Ionisation electrode

[Dave Plowman (News)]

My Viessmann boiler threw a wobbly and tripped out. Press the re-set, and it went through all the normal startup procedure (fans whirring, solenoids clicking), but then tripped again, so I was hoping it was nothing major. The fault code Eb, means 'heat draw off repeatedly too low during calibration' The cure is to 'initiate heat draw off and trigger manual calibration (see page 89)' which didn't help much as there's nothing on page 89 about it. And when I did find it, it didn't seem to be applicable to my version of the boiler as the calibration required you to use a code number which didn't exist. However, similar sort of faults mentioned the ionisation electrode so in desperation I followed the service instructions for removing and cleaning it - and to my delight it sorted things. What does it do?

[Mark]

keeps heating ?engineers? in nice easy beer money.? or it?s a flame detector

-

[harry]

  London SW

Basically it enables the conductivity of the flame to be measured. Air/fuel mix is a good insulator, a flame conducts. You can try this by putting the probes of your ohm meter/multimeter in a flame.

If you have a stable flame, you can test different parts of the flame. In the hottest parts the resistance is less.

With industrial boilers, the ionisation detector is often a standard automobile spark plug with a long electrode.

What mucks them up is when the insulator gets sooted up.

Before ignition they are tested by the control system. if they show conductivity (ie sooted up) the boiler locks out.

So obviously the answer is to clean the soot off.

[Brian Gaff]

Is it not supposed to detect the conductivity of the flame for some reason? Maybe its to do with burning lean or otherwise, if this affects the conductivity of the air in the flame.

Brian

[Andrew Gabriel]

In many boilers, it also doubles up as the ignition electrode. In others, they are separate.

As ionisation detectors, in some boilers they are dependant on live and neutral being the right way around in the supply, and the boiler being correctly earthed. If something has gone wrong there, that can stop it working. Don;t know if that applies to yours.

[MJA]

,

discussing flame ionization probes:

Some boilers use the ionisation current to calibrate the air/gas ratio, and it sounds as if the failure was during calibration.

The European Commission report "Preparatory Study on Eco-design of CH Boilers", Task 4 Report (FINAL), René Kemna, Martijn van Elburg, William Li and Rob van Holsteijn, Delft, 30 September 2007 is an interesting read, and on page 88 there is a description of how this works:

"6.3.1 Measurement of flame ionization

"This technology is based on the measurement of the ionization voltage over flame and gas mixture. This ionization is already used for flame-control reasons (in case no ionization signal is measured, there is no flame and the gas valve is closed). With additional electronic circuitry the intensity of the ionization signal can be measured. And because the flame temperature (ionization voltage) is directly related to the air factor, the ionization signal is a indication for the quality of combustion.

...

"Viessmann uses this technology in the VITODENS boilers, and they gave it the name ?Lambda Pro Control?."

Quite an interesting read on modern boiler design including heat exchangers, burners, control, etc.

Regards,

MJA

[harry]

ticle ,

I never heard saw that one before. The resistance also varies with temperature so I don't see how it works.

Big boilers have an oxygen sensor in the flue gases. I seem to remember you're looking for 2% oxygen in the flues gas to be efficient.

[Bernard Peek]

It probably works in the same way as a Flame Ionisation Detector in gas chromatography although that starts out with a hydrogen flame. When complex organic molecules decompose in the flame they form ions which can conduct electricity. In a lean flame these compounds are quickly oxidised to carbon dioxide which does not ionise. In a rich flame the carbon compounds spend longer in the ionised state before being fully burnt. That means that the rich flame will conduct better than a lean one. If the flame is very rich some of the fuel will be oxidised as far as elemental carbon but no further. That makes the flame yellow and sooty.

[Dave Plowman (News)]

In article , MJA wrote: [snip]

It is indeed a Vitodens, so thanks very much for the info. I have all the user, installations and service manuals that came with it - but they seem to be written in a language I don't understand. ;-)

[geoff]

In message , "Dave Plowman (News)" writes

Senses the flame

[geoff]

In message , Brian Gaff writes

It is detecting the rectification effect which is what such a flame presents

It is also virtually impossible to get wrong (without using electronic components)

>

[geoff]

In message , harry writes

Harry, it is obvious that you know f*ck all about flame detection in domestic boilers

As I have explained on numerous occasions and you have failed to take on board, you are not looking for conduction (which would be quite a dangerous way to sense a flame), but the rectifying effect of a flame, which is not possible to get wrong

give it a rest, eh?

[Andy Burns]

So, is the sensitivity of this effect to earthing the reason for problems when attempting to run boilers from temporary generators?

[dennis]

Do you want to explain the physics of how such a flame rectifies?

[Andy Burns]

Pages 11-12 of this seem to do a reasonable job

[harryagain]

Right? So explain how a flame is a rectifier. And how a recitfier does not conduct electricity? And why determining that a flame is conductive would be dangerous.

[Tim Lamb]

Umm... that appears to rectify due to size choices of the electrodes and not a physical feature of conduction within a flame.

I failed engineering physics so don't shout, please:-)

[Dave Plowman (News)]

Which would makes sense since it seems to be firing, then stops almost immediately. Does this 4 times before giving up and showing the fault light. The insulator did have a very thin coating of a dark material (smoke?) - but would you expect it to be perfect? I cleaned the whole thing and the actual electrode with emery paper as it says in the service manual.

It hasn't tripped out once since.

[Man at B&Q]

It isn't. Just as you need differently doped silicon in a "system" to make a silicon rectifier, so you need a "system" of flame and electrodes.

It does, but only in direction, bar leakage and other transient effects such as reverse recovery. But then I suspect you know that bit.

It's the possibility of false positives that is dangerous.

MBQ

[dennis]

So nothing to do with the flame then.