How does flame detection work?

[david3456]

My Baxi Solo had a very noisy solenoid and it was getting slow to turn on the main burner. The fan, igniter and pilot work and although the pilot is lit, it keeps sparking and won't reach that last step. The problem is intermittent which tends to suggest the fault is not with the electronics.

The main gas valve was replaced (which on dissection shows it had a faulty seal) but the new one does not fix the basic fault. I asked the fitter how the flame is detected and he said something about an earth return. Well... electricity needs two conductors.

Can anybody expand on this? I also note there is a spark gap on the pcb. Does a spark jumping across it mean it is correct operation (ie, the interlock gets an ignition circuit working signal) or incorrect operation (ie, the interlock gets a spark voltage too high signal, which could happen if the spark wasn't reaching the chamber).

I am interested in explanation from someone willing to discuss matter of fact - at whatever technical level. Please do not reply if all you're going to say is to get the fitter back.

 

[footprints]

A flame can conduct electrical current, when the correct current is received by the circuit board sparking is inhibited and the main gas valve opened.

That's as much as my gas fitters brain needed to know, if the flame or electrode ain't right it don't work!

If you really want to get technical, Google flame rectification, lots of people have put up explanations.

Hope that helps,
Best wishes,
footprints

 

[hazetimesfive]

this one explains it pretty well

https://www.google.co.uk/url?sa=t&r...dBNOSAYBJbqr2I4Lw&sig2=lNAz7FB-TF0vgEPj4I339w

 

[Agile]

The mental block most DIYers have is wondering ow a single sparking electrode can both spark and detect a flame!

The answer is that a transformer is used to generate the 14 kV spark.

But at the same time an AC voltage is applied to the earthy end of the transformer. That flows through the transformer winding and is rectified by the flame ( if any ).

The rectification effect causes an offset DC voltage to appear at the earthy end of the EHT transformer. This is detected by a circuit and when present enables the main burner to be supplied with gas and continues to burn as long as the flame rectification voltage is present.

Tony

 

[david3456]

Useful information from all, thanks.

I did try to see what would happen with a multimeter and blowtorch. Nothing. I understand the essential difference now.

When the fitter replaced the valve he did not touch the pilot. As best I can see through the window it looks as though the flame is not properly attached.