Potterton boiler control

[HKB]

I appreciate this is an old thread, but someone may find this helpful. Fan cycling is likely caused by a fault in the -50V low voltage power supply on the 407677 PCB (located just to the right of relay 1), which energises relay 1, and relay 1 keeps the fan running after the air pressure switch has triggered.

If that power supply is malfunctioning (e.g. it has too much ripple because C7 is degrading) this may prevent relay 1 energising. In that case, the fan starts up but when the air pressure switch is triggered, the live is disconnected from the fan, at which point the fan stops; then the air pressure switch switches off, restarting the fan, and on it goes, cycling over and over again.

If fan cycling is happening, the most likely culprit is a dried up capacitor C7 (22uF), but it makes sense to replace all three electrolytics (C4, C6, C7), making sure they are rated a minimum of 63V and ideally 105C temperature rating.

 

[Gasguru]

The sensible advice is that no one should attempt a repair of these boards especially as these boilers have in the past killed homeowners due to faulty ignition sequences (the cases have become distorted from delayed ignition resulting in very poor combustion, leakage from the casing and carbon monoxide poisoning).

 

[ianmcd]

Absolute piles of shoite, and under no circumstances should any one attempt a DIY repair on these