Problem with Vaillant combi boiler

[Rexx Magnus]

We have a Vaillant VCW GB 221h combi (hybrid) boiler. It's slowly developed a problem with the CH. The hot water works fine, but whenever the CH is on and the room thermostat clicks on, the boiler only fires for about 5-10 seconds and then the main burner cuts out.

The pilot light stays lit without problems.

It appears to cut out once the temperature has gone over about 50c (the thermometer shows higher than that a short time after it's cut out, which I just put down to a bit of a delay in the change) but then takes some time before it kicks in again (once the temp has reached about 35-40 deg and then cuts off shortly afterwards.

If I turn the numbered knob on the front of the main switch panel up to a higher number (where you have the CH switch and the main switch with the light on - it cuts out at the 50ish temp when it's on 3.5-4 which is right for the hot water) it stays on for a bit longer, but then the temperature ends up going to about 60-70. It still cuts out after about 30 seconds though.

I have replaced the thermostat control circuit, thinking it was that which was causing the problem - luckily I got a new one for £30 off ebay at the time - but it didn't fix it. I've tested various thermostats (the little ones on the pipes) to see if they cause the problems, but am I right in assuming that if one fails, it generally means the heating won't light at all as a safety precaution?

I'm unsure where to look next - the box of things on the right with three 'pins' and microswitches that stick out seems to move in and out ok when the hot water is used and the valve at the front with the black notched cam seems to turn ok when hot water is drawn as well.

There's a problem with the T shaped pipe where you let extra water into the system to top the radiators up - I think the valve has gone and it's letting water into the system slowly, meaning we have to occasionally drain a bit out of the radiators to keep the pressure down until that's fixed. Could it be that increased pressure buildup has damaged something in the boiler at some point? Maybe a diaphragm in one of the valves? The pump appears to run normally.

[Ed Sirett]

A possible cause of your problems would be a problem with the diverter valve and/or servo valve. Does the flow pipe get hot or remain cool at, say, 1m from the boiler.

[Rexx Magnus]

I'll check when I get home Sunday evening - I take it that if it doesn't get hot then it's one of the valves (would it be one of the two specifically?)

[Ed Sirett]

Yes. The small valve with 4 small pipes attached causes the big valve to move from neutral to HW (rightwards IIRC) or CH positions.

Anyway whatever it is the matter will be serious boiler surgery. YHBW.

[Rexx Magnus]

On Fri, 15 Sep 2006 21:02:44 GMT, Ed Sirett scrawled:

The flow pipe doesn't appear to get hot, other than just from conduction - as it only gets warm near the boiler body. The pump is working, so it looks like it's one of the valves then.

I do seem to remember the valve at the front in the middle had some water drips under it once - so maybe it's that one had the diaphragm go (along with a seal maybe).

[Ed Sirett]

it only gets warm near the boiler body.

drips under it once - so maybe it's that

The one in the front middle is operated by the flow of DHW. It had a diaphragm which moves a piston which does two things. Firstly it throws a microswitch which starts the pump. Secondly it makes that small triangular valve move to the position in which the small pipes make the big horizontal valve (low, right, back), the diverter valve, move to the DHW position.

In CH mode the pump starts which should cause the diverter valve to move to the left which is CH mode. Either the little triangular valve is not doing the business (often because the piston is semi-stuck and won't return under spring pressure to the CH position). Or the diverter valve is stuck, probably due to a diaphragm problem or other more dire problem.

You may be able to find out what is happening by manually moving the servo valve - especially in the direction of the return spring.

HTH

[Rexx Magnus]

So, I take it that if the servo valve goes back to the correct position (I'll have to check this tonight by seeing if I can move it further in the direction that the spring pulls it or not) then it's a diverter valve problem.

I'm sure water has come out of the servo valve section at some point, so it's likely to be that bit that is going funny. Replacing the whole lot should fix it, rather than mucking around with dismantling it and trying to change diaphragms and seals (just in case they're not the problem individually).

I've spotted new diverter valves on ebay for not much money, so I may end up buying both sets of valves as it'll still come to less than £60.

[Rexx Magnus]

May be getting a bit confused here. Is the servo valve the bit with the gas lines coming out of it, just above the thing with the piston on, or is the whole gubbins classed as the servo valve? Or is the bit below the part they call the 'water section'?

[Rexx Magnus]

Ok, after a little further fiddling with the servo valve...

If I push the top of the cam over to the left (triggering the DHW) the diverter valve arm moves out/right and both microswitches release. If I release pressure from the servo valve's cam, the diverter valve arm goes back in towards the left, but further than the original position (when the CH wasn't running but should have been). It doesn't go in far enough to push the second microswitch down, but does cut off the DHW by pressing the other microswitch. If I manually help it so that it triggers the CH, the burner will fire up.

(The above happens if the DHW has been run as well).

However, if the burner has died down (after about 10-15 seconds of running) when running the CH, if it touches that microswitch to activate the CH, it still won't come back on. (There appears to be a problem in the diverter valve that stops the microswitch from moving all the way in to activate - hence the need to help it for testing purposes).

It seems there's some kind of corrosion on the casing of the diverter valve as well - if there's an internal problem or something that has caused a leak, can this be the thing that is causing the heating to cut out, even though the microswitches are connected? I'm assuming that the control lines carry water (I'd thought they were gas to begin with) and if there's something wrong in the valve, it might cause the control lines to muck up as well.

The servo valve cam doesn't seem to go any further to the right when I try to push it down, but the microswitch that it touches is released through normal operation - there just appears to be no more motion to the right.

I guess I could get a new diaphragm for the water section below the servo valve, and also replace the diverter valve in the process. As the CH kicks in after DHW is used - I'm assuming that the servo valve is functioning properly, but there's something else that is causing the cutout.

As I said before, if you turn the knob on the front switch panel, the CH burn lasts a bit longer, but still cuts out after about 30 seconds.

I hope this is all written with enough clarity to give you an idea of what is going on.

[Rexx Magnus]

Also noticed that when the pump runs, the water pressure gauge drops by about

0.25 bar. When the pump stops, it goes back up again - there's probably air in the system by the looks of it. I've bled the system and tried to get air out as much as possible in the past, but until I drain it this time and replace the filler tap I won't know if it's totally free of air. Could that also contribute to the above problems?

I'm wondering if the faulty filler tap has caused a problem due to pressure overload in the past.

[Rexx Magnus]

Ah, and the reason for the latter - one of the stop taps was turned off into the boiler, thereby making the pump create a vacuum. I've replaced the tap, and when the heating runs (by helping the switch) it stays on. I'm going to see if adjusting the foot that presses the microswitch fixes the issue - there may have been air in the system that was causing the diverter valve to mess up.

So, second question - does the air release at the top allow all of the air in the heat exchanger section to escape, or is it best to use the manual bleed nipple at the top of the heating coil to get rid of most of it? Is this done with the pump off? I'm assuming so, otherwise air may get sucked back into the system.

[Rexx Magnus]

Sorry, one additional question - the knob on the front of the water section below the servo valve - what does it relate to? It has a thermometer on, on our boiler. Is it the temperature of the hot water?

[Ed Sirett]

You seem to have got to the nub of the problems. It's still not clear to me whether the diverter valve is failing to function because a) There is air in the system - which the auto vent is not removing - very common problem. b) The diverter valve is playing up c) There servo valve is playing up.

I don't think that amongst all the possible causes the water flow diaphragm is one of them, but the piston is moves not fully returning to home could be, although if the cam moves fully back, it's not that.

Overall I think it's more likely the diverter than not. This is a blind guess from what you've said and is worth what you have paid for it.

The boiler runs and shuts down (but longer when the thermostat is turned up) because that's all it takes to heat the innards, after which the t/stat does it's stuff correctly.

The knob below the water section is supposedly a thermostatic blending valve, the boiler is meant to go flat out and then the valve blends the DHW to the required temp. In practice this control is seized up solid on every VCW 2x2 and SINE I've worked on. You might be able to free it with tools. It won't help your problems although it might alter the DHW performance.

HTH

[Ed Sirett]

the boiler, thereby making

helping the switch) it

fixes the issue - there may

the heat exchanger section

heating coil to get rid of most of

sucked back into the

There should be such pressure (0.8-1.2 bar cold) in the system that there is no way any air can get sucked in pump running or otherwise. Odds-on the auto air vent cap is closed because of its incontinence.

[Rexx Magnus]

I guessed it might be for something like that. I'd managed to free it up and alter the temperature, which was a good thing, as our hot water has never been tremendously hot!

I've ordered a replacement diverter valve, so hopefully I'll change it soon and then know if that's the problem - and hopefully it is!

Thanks for all your advice, you've been a great help. :)

[Ed Sirett]

You will need the instruction book to hand. Keep track of those little 4mm pipes they all look the same once removed! Good luck.

[Rexx Magnus]

Yes, I think I've got that book committed to memory. :)

I'll be marking pipes with permanent marker and tape, I think!

[Rexx Magnus]

Ok, back to the problem once again - I bought a brand new diverter valve (which it looks like it needed anyway, lots of corrosion) and got new washers. Fitted the lot and it still does the same thing, which makes me think it's the servo valve that needs changing, or a blockage in one of the control pipes.

The diverter valve switching mechanism works fine with the DHW, then after DHW is stopped, it moves back through the central position, where it is normally and then towards the left. Again the pump runs as the switch gets pressed, but it doesn't fire up the burner. It then shortly returns back to the central position.

[Rexx Magnus]

Ignore this post now, I've sorted it out finally!

Checked the lines to the servo valve, all were clear. Opened the box of electrics and made sure that the settings for the pump were correct - they weren't, at some point since the boiler had failed, I must have checked the wiring and accidentally reconnected the pump control incorrectly. That wasn't the cause of the boiler not continuing to run though.

I noticed that the pump wasn't starting up this time around, so I checked the thermostat the quickest way possible - shorting it with a screwdriver. Lo and behold! After changing the diverter valve, that must have fixed it, as the pump now started to run and the diverter valve mechanism got sucked in properly by the action of the pump (one of the control lines runs from the top of the pump to the servo valve which made me wonder if it was the operation of the pump that was now causing it to stick).

The diverter valve needed changing by the look of it - some of it's design has changed and has different control line connections, and the old one was covered in corrosion.

Thanks for the help, Ed - I'm sure I'd have not managed to fix it if I hadn't narrowed it down to the valves in the first place!