Buzzing from combi boiler when heating shuts off

13 Apr 2012
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United Kingdom
Boiler is a Viessmann combi. After the CH has been on, and the boiler shuts off, - the fan and pump (?) continue to overrun for a couple of minutes as expected, but when they shut off, there's a loud hum (which varies in pitch) for about 10 seconds or so as the boiler finally shuts off.

Also, like most combis, when there's been no heating demand for 24 hours, the pump (and fan?) runs for a few seconds and the buzzing/humming noise happens then as well. There are no noises when HW is being used.

I've searched old posts which point to a gas valve solenoid causing a loud hum however this wouldn't be activating at the 2 situations described above would it?
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I cannot guess.

Unless the pump is controlled by a relay which is not fully dropping out after running the pump.

Maybe someone else has a better guess.

Does it really bother you?

Transam - you'll have to enlighten me to what the transformer does and when it kicks in.

This buzzing noise appears for about 5-10 seconds when the boiler shuts off - and seems to be louder the longer the CH has been on. I also hear the buzz during periods of inactivity when the PCB instructs the pump to run for a few seconds (for the anti seizure function I presume).

As for does it bother me Tony, well only if it's the sign of impending failure!
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We have not been told the model of Viessmann but if its the usual 100 or 200 series then I dont remember any mains transformer.

We don't know exactly what is causing the noise but my advice would be to ignore it as long as its working.

But when you have the boiler serviced during the summer ask the engineer if he can identify exactly what is causing the noise and tell us here so we can give you our views.

I don't see any point in spending say £200 changing some part just to stop a buzz which does not really bother you.

Cheers Tony, it's the standard 100 series combi WBA-1C I think.

It's under warranty so I'll just leave it and see how it goes. They probably wouldn't come out if it's working OK anyway.
Right, now you tell us its under warranty.

Viessmann are quite good about warranty repairs.

But I would still only advise you to call them about it if its so loud as to be a nuisence.

However, as I said, ask the engineer to see what it is at the next service.


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