Worcester BF 240 Combi CH Fault

[Parablennius]

Mornin' All
I have a 240 BF which has an intermittant fault with CH.The relay on the driver board which switches the gas valve on/off flickers rapidly, causing the burner to flare-up repeatedly but not stay lit.
It works perfectly with hot water, so I reckon it's something specific to the heating side. I suspect a sensor or maybe the CH potentiometer. I suspected the room stat at first but a new one didn't fix it. Something is feeding the relay with a duff signal, Yes?
Can anyone help, Please
Cheers
Steve

 

[Agile]

I have not seen that fault but would conclude that part of the temperature control circuit is fluctuating or oscillating.

Probably a dried capacitor not smoothing. Normally would be new PCB time unless you wanted to trace the circuit from the CH ntc and try changing caps in the vicinity of the CH temp control circuit.

The first part of the circuit controls the modulating valve and the second cuts off the gas valve when the mod signal is at a minimum but the temperature has risen too high.

Tony

 

[Parablennius]

Tony
Thanks indeed for your swift response.
It's a bit technical for me but I can find my way around most things. I've sat and watched it and studied the electrical diagrams for hours and would like to pin it down firmly before I start spending on it, the current economic climate being what it is! I'd hate to buy a new PCB only to find no improvement. Do you think it's worth checking the PCB for dry joints?
Steve

 

[Parablennius]

Afternoon
I've just tested the resistance of the CH return sensor and found it to be within limits but then found one of the rivet-fitted spades was loose. When the wires to it were moved, tester showed open cicuit. I've just dabbed some solder onto it and it appears now to be OK! Do any of you experts out there reckon that loose terminal was the culprit?
Cheers
Steve

 

[Parablennius]

Afternoon
RESULT. Replaced the CH return thermister with a new one and it seems to have done the trick. No relay flickering, just seamless heating 'till the room stat switches off.
Steve