Can you solve my Vaillant combi boiler problem, Sherlock?

  • Thread starter Grimly Feendish
  • Start date

Grimly Feendish

Hello budding Sherlocks.

All the clues you need to solve this mystery are laid out before you. It's not rocket science and I know that some of you guys have the nous to unravel the puzzle.

Boiler: Vaillant Combi Compact VCW GB 240H circa 1993 (ie very old!)

Problem: I've just moved into this house and the DHW is fecked. Here are the facts: -

1. CH works fine.

2. When I open a hot tap, the water gushes out pretty much at mains pressure and the boiler fires fine, but the water only ever gets tepid.

3. I have to turn the tap down loads to get an acceptable heat, ie slow the water through the boiler. Of course, if I turn it down too much the burner shuts off. It's bordering on unusable, especially for a bath. Also it wastes gallons of water.

4. The DHW heat exchange was the original black plastic "four cylinders fused together" type, and it leaked like hell and was encrusted outside with loads of limescale. I've had it changed to a new plate HE. It has solved the leak problem but made no difference whatsoever to the dhw supply.

5. Here is a funny thing - which could be the crucial clue you need: when I have the CH on the DHW works ABSOLUTELY FINE!

6. A heating engineer mate told me that it can't be limescale because I get such a powerful flow from the hot tap. Another heating engineer told me it doesn't need power flushing because the ch works fine.

7. I rang Vaillant (and had to pretend to be Corgi registered - he must have thought I was the most incompetent Corgi gas fitter in the country!) and described the problem. The first question he asked was "Have you checked the burner pressure" and he then gave me some mystical runes to which the burner pressure should apparently conform. I have not checked the burner pressure, but all burners are ignited with a nice blue flame. The supply is admittedly 15mm not 22mm but I'm told that is not so crucial and obviously the boiler worked fine in the past.

8. Another mate told me that, from my description, it is either a "gas problem" or an "electrical problem" - all Greek to me. He insisted it could not be the diaphragm because there is no problem with the boiler firing. When I told him about the DHW working when the CH is on he said "Aha! The diverter valve!"- but he still seemed very unsure and his closing advice was "Get Vaillant in".

I cant afford to get Vaillant in. I've already spent £245 on the bloody thing and Vaillant will cost another £200 plus just to visit and diagnose - we're starting to enter "uneconomical - get a new boiler" territory.

OK Sherlocks! Over to you...............

Grimly Feendish
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Sounds like the PCB's knackered. Have u checked the microswitches?

Could be as simple as the hw temp sensor, or several other things.
How can I check the hw temp sensor, and what other things do u thing it might be?


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So there is nobody out there who can help me with my Vaillant? I'll send a tenner to whoever comes up with the right answer - so long as it doesn't involve scrapping the boiler!

Sounds to me like the divertor valve is passing.........when the system is cold (ie radiators cold) run a bath and feel some of the rads, if they get warm (Even slightly) you have your problem.
This boiler model is the most complex ever built; it incorporates electronic, hydraulic and pneumatic control systems - and in the end it nearly always proves futile to diagnose faults via the web. There are simply too many different reasons to cause your problem. Since you are unable to measure the resistance of a thermister (and this is about the easiest task in boiler diagnostics) theres little point in investigating further.

However this boiler is vastly superior in qaulity to anything made today so I would try and find someone experienced with these older Vaillant models - afterall a new boiler properly fitted is going to cost £1500 - £2000.
Also bear in mind that the boiler you have is only 24kw

It will (roughley) do heat about 10 litres a minute with a 35degrees rise.

Check this using a litre jug times for one minute and a thermometer.
Scrufflepeck. Why the PCB. This item is the first to be changed by a man and his dog. Could you explain why the pcb would be defective when CH works correctly?

If you are passing too much water through the HW heat exchanger, it will not pick up 35 degrees as stated (provided the gas pressure is OK)
Can anyone recommend a heating engineer in the Bristol-Bath-Swindon area who knows their way around these boilers? Or will Vaillant be able to recommend?

Thanks for your help so far.

Harrowgas I just tried running a bath and no pipes from the boiler other than the DHW pipe heated up, and no heat in the rads. The boiler did start to make a funny ticking-hissing sound though, from somewhere low down, it could have been air in the system or possibly an electrical sound.

Any more ideas lads?
Water hot when CH on but tepid when CH off. Isolate both the feed and return for the CH from your boiler by shutting off the service valve on each pipe. Do not attempt to use the CH wlilst in this state! Then turn on hot tap and see if water gets up to temp. If it does then diverter valve is the culprit if not WTF!

PS: I am not in any way qualified to give advice on this matter and my suggestion is almost certainly very dangerous.
GF it could be various obscure things, but why not check the bleedin obvious as suggested? To be honest you aren't going to get yourself far up the learning curve on this one.
If you tell us your postcode....
Grimley, the problem with your boiler may be very simple or quite complicated.

If you dont have the skill or test equipment to even measure the hot/cold resistance of the sensors then you dont stand a chance of finding out what the problem is.

I fully realise that there is a public perception that all "plumbers" are dim wits and that anybody can do the job. The reality is that a modern boiler is a complicated piece of equipment and a competent boiler engineer needs wide ranging skills with electronics at the forefront.

For a start the plumber who fitted the conversion plate HE kit for you is obviously totally unable to diagnose the fault so I dont understand why you think that you will be able to.

Actually Tony I was hoping that you and the other experts might be able to diagnose it - that is why Im posting! No need to be patronising. This is not a jet engine or an super-computer ffs - its a fecking boiler. Whilst I would never seek to denigrate the skills of heating engineers, lets face it, there are a finite number of things that could be wrong and from the information I have given it must be possible to narrow it down to say 2 or 3 problems. I read a lot of ppl saying these Vaillants are so complicated only Vaillant ppl can do it - but I dont buy it frankly. Its a boiler. It works on exactly the same principles as all other combi boilers. If you know boilers, give me a list of what could be the trouble. Based on the empirical evidence I bet its a short list.


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