Loud oscillations from Ariston combi boiler - please help

[mtspark]

Hi,
Boiler is a new ariston ACO 32 condensing combi with vertical flue.
Everything seems to be working, pressure is fine at 1-1.5bar etc,
no air in the system.

The only trouble is that it's making a very loud noise like a
trombone that builds up when the hot water tap is turned on and
then gradually fades away over a period of about 5-15 seconds.
(ie. the system is unstable and oscillates). This also seems to occur
when I turn on the central heating; ie there's no DHW draw off.
So that seems to suggest it's related to the gas burner.

If I keep drawing off water intermittently, I can get the warm system
to start without making the noise, and then everything works as
expected. If I listen carefully though, I can still hear faint
traces of the oscillation when the burner starts heating, so I know
that thing's still aren't quite right.

The boiler itself was idle for a year or so in it's box while
waiting to be installed. The pump was a little stiff, but seems
to be running freely now. The pipe runs in the system are all
pretty short since the boiler is very close to the bathroom/kitchen.
The mains pressure water is fed through a non-return valve, to
a water softener and finally to the boiler.

Does anyone have any suggestions as to the cause?

Thanks,
Mark.

 

[Agile]

Weak mixture !

Caused by incorrect commissioning and often accompanied by an undersized gas supply pipe.

You will be able to instigate the resonance of the inlet tube by turning on a hot tap and slowly turning down the hot water flow rate.

Was the installer not CORGI registered and did he not have a flue gas analyser?

Tony

 

[mtspark]

Thanks for that Agile. I'm guessing the gas pipe is ok, 22mm all the way until the boiler where it narrows to 15mm.

I guess it just needs setting up properly.

Do you happen to know the mechanism for the oscillation? I'm just curious know to know what's actually oscillating.

 

[Agile]

No CORGI installer?

No flue gas analysis?

A 22 mm gas pipe will only be large enough if the boiler is within about 2 m-3 m of the meter?

Has the CORGI measured the gas pressure at the meter and at the inlet to the boiler?

The boiler is set to produce a certain percentage of CO2 at the flue under certain operating conditions as set out in the manufacturee's instructions.

Oscillation is just like in a flute, organ pipe or any other noise producing instrument except that its powered by the fan compressor in a boiler and dependent on the flame speed. If the mixture is too weak the flame becomes unstable and prone to oscillate.

Tony

 

[mtspark]

Ah, so it's actually the flame oscillating, very interesting.

Yes the boiler is about 2m away from the meter. I've done the install myself. I've bought a manometer today to check the pressure and I'm going to hire a FGA next week to do a proper setup as per the instructions.

I'm not a plumber by trade, I'm an electronics engineer, so I can solder

 

[Agile]

Setting the CO2 usually solves any resonance problem as it stabilises the flame.

Its similar to a twin 1/4 wave stub filter with the resonances determined by the inlet and exhaust lengths.

Most manufacturers have found it necessary to lower the "Q" of the inlet resonance by adding a small hole(s) close to the inlet to the fan. Either on the production versions or as a one off cure as required.

Tony

 

[gas4you]

Yes the boiler is about 2m away from the meter. I've done the install myself. I've bought a manometer today to check the pressure and I'm going to hire a FGA next week to do a proper setup as per the instructions.

I'm not a plumber by trade, I'm an electronics engineer, so I can solder

I suggest you call National Grid in to check the working pressures. How did you register the boiler if you installed it yourself. All gas installations should only be carried out by qualified competent engineers.

You may be an excellent electronics engineer but leave the gas to the experts.

PS did you know that your house insurance is now probably invalid through your own installation?

 

[mtspark]

Ok, this Ariston ACO 32 combi boiler is still oscillating loudly and I'm running out of ideas. It has a short vertical flue of about 1.5-2m.

The noise is very loud when cold and continues for about 10-15s before fading away, but after it's warmed up and been running for a few minutes, the noise isn't apparent and everything seems normal.

I've checked the gas pressure at full power, it's 19-20mbar, within the 17-25 stated in the manual.

I've got a FGA and did the low power/high power air/gas ratio setup. This went fine and I was able to adjust the figures to exactly what was stated in the manual, 8.7% @ high power, 9.2% at low power.

The air pressure switch seems to be turning on once the fan gets going, suggesting that the air flow from the flue is ok, but I have noticed it jumps around a bit when the burner starts before settling down to an on position.

Any suggestions greatly appreciated...

Thanks,
Mark.

 

[bripl]

the manual for the aco is very comprehensive and will give you all the info you req to comission it, it must have 20mb min working pressure your readings for co2 are incorrect no digital u gauge?

 

[Rob]

exactly what was stated in the manual, 8.7% @ high power, 9.2% at low power.

Are you sure that aint the other way round...

 

[mtspark]

I've double checked, the manual definitely says 8.7 % at maximum power and 9.2% at minimum power. Is this at odds with the norm?

 

[Rob]

Why would burning less gas produce more CO2..? maybe im missing something

 

[bripl]

pays to use a competent person by the way pumps sticking on these is common nothing to do with your problem.
to help you you MUST have a min of 20mb at the boiler not at the isolation valve.

 

[mtspark]

the manual for the aco is very comprehensive and will give you all the info you req to comission it, it must have 20mb min working pressure your readings for co2 are incorrect no digital u gauge?

Well the manual says 17mb is sufficient.. ???? so why do you say 20?

I used a TPI709 to do the setup. The gas pressure was measured with a non-digital monument mannometer on the gas mixer inlet test point.[/quote]

 

[mtspark]

the manual for the aco is very comprehensive and will give you all the info you req to comission it, it must have 20mb min working pressure your readings for co2 are incorrect no digital u gauge?

So what are the correct/expected figures?

I measured 19.5mbar at the mixer inlet test point at full power, 23 when idle, do you really think a deficiency of 0.5mbar is going to cause this problem?

Thanks,
Mark.[/quote]