it would sit there withg a hot inside and fire up for a nfew seconds every 20-30 minutes to keep the temperature topped up.

The DHW demand SW is just inside where the CW enters the boiler. Its connected on a plug which can just be pulled out to disengage.

When there is a demand for DHW the CH is disengaged.

Tony

 

[jayzedkay]

Hi, thanks for additional info, it makes more sense to me now. I located the demand SW yesterday eve, but typically the boiler has been working properly since sat evening, i.e. no continuing pump run after DHW demand finished and normal start-up of heating etc.

it makes some logic to my problem that the CH is disengaged whilst DHW demand is present. With my continuing pump running, this would cause the CH to fail to start etc.

i'll report back once it fails again with my findings.

Thanks.