Our ancient Vaillant combi (VCW GB240H ) will no longer fire. The culmination of a series of problems I found to difficult to diagnose with certainty.
For some time we haven't been able to get the DHW to fire when we ran the shower upstairs - if we opened another HW tap then it would. Some time after we began to notice the water was getting v hot. Next thing was that the pilot light would blow out when the water overheated. After the pilot was relit the DHW would not easily fire again. We would have to wait until the whole thing cooled down. During this time the CH seemed to operate OK. Finally today, we could not get the DHW to fire at all and something had to be done.
I thought the thermocouple was ok and the pilot problem was linked to the overheating so I decided to replace the DHW thermostat and check and replace the diaphragms in the 'water section' and the diverter valve - (I had previously done this some years ago when we were getting no dhw and I discovered a tear in one of the diaphragms.) Well, when I got home from the Parts centre I found they had given me an NTC sensor not the DHW thermostat - but I changed this anyway. I changed both the diaphragms - although the old ones looked ok and all internal parts were freely moving..
Predictably, this did not improve things. DHW will not fire up and neither will the CH now.
The pump runs and you can hear water running round the system. Now I have the covers off the boiler I can see that the Servo arm is moving ok when the HW tap runs (so I guess the water section is ok) but the micro switches on top of the diverter aren't moving at all. The push rod seems stuck in the same half way position. I can fire the CH with a screwdriver and I can fire the DHW by pulling the micro switch out manually but in both cases the boiler cuts out after about
30 secs. Also when I switch the CH off using the room thermostat control the pump continues to run - although there is no demand. When I then cut it off using the power switch the pump grinds to halt with what sounds ominously like a death rattle.It didn't do that before!!
I'm still going to change the DHW thermostat but I don't know if I should change the diverter valve although it's not moving the micro switches I couldn't see anything wrong with it when I stripped it down. Could the pump have something to do with this? Also I replaced the DHW expansion vessel about 18 months ago after a failure. I've never been happy about it. Would an incorrectly pressured vessel cause the diverter valve to stick? Any suggestions would be gratefully received!