Vaillant Combi problems

[Son of Fred]

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!

[geoff]

In message , Son of Fred writes

Have you checked the diverter valve microswitches ?

[Son of Fred]

I'm afraid the spindle/pushrod isn't moving the switches at all so I can't adjust them. I can fire the boiler by manually making the switches although it wont remain lit. The servo arm on top of the water section moves and returns OK. I'm going to try venting the boiler this morning and if that fails cleaning out the control lines this afternoon.

[Ed Sirett]

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.

[geoff]

In message , Son of Fred writes

you'll have to wait for Ed to appear, he's the hands on vaillant man

[Son of Fred]

I thought I'd update my progress in case anyone might be able to shed any light onto our problems.

First thing I did today was obtain and fit a new DHW thermostat along with the new NTC probe in the hope if I could get the boiler working the DHW would stop overheating. I vented the boiler and bled the rads -- no effect

I then removed the main control line from the servo to the diverter. It was well clogged up with crud. I cleaned it out refitted and Bingo! Boiler fired up first time and stayed lit. I could get CH or DHW on demand. The pushrod was moving freely and tripping the microswitches as required. I ran the DHW for some time and the water temp stayed constant and the pilot remained lit so at least the water temp issue is fixed however the upstairs shower still would not fire the boiler. (The BAth and basin were OK). In view of the crud down the control line I decided to strip the others out and clean them too. Hey, It couldn't do any harm could it? How I wish I had taken a shower first. The other Control lines were cruddy too and I took the servo off and cleaned the holes the lines went into best I could - these too were rather clogged. When I refitted and tried to run the boiler to my surprise and frustration we were back to square 1.No DHW or CH. The situation is as per my first post i.e the servo arm rises and falls at the right times but the diverter pushrod is not moving at all. I can fire the boiler by making the switches manually but it will only run for 30 secs before cutting out - even if I continue to make the switch. If I pull out the spring loaded microswitch trip mech away from the diverter while the H tap is running I can see the pushrod begin to move out but as soon as I release it forces the rod backin. There is not enough pressure in the diverter valve to hold it though the tap is running full pelt. Although I had the boiler up and running earlier ther was one thing that caught my eye. On one occasion while calling for dhw the pushrod was extending out smoothly to the right hand side when all of a sudden it fell back then shot out again like there had been a sudden dip in the pressure. I'm stumped now. I'm thinking of replacing the servo and or diverter valve. Does anyone else think this would be a good idea? What should I try changing first? I'm taking the kids round to me mums for showers now!

"Son of Fred" wrote in message news:Bovqk.147580$ [e-mail]...

[Son of Fred]

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 >

[Ed Sirett]

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.

[1] designed for small flats with a shower but no bath.

[geoff]

A10 seem to have gone down hill a bit lately

Paul seems more concerned with fishing and holidays

shame really

[Ed Sirett]

They seem to be helping me OK. I haven't seem the guy with the glasses around lately? Is he Paul? He is legendary: he spots the broken part as you walk along the street from your van and gets it from the store room ready for when you walk in.

[geoff]

In message , Ed Sirett writes

Kevin - poached from FMT

Paul is tall and a bit fat (who am I to talk ??)

[Son of Fred]

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.