Random Groaning Vibration from Worcester Condensing Combi

2 Aug 2006
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United Kingdom

Moved into this house last year with a "Worcester Bosh Greenstar HE Condensing Combi". We've only had one fault that was sorted by BG on a fixed fee basis - the fan inside needed replacing.

However, since moving in it has produced a loud intermittent groaning, vibrating, rumbling sound. Reminds me of my childhood in Scarborough - Fog Horns!! :)

I used a suitable descaler incase it was jammed up and emptied / refilled with inhibitor recently. No sign of air in the system, pressure ok and stable (always in green).

It occurs randomly when CH on - sometimes a few minutes apart; sometimes only once or twice a night, particularly when working hard / very cold. It seems to be getting more frequent but that might be the weather.

We have a few TRV's that I've fitted since moving in but most Rads are open (although some of the plumbing is questionable: e.g. some "radiators in series" plumbing :( )

I suspect that I need to get someone out to see it, but I'd like to narrow it down - intermittent problems can be expensive I find! :(

Anyone have ideas what might be the problem? Is it major / serious? Any thoughts on repair costs? Any advice welcome!
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One reason for symptons similar to yours that we have encountered on a number of occasions causing loud rumbling vibrations so loud that they annoy the neighbours also, is due to insufficient gas supply at gas valve inlet caused either by meter governor packing up or undersized gas pipe.

The fan will 'hunt' for gas from the negative pressure gas valve under these circumstances and cause huge noise problems.

No idea if this is your problem but worth calling someone in to check working pressures, on our jobs with these problems a new governor or suitable sized gas supply eliminated the sounds at a stroke.
There is a fog horn noise caused by an inadequate gas supply as described above.

There is also a rumbling noise cause by a very short gas supply pipe which causes "pulling" on the meter regulator.

Thanks Chaps.

I was thinking that it was a boiler problem, but I guess it could be gas pressure.

We did have a leak a while back and they moved the meter outside while replacing the damaged underground pipes...this was at the same time the fan was changed. Perhaps that changed things?

The gas is supplied by a run of 22mm pipe about 10-15M long (most buried so I don't know exactly). Does that sound reasonable? Not sure of the boiler size but we have 10 radiators.

I guess that if you are right and I put the gas fire on too then it should occur more often?

Hmmm...interesting! Thanks again!
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Just thought that I'd update you folks on this one...we got it fixed eventually. I tried checking gas pressure but it was "spot on".

It was the circuit board. A fault had developed on the PCB that caused the fan speed control to intemittently change (fast, slow, etc).

New cirucuits fitted and all has been well now for many months. B Gas didn't track it down, but the Bosch engineer spotted it immediately -- expensive, but worth it!
B Gas didn't track it down, but the Bosch engineer spotted it immediately -- expensive, but worth it!

An intermittent fault can be very difficult for an engineer to guess at because he cannot see it happening while he is there.

A manufacturer's engineer has the advantage that he has seen or heard of some rare faults which no one else would have encountered. He also has a van load of spares which he does not have to pay for!

Sorry - that wasn't intended as a criticism of BG...they were nice guys who did their best! :oops:

However, the Bosch chappie immediately knew the problem without having to investigate...I guess that he sees a lot of such problems! He did have one in the van and swapped it to test. If it hadn't worked I suspect it would have done back in the bag..."spare part surgery!".

(He also took one look at the timer and said "oh dear - you have one of those! How do you manage to use it...we only produced a few because they were too complex for users to program"!)
that wasn't intended as a criticism of BG...they were nice guys who did their best! :oops:

BG engineers are generalists who have to try to fix anything that BG cover which is most ( but not all ) boilers. They have the backup of a lap top which has details of boilers and their common faults and if things are still too difficult then a senior engineer to ask or call out.

Manufacturers engineers only fix their firms limited range of equipment and consequently build up a much greater in depth experience of the smaller number of models. They are also advised of rare faults through their firms communication channels.

I had the groaning vibration problem when the boiler was on 'low fire' i.e. when the heating is up to temp and the burner has a small flame to keep it topped up. It would also happen if the boiler was re-started and the start up cycle with the trap filling sequence (which is also accompanied with a low fire flame) is in operation. Fearing the worst I called the plumber who just went through basic checks.
First the flue for blockage then the gas pressure at the meter - both ok. Then the gas at the meter with boiler on high fire and other gas appliances on to check for pressure drop - all ok. Gas analysis at low fire - CO2 less than specified so that was increased. Gas analysis at high fire - CO2 slightly less than specified so again an adjustment was made.
That was the end of the problem and its been ok ever since (3 weeks). Cost £30. Time 1 hour. Parts none. Problem, fuel mixture at low fire. If anybody else gets the same problems it might be a good idea if they or their plumber makes the same basic checks before more expensive options are considered.
Steve, why do you think that all boiler engineers ( or plumbers as you call them ) are uneducated morons who dont know how to set up and adjust boilers?

I suppose that you have never bothered to get your boiler serviced annually when these adjustments would be checked?


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