Hot Water Turning Cold With Vaillant TURBOmax 828E Combi Boiler


Postby TheGingerWinner » Wed Nov 07, 2018 12:11 pm

I've got an issue with my Vaillant TURBOmax 828E combi boiler and I am hoping someone might be able to help me understand the likely cause of the problem...

The hot water occasionally goes cold mid flow. I first noticed this when taking a shower, so I had a look at the boiler (without turning the shower off) and noticed that the digital temperature gauge had dropped down from around 75 to 40 degrees. Turning the boiler off and on again fixed the issue.

Initially this wasn't happening very often, but now it's happening 2-3 times a week. I also noticed the other day that it happened for the central heating.

The boiler was in the flat when I bought it in 2007, so I guess it's probably around 15 years old. I am not familiar with the inner workings of gas boilers so I would be really grateful if anyone could let me know what they think the cause is most likely to be...
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Postby DocMartin » Fri Nov 09, 2018 11:56 pm

Bit lengthy, but hang on to the end...
I have a Vaillant Turbomax 828, 2003 vintage. I have had two problems with it. The first was it chewing through diverter valves, largely I believe because I did not fix a leak in the system and used to pump up the pressure with mains water every so often, but no inhibitor. The radiator water was a filthy black colour and I think it just to wear out the sliding seal oif the diverter valve. But I digress...

The second problem sounds like yours. Every so often it just used to run cold. It would usually come back again after a while but there was no pattern to it.

Anyway, I turned it off at the mains with the two pole isolator switch, flipped down the front panel, then undid the plastic screws on the front control panel and flipped that down. Inside is a plastic box clipped to the back of the control panel that protects the pcb. I managed to prise the clips out (not easy) and took off the plastic cover. Inside is the pcb with some fearsomely complex looking integrated circuits and stuff. No obviously burned or charred components. I was about to give up but the error codes (you need the manual to understand these) had already told me that the start up interlock sequence wasn't going right (can't remember the exact codes, but it wouldn't proceed with flame ignition as it wasn't satisfied everything that needed to work was working on start up).

So, there are only so many things the boiler wants right before it lights up and one of these is the exhaust fan. The fan has to start up before ignition and the boiler uses a diaphragm pressure switch to detect the low pressure that an operating fan causes. I dismounted/unplugged the fan switch. Depressing the diaphragm did cause a circuit to be made between the two electrical pins on its connector, but it was 6 Ohms ( I used a multimeter) which I thought was quite high for a basic switch. I replaced it with a new one - lower ON resistance but still no joy. I then wondered if the fan was operating. It wasn't when I demanded hot water*. But was it the fan not working or something else blocking the fan from starting?

(*obviously I had the boiler switched back on at this point)

I traced the fan power wires back to the pcb and (I may have taken the pcb out here) traced them back to a relay on the pcb. Mid-to-rear rightish, IIRC...

Now the bit to be wary of - there is live 240V inside the control panel and in some parts of the pcb. The worst offender is the transformer on the front right of the rear of control panel, when the panel is flipped down. It has exposed solder joints at 240V you can easily touch. I plastered that with electrical insulation tape but you shouldn't poke around with live mains stuff unless you know what you are doing.

Anyway, I demanded hot water from the tap ( I have a hot tap a few feet away from the boiler - outside loo - you may need someone to turn a tap on and off for you) and noticed (a) the smell of ozone and (b) an intermittent green arc inside the fan realy (it has a transparent cover). It also had Deleted on the inside. I isolated the boiler from the mains via the double pole isolator that feeds it, prised the relay cover off (snap clips again) and saw one of the contacts was burned and in a bad way. I inserted some wet and dry 600 grit sandpaper into the gap between the two contacts (use a screwdriver blade and twist the contact arms apart a little - DO NOT PRISE THEM APART MORE THAN IT NEEDS TO INSERT THE WET AND DRY (else you can destroy the gap aligment - ask me how I know that ;0) then do a little light sanding. Not much. Turn the wet and dry over and do the other contact. Maybe give the contact area a clean with a sable brush or a blast of air from a straw or something, then reassemble the relay cover, put the pcb cover back on, and switch the mains back on and test.

It worked for me. No arcing thereafter, and reliable hot water. The thing about relay contacts is they can be a little random when they're dirty and decide to work or not work on a whim,,,
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Postby TheGingerWinner » Sun Nov 11, 2018 11:52 am

Thanks for taking the time to put down such a detailed reply.
I don't think I'm going to tackle this myself, but it's good to have your insights before discussing with an engineer.
Once again, many thanks for your reply. It's appreciated.
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