Vaillant VCW 242 EH hot water query

[paddy69]

Hi! I am having an issue with my hot water on a Vaillant VCW 242 EH boiler. The shower has started suddenly going cold. I can see what is happening and wonder if someone can point me in the right direction!
When the hot water is switched on the microswitch on the water section is activated and the diverter valve is pushed outwards and we have hot water. However, if someone draws water on our system (unfortunately it is also shared with next door) the water pressure can drop enough to turn the boiler off! HOWEVER, when the pressure comes back, the microswitch on the water section is reactivated, so the boiler is running, however, the diverter then goes into CH mode and the hot water goes to the radiators.
Is the problem here likely to be in the servo valve OR will it be in the water section (the microswitch is working fine)???
Thanks!
Phil

 

[paddy69]

All the small tubes are clear and servo has been swapped (I keep spares) However, when having a play I discovered that if I screwed the small tap (middle bottom) that looks like it controls the flow of hot water a bit, in, I could end up with NO hot water activating (switch on top of water section operates BUT diverter goes into CH mode). However, if I screw that tap out then I get hot water mode working again!

 

[Gasguru]

Is the adjustment on the microswitch and vertical pushrod correct?
These boilers are just too complex to diagnose over a forum.

 

[paddy69]

Are you talking about the microswitch on the servo?? It is adjusted such that only very small movement of the cam is needed to switch it. Not sure how these should be set.
As for the vertical push rod! This comes from the top of the water section? Not sure about that! What exactly does it do?? The boiler has been working fine for years so I can't imagine a setting has suddenly gone. More likely something upsetting the hydraulics.
As you say, a complex, highly over engineered boiler, but incredibly reliable (most of the time) AND, if you know how to diagnose and fix them, they go forever!

 

[paddy69]

DOH! Of course the vertical push rod operates the cam!!! This DOES work, operates the microswitch. The microswitch stays activated when the water pressure drops, however, the diverter valve gets drawn in and then goes into CH mode!!! And it doesn't always go back to DHW mode, even when the pressure comes back up!!
Confused!!

 

[Gasguru]

I suspect that the cam is rotating just sufficient to operate the microswitch lever but not sufficient to ensure the ceramic valve blocks (inside the servo valve) are
aligned to allow full differential hydraulic pressure to be applied to the diverter diaphragm to move it to DHW position.
Have a look in the manual for the setting clearances.

 

[paddy69]

 

[paddy69]

Thanks Gasguru! That sorted the problem.
I could really do with someone who knows these boilers inside out, to sit down and give explanations of how all the parts work. I know it would end up being a huge book but would be really useful for people trying to diagnose things on these boilers.
Phil