Boiler vibration/oscillation (not the fan or pump)

[Richard Russell]

Our boiler (Baxi Megaflo System 32 HE) has, since new, been prone to vibration at certain levels of demand. The oscillation is at quite a high frequency, I would estimate 15-20 Hz, and doesn't have a mechanical source (like the fan or pump) but seems to be a rapid modulation of the burner. When it's happening, the flue gases have a distinctly different smell from usual, presumably because of incomplete combustion.

It feels to me like a control-system instability, just like an insufficiently-damped negative-feedback loop. Changing the demand (e.g. opening the valve to the radiator circuit) will usually supress it, after a few seconds, and as the oscillation gradually dies down the frequency increases. The boiler service engineer hasn't really a clue, especially as we've never managed to provoke it to happen when he's been here. Any thoughts?

Richard.

reply by email change 'news' to my forename.

[Andrew Gabriel]

My Keston did that when new, at low modulation levels. It supposedly came with the mixture pre-set to the required value. On checking, it was miles out. Once set correctly, this stopped. Has the service engineer put a flue gas analyser on it to check the mixture at all modulation levels (at least, both max and min)? Some people with the problem on the Kestons got Keston themselves out, and they fitted a restrictor on the air intake inside the boiler. That's one strictly for the manufacturer to do though, or the boiler's gas certificate is voided. In my case, correcting the mixture fixed it.

In the Keston, the modulation is done by varying the fan speed. The gas rate is controlled by the pressure at the gas mixer venturi (which is in turn a function of the fan speed). It's not difficult to see how you get a positive feedback loop here, with a time constant of the order of the response time of the pressure reducing valve (which is what adjusts the gas flow rate), which is very likely 15-20 Hz. On mine, it sounded like a 32' organ pipe.

[Richard Russell]

He certainly used a flue gas analyser, but at what modulation levels I can't say. I know he telephoned Baxi for their advice when he was here. Unfortunately at the time of year our annual service is due (April) the problem is less prevalent; perhaps we need to call him out.

Richard.

reply by email change 'news' to my forename.

[RobertL]

My boiler starting doing somethng vaguely like that. it woul make a chugging sound at about 5 Hz, and evenually the flame went out and it had to restart.

it turned out to be that the flue had come apart in the loft!

Robert

[Peter Scott]

Usually a problem with air flow up the flue. Either too much or too little. Take a look at

then click Downloads. Fault-finding is at the end. This is for an oil boiler but the principles are the same.

Peter Scott

[Richard Russell]

I'm pretty sure it's not that kind of problem, because the oscillation frequency is much higher than I would expect from such a cause. The boiler has a conventional through-the-wall concentric flue. Thanks for the suggestions, anyway.

Richard.

reply by email change 'news' to my forename.

[Fi.plowman]

Hi I have just read your note and we are having the same problem and no one seems to know what to do. They're has been talk of too much gas being drawn and we have noticed that . Did you resolve the issue. Thank you

[Tim+]

Better to start a new thread than trying to chase up replies to a 5 year old thread.

Tim

[Andrew Gabriel]

The 5 year old thread you're looking at has two different resolutions for the issue. Is there something else you know about yours which makes those two fixes not relevant? Otherwise, it's already answered.