Ionisation electrode

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?

 

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


-

 

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.

 

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

 

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.

 

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
<www.ebpg.bam.de/de/ebpg_medien/001_studyf_07-11_part4.pdf> 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 Controlâ€."

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

Regards,

MJA

 

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.

 

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.

 

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. ;-)

 

Senses the flame

 

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)

 

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

 

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

 

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.

 

So nothing to do with the flame then.

 

I'd like to see you make it work without the flame.

MBQ

 

Ask anyone whose boiler won't light on Christmas Eve. ;-).

Tim

 

Thermocouple failure is one of the most common things to stop a boiler
working, IMHO. At least in my case just cleaning the electrode (or more
likely the insulator) got it working, which is a bit cheaper. And it's
rather easier to remove/replace.

 

When boilers became efficient they needed fans to push the combustion
gas out because there was no convection.
They also had no pilot light and had to be lit every time they fired.

The danger was that the gas valve might pass gas and on ignition, the
boiler would be full of gas and explode.
This couldn't happen when there was a permanent pilot light as any gas
leak would be quietly burned off.
So the boiler has to be purged (with air)on start up to clear any
gas.
The ionisation detector detects the flame. If there isn't one when
there should be, the boiler goes to lockout. (And gas is cut off)
The detector is tested before start up on every occasion and can only
fail "safe".

 

There are many ways to ionise the air/gas.