On Mon, 18 Aug 2008 23:08:30 +0100, Son of Fred wrote:

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!

The pump noise does make me think this might be a pump problem.

All the symptoms from your description point to the pump. The microswitch
assy is porbably OK. I'm not sure but I think you can change the just the
pump head if so that will be fairly inexpensive to repair and try out.
If the pump was OK and you pushed the diverter to the left to get the CH
to start it should have continued to run OK.

This boiler is about 13-20 years old and should be possible to repair it
to give you quite a few more years.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html