Loud droning from new Vaillant system boiler

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We had a new Vaillant EcoTec Plus 612 system boiler installed in February.

Occasionally, about once per week, we have experienced a very loud droning resonance in the pipework. This is savage enough to be heard through the house and felt on the pipes and boiler casing.

It appears to be caused by the water flow shortly after the system fires up and usually stops after a couple of minutes. It is bad enough that my reaction is switch the boiler off instantly. On restarting the noise does not start again.

The boiler was a direct replacement (no changes to plumbing) for a 10 year old similar boiler which developed a leak in its heat exchanger.

Our unvented system has 8 underfloor heating loops and a Gledhill water storage tank on S-plan.

Vaillant’s warranty engineer could find no fault. It did not “perform” while he was here, though it made a loud humming for some minutes as it warmed up – sufficient for him to decide there was something wrong somewhere.

It is difficult to localise the fault because everything is very close together. I cannot be sure which valve is open when this happens though I’m fairly certain that on one occasion the tank circuit was open.

Could one of the Honeywell 2 port valves be causing this?

Does anyone have any other suggestions as to a cause?
 
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Have you got an automatic bypass valve in the system. Not on the boiler it doesn't count.
 
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Thank you for you reply.
No, there is no external automatic bypass.
I was suspicious of this 10 years ago but our plumber assured me that Vaillant's internal one was adequate and if there were 2 bypasses, "they would fight one another....."
Your experience tells you that's not going to happen - maybe I should find a plumber to install an auto bypass. I guess that can only help.
 

CBW

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Regards the Honeywell valve, it could be the noise, are you able to get boiler to fire up and listen to the valves simultaneously? If there’s only yourself, you could try setting a timed period and listen at the valves.
 
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Thanks for your suggestion.

Yes, I can fire up the boiler and listen to the valves - they are all close together.

Unfortunately, I struggle to tell where the noise is coming from.
I tried listening though a screwdriver against the valve bodies, against the pipes, even against the boiler casing but I could not say where it was worst.

The 22mm motorised valves are both 10 years old.

I just checked the valves and the original paperwork. One is a Honeywell V4043, the other (a surprise to me) is an Iflo TP985384.

I have no idea why he fitted 2 different makes. Are Iflo valves durable?
 

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