I think I have celebrated too soon as I still have one of my original
problems. That is the pilot is still blowing out AFTER I have run DHW for
some time.
About 20 seconds after the HW is turned off after a long run (Bath fill or
long shower session) then I hear the thermocouple shut off the pilot.

So far I have replaced the DHW thermostat and the NTC probe thinking this
was linked.
There is a DHW flow regulator on the water section but this has been seized
up in the same position for the last 15 years!
I had to replace the thermocouple 2 years ago as it was failing - but it was
behaving v erractically - dropping out v frequently in CH, DHW or idle modes
so I dont think it's the thermocouple- but I could be wrong.
Is there anything else it could be?
Otherwise I will have to replace the thermocouple for elimination purposes.


"geoff" wrote in message
...
In message , Ed Sirett
writes
On Tue, 19 Aug 2008 23:37:30 +0100, Son of Fred wrote:

Cheers Ed!!!
I got back from my ablutions at my old mum's and when I read your
comments re pushing the M/S assy all the way to left to fire the CH I
realised this is not what I've been doing. I've been firing it by
holding in the black switch with a screwdriver blade. The boiler would
fire for 30 secs then fade. This made me realise the other microswitch
needs to be depressed at the same time.
As soon as I read your post I pushed in the M/S assy fully to the left
as you described and the boiler gave an almighty gurgle, fired and
stayed lit.

The pushrod is moving freely once again and I can get CH and HW on
demand. Even the shower is firing the boiler now (and that hasn't
happened for a while now without opening up the basin HW tap at the same
time.)

I guess there must have got an airlock somewhere in the diverter (or the
pump?) after I refilled. Everything is running tickety boo now so the I
think key was cleaning out the control lines and the servo after all.

As you noted the pump dosen't sound too healthy. If I flip the mains
power switch when its running it comes to a halt with a clatter so maybe
thats going to be the next thing to go wrong. I was interested to learn
from your post I might not have to change the whole thing, and I'll keep
that in mind.

Many thanks to you and Geoff for the Input.

"Ed Sirett" wrote in message
...
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.


I did battle with the 18kW [1] version today. Having stripped the whole
of the domestic water switch unit down and found that it just needed a new
sindle gland...

So.. I was fully bracing my self for c. £100 (actually 75+VAT) for a new
'wasser schalter' but was well pleased to find that for £1.57 I could buy
a new 'stopfbuchse' (spindle gland) on its own.

Full marks to Vaillant and A10 boiler spares for making and stocking just
the part that needed replacing.

A10 seem to have gone down hill a bit lately

Paul seems more concerned with fishing and holidays

shame really


--
geoff