Vaillant 242 Gas Water heater. Shortcut in board anc compatibility with model 282

[edugimeno]

Hi!!
Yesterday I was installing a new Wifi thermostat for my domestic gas water heater model Vaillant 242/2-3, and accidentlly one wire from mains touched one wire from the thermostat output contacts. This shut off the heater so I finished installing the thermostat and then went out to check the heater

I found one fuse blown (T2 250V). Replaced it with another equivalent fuse and it blew right away. I measured L-N in mains input and I sometimes got 4K but sometimes, not sure why, I got 100 Ohm...
At a certain point, moving stuff around, I put all wires back in place and turned the heater main switch on. I got PWR-ON led lit green. Then I used my cell phone to turn the thermostat ON, and the fuse blew again. It was not like a "gentle blow", it was a huge spark... Changing stuff and moving wires one of the times I put a new fuse in place while the heater was in ON position and I got such a large spark that part of my finger got black smoked with a loud and bright spark

I measured some parts but could not find a clearly shorted component.

Then I remembered that I had a lot of parts from one time I bought a decomisioned working heater, its apparently like mine but this one has a digital LCD. Model 285/2-5. I checked all sockets and wires and they all look the same
The labels on the board cover show different values for P, Q & D, but Im not sure if these are about stuff in the heater mechanical parts (blower, gas valve, etc) or about the electronic board specs.

Would you swap the board right away?
Or any idea on how to diagnose the broken board?

Photos include shorted board labeled "1", model 242/2-3, and replacement board labeled "2" model 282/2-5, with label comparison and front panel for model "2"

Thanks in advance!

[petehall347]

get the manuals for both and compare .
also you could ask vaillant .

[edugimeno]

But the manual provides the same specs (total heating power, etc..) which is the specs for the whole thing (board+ boiler parts), and I can already see those differences in the stickers I attached.
Also if anyone can suggest what to try to repair it, I would also appreciate it!
Thanks

[edugimeno]

Ok I brought home this other board from the "alternate" model of boiler (model 282 instead of 242). I compared side to side and the only difference aside from havnig a socket to plug the LCD at the rear side, is 3 var resistors close to the white plastic 2x12 socket, which are missing in the 282 model, probably because these parameters are setup thru the LCD (burner delay, gas Hz and heater pump time)
Assuming that being a gas driven applicance I should probably not swap these boards being not 100% equal, to avoid any risk, but I can compare components.
I compared one by one all components (continuety, resistance, diode voltage, etc) and they measure the same from one board to the other.
So at this point Im lost.
How can I test this board and not having to replace 100 fuses? I remember there was a method of using a filament bulb in series when there was a shortcut but I don't even have one of those anymore...
Any other idea on how to prevent too many fuses to blow while I test it?

Thanks!

[petehall347]

firstly check for shorted semiconductors . no power needed for that . neither should there be . yes you can use and old school light bulb or a variac . its pointless blowing fuses it will only cause more damage anyway .

[petehall347]

i take it the fuses are blowing with it wired as it was before you did anything ?

[redwire]

Hi!!
"... accidentlly one wire from mains touched one wire from the thermostat output contacts. [...]

Is this the 24VAC thermostat side you are talking about, that it touched mains?
edit: what it's 21VAC here, from the PCB label.

[edugimeno]

firstly check for shorted semiconductors . no power needed for that . neither should there be . yes you can use and old school light bulb or a variac . its pointless blowing fuses it will only cause more damage anyway .

Yes as you can see above, I already checked all components one by one. No semis shorted, resistors show correct value, diodes probe right, caps are not shorted. I will try to find an old filament bulb but they are not common here anymore.
Thanks

[edugimeno]

i take it the fuses are blowing with it wired as it was before you did anything ?

Yes, I put everything back in place with all wires and sockets and it blows but I want to believe that there was one time it blew right away and then it ended up letting me turn it on and then blew when I turned the thrmostat on... There could be a bad contact somewhere. This boiler has suffered a lot of corrosion as I used to store chlorine under the boiler and one day there was a water leak so lots of fumes ended up getting into the boiler...
I will probably try to see what happens if I replace the fuse and put everything back in except the large outputs (blower, pump and gas body) and then plug them in one by one

[edugimeno]

Is this the 24VAC thermostat side you are talking about, that it touched mains?
edit: what it's 21VAC here, from the PCB label.

My house is using the 230V thermostat lines (pins 3&4), which you would see jumped by a red wire in the "new" (alternate) 282 board
I've checked this line and it bring mains LINE, drives it to pin 3, returns at pin 4, 2 R (R400&401) in series, D400 in series, and then it enters an opto IC400 by the red fuse holder
I've never used the 7&8&9 pins ("21V") which are supposed to be for 24V thermostats

On the other hand, manually driving 230V from main to the thermostat leads doesn't seem to be likely to damage anything...I mean if I touched L from mains into N from thermostat it would have blown a fusem which it did, but what else could have been damaged??

Thanks!

[redwire]

Between the two boards you can take comparison ohm/diode test readings with a multimeter.

I don't see much on the mains side. Some relays, not sure what D104-D107 are doing.
Is the igniter powered from mains? Check T100 (SCR?) if shorted maybe. R401, R402 and that middle PCB trace look a little smoky but this might be dirt.

[edugimeno]

Between the two boards you can take comparison ohm/diode test readings with a multimeter.

I don't see much on the mains side. Some relays, not sure what D104-D107 are doing.
Is the igniter powered from mains? Check T100 (SCR?) if shorted maybe. R401, R402 and that middle PCB trace look a little smoky but this might be dirt.

Yes like I mentioned above I already measured all components, one by one, comparing both boards
The igniter is pulsed by T100, which was tested with same values as working board
I think those diodes D104-107 are rectifying a reduced voltge (C+R network) from mains after fuse and white filter caps. At least they are connected in bride rectifier configuration
R400&401 measure 22K as they should be. They are used to lower the 220V from the thermostat 230 contacts to drive an optocoupler

[redwire]

This is a Vaillant TURBOmax 242?
To make sure - the problem is the board keeps blowing the mains fuse T2 250VAC, whenever the thermostat is turned on? The thermostat is external, a wireless box?
Something is overlooked. The controller board has no mains loads other than the small power transformer (T1,25A?) and you say the board powers up though.
Demand for heat, it should close the relays and switch on mains power for the circ pump, fan, start the igniter etc.
That leaves - I wonder if a relay is damaged, welded contacts shorted to something? Otherwise there's nothing else I can see that could short mains.
You must have the thermostat wired wrong. Neutral is pin 5.

[edugimeno]

This is a Vaillant TURBOmax 242?
To make sure - the problem is the board keeps blowing the mains fuse T2 250VAC, whenever the thermostat is turned on? The thermostat is external, a wireless box?
Something is overlooked. The controller board has no mains loads other than the small power transformer (T1,25A?) and you say the board powers up though.
Demand for heat, it should close the relays and switch on mains power for the circ pump, fan, start the igniter etc.
That leaves - I wonder if a relay is damaged, welded contacts shorted to something? Otherwise there's nothing else I can see that could short mains.
You must have the thermostat wired wrong. Neutral is pin 5.

Thanks. Yes this is the schematic for my boiler.
My thermostat was mechanic but I replaced it by a "wireless tuya compatible" thermostat. Anyway its output is a "dry contacts relay", so it should not matter what voltage gets into the thermostat on pins 3&4
Yes when heating is demanded, after checking for some conditions like no overheating, no water underpressure, etc, at the sensors the boiler has, it should start the air blower, the water circulation, open the gas valve and ignite the electrodes.
I will check the relay connections and probably apply DC to the coils and see if they toggle between NC to NO leads.
Also like I said something changed while I was checking this boiler, because first thing when I put the new fuse it blew right away and then later on after many tests, it would hold power on and would only blow when I demanded heat from the thermostat.
Im going to hook up a filament bulb in series, have all the outputs (pump, fan, gas) disconnected for the first time, and power it up, and then start plugging stuff in one by one.
Maybe one of the times the fuse blew it broke whatever short it was having...who knows.
I can't do this until next weekend as the boiler is at my town house, I will hold this for now and get back when I get results
Thanks!

[edugimeno]

Ok I have some information today. I went to my town home and spent some time checking this.
First of all, I still got the spakr and blwon fuse when I triggered the thermostat. But it wouldn't blow if I manuall would jumper the trigger points (pins 3&4). I assumed that my thermostat was bad and it was incorrectly putting mains voltage into what should be a dry contacts output, and then when I got home I checked the manual and the ouputs I was using in the thermostat were labeled "LOAD" and I had other output labeled "DRY CONTACT", so it was my bad.
No back to the boiler, by using just a wire jumper between 3&4, I tried everything. Boiler would start and into stadby mode. BUT it would start the igniter all time right away as soon as I powered it, non stop. Then I demanded hot water, the led indicating it would light up, but the ignitor won't stop clicking and it looks like it wasn't starting all the other stuff (gas, blower, etc)
Same with heating, I would jumper 3&4 to request heating, and it would light up the corresponding LED, but it wouln't do anything else

Then I tried what I didn't want to try. put the other model board (Vaillant 282 instead of 242) in place, same board format, same sockets at same place, just one additional LCD screen.
It would start fine in standby, and when hot water or heating was demanded, it would take some time to start, like "trying" and then after 10-20 seconds it would do all its things correctly, hot gases out of the exhaust pipe, hot water if I touched the exchanger, and stable. BUT I stayed there for a while and I realized that it was turning off and on all the time the gas fire, not sure if the fire itself had extinguished and it was lighting it again, or it was incorrectly feeling the fire was off, I will open the fire chamber next time and see what's going on inside

Option 1 is this board uses different levels for some of the sensors or actuators and the gas level is low, or the flame ionizer detector is weak
Option 2 is that after not using this boiler for several months, the gas noozle is a little dirty...

Does anyone have experience enough to assist no this?
Thanks

[redwire]

The boilers can have different size flame detectors, the probe current might be different- or your flame rod is dirty. It's just sandpaper to clean it and the wire must be in good shape, away from other things and no water or rust on the probe.

On the dead board, I think the board's thermostat input pins 3&4 is damaged from mains going there. OUCH!
Check if opto-coupler IC100 MOC3062 is what gives power to the igniter, wild guess- it might be shorted pin 4-6. The MCU drives it on pins 1,2. Also the 4 diodes nearby D104-D107 see if they are OK.
This would explain the igniter being stuck on.

[petehall347]

It would start fine in standby, and when hot water or heating was demanded, it would take some time to start, like "trying" and then after 10-20 seconds it would do all its things correctly, hot gases out of the exhaust pipe, hot water if I touched the exchanger, and stable. BUT I stayed there for a while and I realized that it was turning off and on all the time the gas fire, not sure if the fire itself had extinguished and it was lighting it again, or it was incorrectly feeling the fire was off, I will open the fire

water pressure would do this is water pressure set and holding on the gauge ?

[edugimeno]

Actually my old front panel had an analog pressure meter that was stuck, so I've been blindly adding or removing water to the system for some years.... When I first plugged this other new panel with alternate board fist thing I realized is that it shows the pressure in the digital display, and I believe it was showing 5.X bar....way above required. I just downloaded the manual and it says 0.8 - 2.0 bar working pressure.
Maybe my old board woulnd't cut due to overpressure and this one does. I will empty some water and try again.
I understand that if the water pressure is too close to the maximum, when heating it would increase (due to some air bubbles that dilate) and would get to the cut point, then cool down and get back to working point...right?
Will try soon
Thanks!

[edugimeno]

Ok, last one (hopefully) update on this.
I released some of the water pressure out, tested the heater for a while and seems to be running ok, continously with the new alternate "digital" board, so everything fine for now.
Thanks everyone!