Loud oscillations from Ariston combi boiler - please help

[bripl]

Well the manual says 17mb is sufficient.. ???? so why do you say 20?
because i am familar with this boiler and competent thats why your min co2 is correct 8.7+/-0.2 adjust max down to 8.7+/-0.2,you will need a digital u gauge as well to complete the commisioning.

 

[mtspark]

your min co2 is correct 8.7+/-0.2 adjust max down to 8.7+/-0.2,you will need a digital u gauge as well to complete the commisioning

My maximum is already 8.7 +/- 0.2, its the minimum that's at 9.2 +/- 0.2 as per the manual.

So to clarify, are you telling me to set the minimum down to 8.7 as well?

Thanks,
Mark.

 

[Agile]

At a CO2 percentage between 8.7 and 9.2 the boiler should not be making any noises.

As usual we cannot be absolutely sure exactly how and where this fellow is doing the measurements.

He talks about a "gas mixer" and I assume he is refering to the gas valve!

An important aspect of FGA is the normalise the instrument in fresh air. We cannot be sure that he has done that properly as he has not told us what he did.

Was it this model that someone told us a couple of months ago was prone to resonance with a vertical flue and the manufacturer's could not find a cure?

Tony

 

[bripl]

co2 setting min %vol 8.7+/-0.2 after 4mins of operation working pressure 20mb

 

[mtspark]

As usual we cannot be absolutely sure exactly how and where this fellow is doing the measurements.

He talks about a "gas mixer" and I assume he is refering to the gas valve!

An important aspect of FGA is the normalise the instrument in fresh air. We cannot be sure that he has done that properly as he has not told us what he did.

Yes sorry, it's the "gas valve" BTW the manufacturer calls it a "multifunctional 1:1 gas/air control for gas burning appliance"

Yes I did normalise the instrument in fresh air during turn on as per the instructions for that instrument.

I've noticed that if I unscrew "screw 4" at top of gas valve (according to ariston manual), the noise reduces. Now, according to the datasheet for the gas valve, this increases the gas outlet pressure, so I am coming around to the suggestion that the supply gas pressure must be at least 20mb for this unit to work properly.

I also think it's probably a design fault that it needs such high gas pressure. Probably the design of the combustion chamber/fan combo. Does anybody concur?

 

[bripl]

ariston have changed a few things with this model positioning of condense trap being one and the manual vent on the lhs of the heat ex early ones were upright new ones tend to drip on the stat as they face you and the early ones tend to split heat ex on the lhs in the middle of the casting yet another case of lousey r&d and poor quality materials in boilers welcome to the green world of boilers,and the instructions have been amended as well

 

[Agile]

I've noticed that if I unscrew "screw 4" at top of gas valve (according to ariston manual), the noise reduces. Now, according to the datasheet for the gas valve, this increases the gas outlet pressure, so I am coming around to the suggestion that the supply gas pressure must be at least 20mb for this unit to work properly.

I also think it's probably a design fault that it needs such high gas pressure. Probably the design of the combustion chamber/fan combo. Does anybody concur?

I cannot agree with any of the guesses or assumptions you have stated above. But then you work with electronics bit boilers so thats to be expected.

The resonance is symptomatic of a weak gas mixture!

The gas valve contains a basic regulator so the output is largely independent of the inlet pressure within limits, perhaps 17-23 mB !!!

You dont say what "screw 4" on the GV is for ! I will assume that its the outlet pressure adjustment. This will control the gas/air mix at any particular fan speed I expect.

You should not be tampering with that unless monitoring the % of CO² emissions. What effect does altering it have on the resonance and co² % ???

Resonance is a common problem on premix boilers. Many manufacturers agree that the mixture can be run richer within limits to reduce the problems. A typical suggestion is to increase the CO² % up to 0.5% above the specified figures.

Be aware that I do not officially act or give advice on behalf of MTS although I would be happy to discuss the matter with them.

Tony Glazier

 

[Crystal]

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[mtspark]

Hi Agile,

You dont say what "screw 4" on the GV is for ! I will assume that its the outlet pressure adjustment. This will control the gas/air mix at any particular fan speed I expect.

You should not be tampering with that unless monitoring the % of CO² emissions. What effect does altering it have on the resonance and co² % ???

Yes screw 4 is the outlet pressure adjustment. If you care to scroll pack a few posts, you'll notice that I have a TPI709.

I'm of the same opinion now, that the gas pressure simply isn't high enough for this boiler, even though the manual says min 17mbar and I'm measuring 19mbar at full power.

Other boilers seem to cope with much lower pressures, that's why I'm saying it sounds like a design issue.

Just curious, but you have to fit v.large diameter gas pipes very often?

The current pipe diameter is 22mm, so I'm thinking to upgrade it to 28mm. But first I'll check the pressure at the meter, it's a pretty old banged up meter so it might need replacing.

 

[Agile]

I'm of the same opinion now, that the gas pressure simply isn't high enough for this boiler, even though the manual says min 17mbar and I'm measuring 19mbar at full power.

Other boilers seem to cope with much lower pressures, that's why I'm saying it sounds like a design issue.

Just curious, but you have to fit v.large diameter gas pipes very often?

The current pipe diameter is 22mm, so I'm thinking to upgrade it to 28mm. But first I'll check the pressure at the meter, it's a pretty old banged up meter so it might need replacing.

I have NOT said that your gas pressure is not high enough! Please consider exactly what I have said and without jumping to wrong conclusions.

The gas pipe size has to be calculated BEFORE the boiler is fitted ! 22mm is only just about sufficient for 35 kW if the distance from meter is less than about 4 m.

What is the pressure loss between meter and boiler?

Tony

 

[gas4you]

For gas pipe sizing you need to look in M I's tech table for gas consumption at m3/hr. This is the figure you work it out by not the Kw.

As a guide that Vaillant 831 I fit uses 3.3m3/hr max, whilst the 837 uses 4m3/hr

You should find that 3.3m3/hr is ok on 22mm copper up to 12/15m (equivalent length) whilst 4m3/hr I would probably not go more than 9M (equivalent length)

 

[mtspark]

What is the pressure loss between meter and boiler?

OK, i've made some measurements. My gas pipe is about 2m long and has about 5 L bends, 22mm dia all the way until it reaches the boiler connection, where it narrows to 15mm. There is a 22mm T just before the boiler leading to the stove. The reason it has so many bends is that it comes out from under the meter, then goes up the wall.

I measured the gas pressure at the meter unloaded, its 24mbar. When I turn on the stove hot plates and the boiler, this drops to 22mbar.

By the time it reaches the boiler gas valve inlet test point, it's 19mbar (without the stove on) at full power. I notice that Ariston supply 15mm connections for the gas, so this may be where some drop is occuring.

Does this sound acceptable?

I have NOT said that your gas pressure is not high enough! Please consider exactly what I have said and without jumping to wrong conclusions.

OK, so you want me to increase the CO2 reading by 0.5%, is that right? Both high and low power settings?

BTW Agile, you're more than welcome to speak with Ariston about this to see what they have to say.

Best Regards,
Mark.

 

[Gasguru]

Have you got the FGA set to the correct fuel? What oxygen reading are you getting - should be around 5%.

Have you ascertained that it is definately the burner causing the resonance? Once the noise starts shut off the gas valve to see if it continues (before lockout).

I only say this having read the manual - the ridiculous service schedule (required each year for the warranty) dictates the diverter valve is stripped and replaced.

Perhaps the boiler having sat around for a year or more has resulted in degradation of the diverter O rings.

There is also mention of high temperature silicon grease on the combustion cover seal to provide an effective seal. Maybe a leaking seal will promote reasonance as well as overheating the fan etc.

Also worth talking to Ariston (don't be fobbed off easily - their boilers often have basic design flaws that they try and hide). Perhaps the distribution plate(s) behind the ceramic burner have been modified.

Manual here http://www.centralheating.co.uk/ind....memberDetail/con_id/6444/directoryGroup/5367

 

[Agile]

What is the pressure loss between meter and boiler?

I measured the gas pressure at the meter unloaded, its 24mbar. When I turn on the stove hot plates and the boiler, this drops to 22mbar.

By the time it reaches the boiler gas valve inlet test point, it's 19mbar (without the stove on) at full power. I notice that Ariston supply 15mm connections for the gas, so this may be where some drop is occuring.

Does this sound acceptable?

You have frustratingly managed to NOT give the relevant information I need to consider your problem.

Please set the boiler on full power by opening ALL the hot taps in the house AND turning the DHW temp to maximum. Turn on ALL hob burners and open window.

Measure pressure at meter AND at boiler inlet and tell us what they are! Its the presures that count !

Tony

 

[Agile]

As a guide that Vaillant 831 I fit uses 3.3m3/hr max, whilst the 837 uses 4m3/hr

You should find that 3.3m3/hr is ok on 22mm copper up to 12/15m (equivalent length)

David, you often mention this "rule of thumb" that you seem to work to.

I do have to say that my experience is that 35 kW or 3.4 m³/hour will not provide an adequate supply with 12/15 m of 22 mm tube in fact in my experience 5-6 m equivalent is usually the maximum.

Are you really saying that you can achieve less that 1 mB pressure loss at these lengths? Can you give me an address where I could see this in operation for myself?

Tony