Worcester 280RSF - main burner won't light - HELP PLEASE!

25 Nov 2007
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United Kingdom

My 1994 Worcester 280RSF has started playing up.

When a hot tap is opened, the demand light comes on, the fan and pump run, the pilot gas valve opens, the ignition sparks and the pilot lights and stays lit. So far so good.

BUT the sparking just carries on and the main gas valve stays closed, so nothing gets hot (obviously!).

Any suggestions? In particular, does anyone know how the ignition board senses that the pilot's lit? I can't work it out, even with the excellent technical manual.

The ignition electrode and wire seem to be in good shape and the spark is nice and healthy. The ignition board looks fine with no signs of overheating, bad solder joints, water damage etc.

Hope someone clever can help :oops:

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the same lead that ignites acts as the heat sensor, could be the sensing probe but chances are its the board.
Check for 240VAC (think it is 240V on this one :confused:) at the gas valve terminals whilst it is attempting to fire.

As the burner hasn't yet fired the flame rectification won't even come into it yet.
Need to check working pressure and burner pressure as it is trying to fire, needs about 2mb to fire
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Thanks, all three of you :D

As the burner hasn't yet fired the flame rectification won't even come into it yet.
Ah, flame rectification is the clue I was after.

But isn't the pilot (which does light) meant to cause flame rectification and tell the board to stop the sparking and open the main burner's gas valve?

I'll look into the other suggestions too, of course.

Thanks again,
A progress report... No different with a new electrode and lead (as suspected) but replacing the ignition board (the smaller, simpler one) did the trick.

I was pleasantly surprised by the price of the board. Not much more than £50, and the boiler's back on the road again.


If you look on the old PCB you will see a high value resistor in series with the ignition transformer earthy connection.

Thats probably gone open circuit.

I dont have a PCB to hand so I cannot identify exactly which one.

The flame has to be detected at the pilot flame before the sparking stops and the main solenoid can be allowed to open.

Thanks Tony, that's the kind of hint I like ;)

I repair electronic equipment for a hobby and know a bit about that kind of thing, so as I had no circuit diagram, the high-value resistors were the first thing I checked - suprprisingly, all of them read OK on digital meter (tested with one end lifted and fingers out of the way to prevent false readings).

I also checked the three transistors with a dedicated semiconductor tester and they were fine too, as were the relay contacts. I suspected breakdown/leakage in the transformer/coil, or possibly the VDRs. I suppose it doesn't take much to stop things working when you're dealing with microamps.

I've made a mental note of your advice in case it comes in handy later though.

BTW, there seem to be at least two versions of this board about. The replacement supplied (genuine Bosch/Worcester in a sealed box) was marked Pactrol, and had a few extra components including an SCR which the original (marked Worcester Heat Systems) didn't have. The ignition coil pinout is also different.

There will have been an SCR on the old PCB as they are used to discharge the cap through the transformer to provide the ignition voltage.

If its not one of the resistances gone o/c then a cap may have gone s/c or might just be breaking down when the voltage is applied.

Most unlikely to be the EHT transformer.

Hi Tony,

There will have been an SCR on the old PCB as they are used to discharge the cap through the transformer to provide the ignition voltage.
Good point.

But I think the original board uses a little gas discharge tube instead, marked SG1 (= spark gap??). You can see it flashing as the electrode sparks. The one on my board looks rather black inside the glass, and the rate of sparking with the old board seems much slower than it used to be.

Sadly, there are no markings on the tube, so I'm not sure what I could try replacing it with. Farnell and RS sell loads that all look identical, but with a range of different breakdown voltages from 7V to 10kV, e.g.

Cheers, and thanks again for your words of wisdom,


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