Probably blown-up Combi PCB with iolloi thermostat !

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aroundM21

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I looked online before I bought the Chinese "iolloi Wifi Smart thermostat 5+2 days" and found that someone had got it to work with my exact type of combi (Vaillant 831 Ecotec). So although the online iolloi instructions aren't exactly the same terminology & diagrams as my ancient Honeywell remote control, I was confident enough.

When I turned on the main supply with the iolloi wired-up there was no reaction but the boiler wouldn't come on. I've since identified that the external 3A fuse had blown and replaced it {there's a heated mirror also on that sub-circuit}.

I'm not sure that I should look inside the Vaillant at the PCB to look for signs of burn-out as I feel I've already gone further than I should. But it's possible that the old set-up was using the 24V call loop and my new wiring could have been for 240V. Sure I'm guessing, but I cannot see any other explanation why the boiler does not start at all (blank screen) but the mirror and even the old thermostat base show as having power.

Any suggestions perhaps welcome but mainly posting to warn others. I think when I've found someone who is confident with Vaillant electronics, I may look at one of the top-brands of Wifi Smart controls, even though more expensive than iolloi.
 

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From the wiring diagram, it does appear that the switching on the new thermostat is directly connected to live.
Screenshot_20221023-154943_Chrome.jpg

...so PCB damage could have occurred (The Vaillant could be using either 24V or 230V for the thermostat).
However, out of intrigue, you have a connection into A and B from your current Honeywell. Did you connect both of these wires to the new 'stat, and if so, which terminals did you use?
 
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"However, out of intrigue, you have a connection into A and B from your current Honeywell. Did you connect both of these wires to the new 'stat, and if so, which terminals did you use?"

I expected the yellow/green wire to correspond to normally open so put that to 6; and the corresponding blue to 5.
 
RandomGrinch said:
I believe your boiler is room-sealed, so access to the PCB should only be attempted by someone suitably qualified!
Click to expand...
Oh no: that suggests very specialised / limited potential ....

I saw 831 PCBs for sale online for £170 so was hoping it wouldn't be too difficult to find someone who is skilled enough to do it.
But clearly I need to think again about going for things to quickly having got my fingers burned once!

Do you know whether I can tell myself whether it's sealed-up ? I'm probably over-compensating by now not opening the cover at all, which should be straight forward ...
 
Do you know the age of the boiler? Did you accidentally cross any of those wires over?
 
CBW said:
Do you know the age of the boiler? Did you accidentally cross any of those wires over?
Click to expand...
I estimate its ~12 years old [it was here when we moved in, but I remember talking with Vailant about the guarantee ending].

Is age a consideration to replacing a PCB ?

I definitely did not cross L/N with the 2 controlling wires. But my concern is that I may have not limited the controlling wires as low/no voltage - the Vaillants CAN use 24V as the "seek/stop" wires. I should have checked how it's wired inside first to be certain. The diagram on the original thermostat isn't enough.
 
No diagram on original isn’t enough, but given the age (why I was asking) it might be mains voltage. Your pcb may have been replaced, but if not, might have failed soon anyway (if that’s some comfort?).
 
CBW said:
No diagram on original isn’t enough, but given the age (why I was asking) it might be mains voltage. Your pcb may have been replaced, but if not, might have failed soon anyway (if that’s some comfort?).
Click to expand...
Well ... like Boris, the plot twists and turns ....

The boiler is now working. I have no explanation for this because it refused to start many times for an hour or more (with me pressing its on/off button quickly or for long presses, and the reset button) after I'd replaced the external 3A fuse.

I'll get a regular service and post any conclusions.
 
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