My 10 year old potterton puma 100 combi boiler will not work, no hot water, no heating (its b????? cold), replacing the relays on the main pcb seems to resolve the problem for a short period of time, but when it goes wrong, it trips the electric out.
Its could work for 2 days,2weeks, 2 months or more.
Has any body got any ideas.

Mark, December 2008

Ahhhhhhhh. i have exactly the same problem. British Gas have been round 5 times but cannot solve the problem. Each time they come - it works again! Very frustrating. Did you find a solution / cause?

Francis, January 2009

Not a "solid" problem - very intermittent and widely varying failure frequency.

1) May not be the boiler at all - could be some other device sharing the same circuit with bad habits and your boiler could be pushing the breaker over its limit - i.e. the combined power rather than the boiler power causing the trip...

2) Solenoid coils behave strangely especially when power is lost - the decaying field in the windings tends to self generate more power in the coil (transformer effect) and the overall result is one of the coil "hanging up" so the circuit designers put bleed resistors of relatively high value across the coil but on the PCB to dissipate the unwanted current. If the bleed resistor is not doing its job, bad joint say, the power generated in the isolated coil can only go along the neutral line (phase already being switched off) - this might be enough to trip an RCD.

These are purely guesses and probably have little value in the real world.

Peccavi, December 2008