Vaillant Thermocompact diagnosis

[bertiebooth]

This boiler is firing when not in use because the main PCB is faulty. Thus is about to be replaced, but that might just mask the other fault(s), hence I'm keen to find those faults now.

This boiler has neither a secondary heat exchanger nor a pre-warm feature - it is a very simple and, formerly, very reliable boiler.

I'm coming to understand that there is PCB dedicated to temperature control, so there's a distinct possibility that this is also faulty, but I'd like to find a way of testing/proving the three sensors before investing in a PCB.

Is there nobody on the site who understands this boiler? At this rate I'll be the resident expert in about 8 days from now...

 

[plumb46]

oh dear ???

 

[picasso]

as this is a system boiler I assume its controlled by 2x2 valves? if the boiler is firing without a signal from the valves , the valves wont be open to allow the water to flow and it will overheat, I thought bunnyman had said how to check overheat stats?

 

[plumb46]

sorry i miss read the first replies didnt know it was a system boiler
frost stat conected to system ?

 

[bertiebooth]

Yes, this is a heat-only boiler, and yes, there 2 x two-port motorized valves, which have been installed exactly as per the Vaillant instructions and are in perfect working order.

The boiler, which I'm fast realising now that nobody else here knows anything about except for me, has an internal auto-bypass, and the bypass valve was replaced in 2009 because it was permanently open.

Bunnyman has not indicated how to test anything - he said "it won't be a faulty overheat", on the grounds that they can't both be faulty. But they can.

There is no frost stat, and even if there was it wouldn't matter because the boiler fires with no connections on
terminals 3 & 4, which means that the main PCB is faulty, which was diagnosed and confirmed by Vaillant before my first post.

 

[Andygasman2010]

Hi OP. How did vaillant confirm it was a main PCB fault? Did they take you through the test that bunnyman described? It's very rare that the main PCB would give you this fault so if you haven't already done so check what voltage you've got on terminal 4 when the boiler is at fault. Just like bunnyman described. If you're lucky it'll just be an external control fault (motorised valve etc) and much cheaper/easier to fix! The fault you're describing would typically be a stuck microswitch in a motorised valve.

Just let us know how you arrived at a fault PCB and we'll shut up about it

I've got loads of really good tech info on this range of boilers but it's good old pen and paper so i can't upload it. The single wire sensor on this boiler is very reliable so I doubt thats failed, the other two are make/break contacts and again are very reliable.

Are you sure that when the boiler is banging/boiler that there is adequate flow around the system?

 

[Bunnyman]

oh dear lads another arrogant numpty...if you want advice from the experts listen to them you blew it asking vaillant technical they know nothing trust me i am in a position to know...if you do the test i suggested it will prove if the mother board is def faulty this is rare but not unheard of the optocoupler maybe has packed up..if this boiler was overheating one of the safty devices would have tripped...do you thin

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• Bunnyman
• 26 Mar 2010
• 499

k the leading manufacturer would have fitted a belt and braces overheat protection that would not have failed to safety if the stat broke for gods sake!!!

 

[Andygasman2010]

If you've got no wires at all in 3 or 4 then you're correct, it shouldn't fire up and it'll be the mother board that's failed. I'd sort that out before delving any deeper as it may all be tied into the same fault!

When you say your boiler is banging/whooshing is this during demand or when it's firing up of it's own accord?

Also, you said it doesn't actually overheat so what does it eventually do if you leave it banging? It would be normal to get a noise before the stat actually tripped out.

If it's not overheating it's most likely that it's not getting hot enough to.

 

[bertiebooth]

Hi Andygasman2010

Vaillant confirmed that the main PCB is faulty if it fires with no wires in terminals 3 & 4.  If you disagree then please go ahead and explain why.

I hear your advice about replacing the main PCB before delving deeper, which sounds wise, but the only reason for delaying was the general approach of changing nothing until all information has been gathered.

The banging occurs with no demand, i.e. the boiler firing of its own accord.  After a handful of seconds the gas valve shuts off completely and then the boiler of course calms down.  Nothing trips out at all - if left alone for long enough to calm down, or the re-ignition delay of 5 minutes, which turns out to be the longer, the boiler will then reignite.

While investigating, as soon as I hear the onset of the overheating I turn the thermostat, which is commonly left on 9, down to 1, then the gas valve shuts off sooner, which implies that there is still control via the relevant PCB.

I believe that this overheating doesn't happen if there is a genuine demand for heat, from either roomstat or cylinder stat (via the relevant motorised valve).

What I'm dying to know is:

1. If 2K Ohms at room temperature is reasonable for the NCT thermister.
2. What resistance values to expect at higher temperatures.
3. How I should test the flow thermostat.
4. How I should test the overheat sensor and cut-out device.

If the technical information that you have will yield those answers then I would be very grateful to you for providing them!

 

[Bunnyman]

also if the boiler is firing with nothing in 3 an 4 an the mother board has gone make sure you get the right one ie hybrid or non hybrid ,,the same boiler has 2 different types of pcb an they r not interchangeable if it is firing with no wires in 3 an4 the motorised valves will not b open so boiler will n pumping around its own internal bypass which it will do quite comfortably without overheating,,if you get a banging it could be the bypass that was changed in2009 is the wrong way round check the arrow on it for flow direction this is a common fault,if this is ok an its still banging it wont be pump as pump has to operate to fire the gas via the pd valve so probably blockked main ht exch,,,please donot waste your time testing res on sensors or overheat if any of these are faulty they ALWAYS FAIL TO SAFTEY AN TURN OF GAS UNTIL YOU MANUALLY RESET THEM!!!

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• Bunnyman
• 26 Mar 2010
• 499

 

[bertiebooth]

Thank you Bunnyman for some sound points.

I had already identified that this is a hybrid model and Vaillant gave me the part number for the main PCB, but thanks for the tip-off.

The bypass valve for this boiler cannot be fitted the wrong way around because it's incorporated into an asymmetrical elbow.

Your reasoning about the pump is sound, so if you're right about the sensors, and if it isn't any of the sensors then it has to be the heat exchanger.

My concern about the temperature control PCB is that the flame doesn't appear to modulate down at any point - it's either lit or not lit.

 

[Bunnyman]

Chuckle!! It certainly can be fitted the wrong way mate check again it is an elbow which can be fitted either way round sometimes with a extension union which can be screwed either end

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• Bunnyman
• 26 Mar 2010
• 499

 

[Bunnyman]

The boiler won't modulate as it's temp rise will b to steep for a single Ntc on these old models to recognise open the motor valves an let it heat when the temp gauge gets to about 60deg turn the stat knob down should modulate at around 5 or 6

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• Bunnyman
• 26 Mar 2010
• 499

What part numb they give u 130331 or 130330?

 

[bertiebooth]

http://tinyurl.com/VCGB242EH

So is it now the concensus that the heat exchanger is scaled/blocked?

 

[Andygasman2010]

I don't have the figures for testing the NTC, in all my years i've never had to change one ( hint ) i think it's something like 2.5kohms when cold, 350ohms when hot (80C)

The other two stats are make/break so the only way to properly test them would be to get them up to their max temperature and see if they trip. The following info is by memory so guys shoot me down if you want! The manually resetable one behind the control box trips at 97C and the main heat exchanger overheat stat goes at 128C.

I've never done these tests before (other than continuity) so I really think you'll be wasting your time. Change that mother board and go from there, i'm sure that'll cure your problem.