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.
http://www.rtrussell.co.uk/
To reply by email change 'news' to my forename.

In article ,
Richard Russell writes:
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?

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 23 Nov, 11:56, (Andrew Gabriel) wrote:
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)?

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.
http://www.rtrussell.co.uk/
To reply by email change 'news' to my forename.

On Nov 23, 9:56*am, Richard Russell wrote:
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.


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

Richard Russell wrote:
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.
http://www.rtrussell.co.uk/
To reply by email change 'news' to my forename.

Usually a problem with air flow up the flue. Either too much or too
little. Take a look at www.morganscott.co.uk then click Downloads.
Fault-finding is at the end. This is for an oil boiler but the
principles are the same.

Peter Scott

On 23 Nov, 14:48, RobertL wrote:
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!

On 23 Nov, 16:03, Peter Scott wrote:
Usually a problem with air flow up the flue. Either too much or too
little.

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.
http://www.rtrussell.co.uk/
To reply by email change 'news' to my forename.

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

wrote:
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

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

Tim

In article ,
writes:
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

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]