Vaillant Boiler not starting Central Heating

[LanceB]

Hi, I have a Vaillant Thermocompact VC GB 242 EH which is not giving me heating. It must be at least 18 years old but has been in generally good working order.

The symptoms started as the pump going on and off. The boiler would start fine but then I noticed the pump cutting out and coming back on. This was accompanied by a microswitch type clicking which I think was coming from the main control unit. Now the CH doesn't start at all. By this I mean the programmer is on, the room stat is turned up but the pump and fan do not start. There is plenty of water in the system.

If there was a lack of water of air pressure etc I would still expect to get some action out of the boiler just not ignition. I am getting nothing. I know the pump an fan are fine as the DHW works fine.

I have checked the feed from the room stat and there is a signal getting to the boiler fine. I have also 'fooled' the system by depressing the microswitch on the diverter servo and the boiler fires up fine and starts to heat the rads.

The diverter valve, servo and the micro switches on the diverter are all relatively new. The diverter is moving ok when the DHW is sensed and returns ok when the tap is switched off.

Due to the initial symptoms and the fact I am now getting no ignition sequence at all I am starting to think it may be pcb related (or one of the switches on a pcb). Trouble is there are 4 boards - the main one with the on/off switch and 3 smaller ones connected to it. It is not possible to see anything obviously wrong with pcbs and they are not particularly cheap so don't what to play guess the pcb to replace.

Any ideas on other causes or feeds that may be preventing the ignition sequence, or any idea which pcb may be at fault (if at all).

many thanks
Lance

 

[Agile]

The last time I went to one of those the owner left me to fix it while he went out for the evening.

Trouble is he double locked the door and there was no way out for me and the fridge was empty!

Very luckily he saw the text and came back. I had even reminded him NOT to lock the door!

You have given quite a good description and it sounds unusual. Perhaps one of the others will have encountered it before.

Tony

 

[LanceB]

Me and this boiler have a love/hate affair lol

I have repaired it a number of times over the years and experienced most of the mechanical issues with the diverter, the servo, the fan and the heat exchanger. I am therefore familar with how most of these parts work. Usually when there has been issue, I have had the fan or the pump start up just no spark. This time I am getting nothing. the fact the DHW works fine shows there is nothing fundamentally wrong.

Trouble is trying to find an engineer who knows something about this particular boiler. I often find I know more about some of the specifics (which has saved me money when a worn servo was the issue and not the whole diverter valve itself).

 

[Agile]

You have two problems!

Few engineers have the desire and skills to diagnose these more obscure faults.

Then faced with such an old boiler they take the view that its not worth repairing and the sensible solution is to replace it.

I am one of the few who will repair old boilers but thats not to my advantage as I do repairs on a fixed price basis!

Tony

 

[gazdaz36]

Nice complex boilers to work on these lol.

One thing to check for is that the diverter is moving into the central heating mode properly. The purpose of the servo valve is to move the diverter into the correct position. If someone has increase the length of the pin above the watersection that activates the dhw microswitch then the servo does not move the diverter valve fully into the central heating mode.

On the end of the diverter valve there are 2 microswitches. I think when there is a dhw demand the diverter pushes the plate away from the microswitches and for central heating it pulls the plate towards the microswitches.

If both microswitches on the diverter are not made then the central heating will not fire up

 

[DP]

OP, have not read your put fully, so excuse me if you have already checked boiler for HW demand.

Do you get HW when you run a tap?

If yes, then Have to say nothing wrong with ignition sequence or the pump

 

[LanceB]

Hw is fine. pump is fine. fan is fine. diverter is doing what it should. I think either control unit is faulty or another signal is telling it not to start. all the external signals appear ok ie room stat and timer.

 

[gazdaz36]

Are both microswitches being operated on the diverter?

If they are then possibly its a motherboard fault. With dhw ok it rules out the fan and ignition pcb. Try giving the ch switch next to the on off switch a prod to see if it makes it work

 

[Agile]

Statistically the operation/settings of the microswitches are the most common cause of these type of problems in my limited experience of this model.

Have you done a contact resistance measurem,ent of the NO and NC ?

Have you done a voltage across contacts measurement when they have been operated? In fact are they all apparently being mechanically operated fully?

Tony

 

[LanceB]

Both microswitches appear to be working. i also replaced the whole assembly last year. there is a black switch next to the main power one which has a picture of a rad on it. that is in the on position. my guess was motherboard as i couldnt see any thing mechnically wrong (and most mech bits have been replaced at some point). Trouble is there are 4 circuit boards in there. i have a copy of a service manual which i will read to see if it sheds any light.

 

[LanceB]

Hi Agile
what are the NO and NC?
Thanks

 

[gazdaz36]

Pcb's

Top - Igntion pcb

Middle - cpu board

Bottom - Fan pcb

the one they all plug into is the motherboard with the on-off switch

Be careful also as this boiler has different cpu board and motherboard depending on age.

This is classed as hybrid and non hybrid pcb's

 

[Agile]

Merely physically being operated is no guarantee that its being electrically operated.

Needs low resistance measure of the resistance on Normally Open contacts when switch is operated.

Also resistance of Normally Closed contacts!

Needs to be less that 1.0 ohms. Waste of time using a 10 K range as some nupties do.

Tony

 

[The831Bunny]

First of all to confirm, I guess you mean you have a VCW 242EH Combi. A VC is a system boiler.

From whats been described the problem sounds like a fault on the mother board. A couple of checks would be to confirm there is 230V mains live to T4 (switched live from controls), and the pump mode jumper plug is in 1. The pump mode plug is on the left hand side of the terminal box access. It may be a triangle plug with one wire, or an oblong plug with a loop wire. Either way set it to 1.

With 230V confirmed to T4 the pump should run immediately. If Dhw is functioning OK, and the pump still dosnt run, then you are looking at a faulty opto-coupler on the mother pcb, or possibly a faulty pump relay. The fix is to replace the PCB Part No. 130331 (Hybrid version with HY on the control box)

 

[Bunnyman]

Arr u beat me to it 831 the dreaded optocoupler, what a great invention! I agree prob mother board, pull off the black knob if it's plastic it's hybrid if it's brass it's non hybrid an u have a top Vaillant bloke it Essex mate google it. http://www.vaillantservice.co.uk/VAILLANT/Vailliant_Home_Boiler_Repairs_Servicing_Faults.html