Fanfare 30/50Si No spark

[iwbnwif]

Hi, its the usual story I'm afraid, cold weather starts and the boiler stops

Yesterday set the switch from "Lo" to "Hi" and programmer from Hot Water to Hot Water + Heating, everything was fine last night and this morning but this evening it fails to spark.

Starting sequence seems normal, power on, fan runs, pcb relay clicks + pilot solenoid clunks but no tick, tick of the ignition.

I can light the pilot with a match, main burner fires and the boiler runs fine until the boiler stat or tank stat say stop.

It looks to me like there is nothing happening on the PCB to generate a spark (I read that you will see flashing on the high voltage coil if its working??)

Before I shell out £££ for a new PCB is there any way this could be the ignition lead or electrode?

Many thanks to anyone who can help

 

[iwbnwif]

Short follow up. I bought a new PCB for £40 of fleabay and low and behold everything is working again.

 

[Agile]

You seem to have had the PCB delivered much quicker than the usual 3-6 days associated with Fleabay.

Probably just as well as we would have been telling you that you should not have been opening up the boiler's combustion chamber!

Tony

 

[iwbnwif]

Thanks Tony, as luck would have it I found someone selling a new one just 0.5km away so was able to drive over there.

No, I certainly wouldn't open the combustion chamber! The PCB on this old model sits underneath, and the spark electrode is accessible from the front without taking the cover off.

One thing I am curious about - how does the board know the pilot is alight before opening the main burner solenoid? I can't see any cable for a thermocouple (the pilot just has a gas tube and the ignition lead).

 

[MeterMonkey]

Congratulations on supporting your local economy.

 

[Agile]

An ac voltage is applied to the spark electrode and the ignition voltage of about 14 kV is added to this.

When the pilot flame is established the ac current flows through the flame but not to the same extent on the positive as negative peaks of the applied voltage. The circuitry senses this unbalance and uses it to sense that a flame has been established and then allows the main flame to be turned on.

Tony

 

[iwbnwif]

Ah, got it - thanks for the clear explanation. Clever bit of design.

Now I understand why if the electrode has degraded then the pilot will still light but the main burner solenoid won't open - obviously the sense circuit is registering (enough of) a change.

 

[Agile]

The 14 kV spark voltage will jump across small gaps of say 1 mm in the path to the spark electrode.

But the flame sensing voltage of about 200v needs a low resistance path to the electrode ( as well as good earth return resistance ).

Tony