No big flames inside Vaillant VCW Sine 18T3WFH !!

[Beavis]

Moved into our house in Dec03. The boiler is a Vaillant VCW-Sine 18 T3 WF H 'combi' boiler which does heating and on-demand hot water. I was told by a plumber (CORGI) guy who called to connect our new cooker that this boiler is "a decent one". It has worked fine for months and now it seems to have packed in completely! The symptoms are : Pilot light is alight, but no 'big flames' ignite when the heating is turned on (wall mounted computer), or a hot tap is turned on (cold water only comes out). The problems started when I turned off the mains water to plumb in a dishwasher under the kitchen sink. Did that ok, but stupidly thought I needed to turn boiler pilot light off while mains was off. Big mistake! At first on restarting the pilot wouldn't stay alight (it kept going out after lighting, until I held the green button in for a 1min instead of 10secs...borderline thermocouple!), then when I got that going, no hot water or heating. The system is pressurised at spot on 1bar (I have tried 0.5/1/1.5/2 with no luck). I have bled all rads, and all air from boiler (including topmost bleed valve). The main control unit makes the correct relay clicks when CH/HW asked for, but the pump does not start. At first I looked at the water-difficiency venturi/diaphragm/valve, but then realised that this would probably work if the pump was running. I have checked for power to the pump and also at gas valve solonoid. Nothing at either (suggesting both are ok, but the control PCB is not sending them a signal). I have also fiddled with the NTC and the flow-stat. EVERYTHING I do gets me nowhere! I am running out of ideas. How could plumbing in a dishwasher knock out the main PCB?!?! It's a totally seperate system!! Fuses are ok. All looks ok. Any ideas? House is freezing! Wife is complaining!! I am an engineer by trade (with degree) so I'm one of those "I'm not paying to get a plumber in, I'll fix it myself!" people! Trouble is I've been messing with this thing for 5 days and I'm getting nowhere!! Help!!

 

[weargas]

on the right hand side of the pcb housing box is the most awkward to get to overheat reset. 1st thing to try.

 

[Beavis]

Thankyou. I will look for this!

I don't think the thing has overheated, but you never know.

I assume I am looking for some sort of push button.

I'll try it and report back.


Scott

 

[galwag]

The problem lies with the mechanism behind the pilot light green push button.

From memory there is a microswitch that makes once the pilot has lit.

Try turning the pilot light off with the red button & relighting again to see if that clears the fault - repeat a few times if necessary.

If this fails you will have to gain access to behind push button assy to reset mechanism.

 

[Beavis]

Thanks for your suggestion, I think that this must be on the right track. I had high hopes about this mysterious overheat reset button, especially when I got home, looked at my boiler, and sure enough it had one! I pressed it, but alas, nothing has changed.

Since the only thing I did during the dishwasher plumbing episode was to stop and restart the pilot, this microswitch does sound like it could be the cause.

I have read the installation manual and looked at how to remove the control unit panel. Since my boiler is 'hard wired' I'll have to shut of the house electricity to be safe. I'll take a look at this later tonight or tomorrow and report back.

Thanks! Scott

 

[DP]

It would be easier if you tested with a multimeter

 

[Beavis]

I have been using a multimeter for most of my tests so far. eg: AC volts for pump/solonoid, continuity test for microswitches, and ohms for NTC (~4.7k...not sure if that is about right). Tonight will be my fifth night going home to a cold house with a 'new lead' in my invetigation. So far, every exciting new idea has resulted in dismay. It's so depressing when you think you've cracked it and then, no, that's not it.

I'll shut off the mains electricity tonight and open up the control box.

The pilot is staying alight, so I'm pretty sure the thermocouple is working ok. I assume the thermocouple 'signal' is the thing which keeps the pilot gas supply flowing, but there must be an electrical switch as well as a gas valve behind the green button. Sounds like the gas part is working ok, but the electrical switch is not telling the main PCB that the boiler has gas. I've tried running hot water with the green button pressed. That didn't help.

I've moved the pump connection to "I" so that it runs continously when the heating is on. I tried pos III too (always running) but that didn't make it work. I removed the pump centre cap and checked that it spins ok. No debris it it.

Looking at the circuit diagrams there are only about 3 or so 'switches' which can stop the pump running. One is the heating on/off switch, one is the overheat switch, and then the other is the green/red button gubbins, "FFD" I think it is labelled.

I'll look for this microswitch tonight and report back. Hopefully it will do the trick!

Thanks, Scott

 

[Beavis]

HOORAAAAYYYYY!!! It works!! **What a polava!!** Thanks to galwag and weargas for the suggestions. The microswitch idea was almost there. I took the control box apart and found that the small barbed copper spring which locks the mechanism in the on state and holds the two microswitches in the 'closed' state (ie: switch buttons NOT pressed, they are "normally closed" switches) was not doing its job properly. The pilot was ignited, and the thermocouple then kept the gas on, but the mechanism was flicking back to the 'red button' (stop) state and pressing on the microswitches. I bent the spring slightly to ensure the metal plate did not slip over the barb. Re-assembled and it fired up fine!

As a final bit of excitement to the whole episode, the pressure gauge then started to rise to over 2.5bar! I quickly bled off some water and got it down to 1.5bar hot / 0.8bar cold. I had started the boiler with 1.5bar cold pressure. It seems that this is too much and when it all heats up (even with flow stat only set to 2) the pressure builds up too much. After some careful bleeding of the rads it is now running happily at around 1.3-1.5bar. House is warm, wife is happy. Excellent!!!

 

[Beavis]

Pix and text here
http://www.beavispits.co.uk/boiler/boiler.html