Vaillant ecoFIT pure 825 thumping noise

[Intoxicat]

My 7-year-old boiler has been regularly serviced. Just over a week ago I had TRVs fitted to all but one radiator at the same time as the service, although this is probably a coincidence.

All was working fine for a few days afterwards, then suddenly, whenever I turn on a hot tap or the shower, the boiler makes a very loud, rhythmic thumping sound, which stops as soon as the tap, which does produce hot water, is turned off. The central heating runs smoothly and silently. The thumping noise occurs if the central heating is or is not running at the same time.

My heating engineer is baffled by it and had never come across it before. He phoned a Vaillant specialist who had never come across it either. What worries me is that he has ordered a new condensate trap, but the current one is spotlessly clean and has no mechanical parts. There is really nothing that can possibly go wrong with it. He sawed through the external section of the drainage pipe to inspect it and found that it had unnecessary insulation that was reducing the diameter, but it wasn't blocked in any way.

He has also ordered what I can only describe as a kind of spark plug as there is some very slight orange discolouration, but although I'm no expert I'm not convinced, and think I could be paying for new parts that aren't necessary, and that the problem still won't be solved.

Has anybody experienced this before and is confident enough to offer a solution?

 

[Stuckinarut]

It sounds as though your heating engineer is chucking parts at it in an attempt to resolve the issue, without knowing what the “fault” is. The drainage pipe, if it’s the condensate pipe should be insulated. Ordering a new condensate trap won’t solve the issue imo. Can’t really comment on the electrodes.

Do you have any dripping taps?

 

[Intoxicat]

It sounds as though your heating engineer is chucking parts at it in an attempt to resolve the issue, without knowing what the “fault” is. The drainage pipe, if it’s the condensate pipe should be insulated. Ordering a new condensate trap won’t solve the issue imo. Can’t really comment on the electrodes.

Do you have any dripping taps?

No dripping taps anywhere. He said the reason it doesn't need the insulation is because it's of a width that makes insulation unnecessary. Please don't ask me to go outside at night to measure it in this horrible weather, but I'd guess it's in the region of 5cm. In any case, if that was the problem it would surely take at least several minutes, if not a lot longer, to be the cause of the problem if the pipes above were clear. He diagnosed a faulty condensate trap by not being able to blow through the pipe above it, but surely that would force the floating valve to close anyway? When he cleaned the trap out there was only discolouration rather than actual particles, but it had only been serviced the week before.
I can't remember the actual term he used, but the suggestion was that the fault was happening when the boiler was calling for 'super-heat' (something like that). Even then, it doesn't make a noise when the heating first comes on.

p.s. I went on the Vaillant website and tried their chat 'assistant engineer'. What a joke. It's enough to give robots a bad name. I hope it wasn't an example of AI.

 

[Stuckinarut]

No, I wouldn’t ask you to measure it, it’s nothing urgent. Afaik those Vaillant traps don’t have a floating valve, well the Ecotec pro 28 doesn’t anyway. Are you able to record the noise and upload on here? Might have to use YouTube or some other platform. Doesn’t have to be done now.

 

[Intoxicat]

It definitely has a floating valve, which has to be filled with water before being put back after cleaning. I made a video of it to send by email, but even though only a few seconds it was too large at over forty megabytes. I'll see if I can upload something to YouTube but it will take a while.

Here you go. It sounds disappointingly benign here. It has been louder in the past, so turn the volume up full to make it sound more dramatic.

 

[Intoxicat]

No, I wouldn’t ask you to measure it, it’s nothing urgent. Afaik those Vaillant traps don’t have a floating valve, well the Ecotec pro 28 doesn’t anyway. Are you able to record the noise and upload on here? Might have to use YouTube or some other platform. Doesn’t have to be done now.

Sorry - my mistake. It's actually an ecoFIT pure 825. I'll see if I can amend the title. If it's something to do with the ignition electrode, why is it only doing it when the hot water tap is turned on, and not when the heating first strikes up from cold? Surely it's the same principle?
Editing now rather than stretch out the thread. It's okay for a few seconds when the hot tap is first turned on, but then the problem starts when you hear it putting on the turbo-chargers to provide the hot tap and shower water. So why doesn't it do this if I suddenly turn another radiator on, for instance?