Richard Faulkner wrote:
You will recall my ongoing intermittent C/H boiler problem, and that I
proclaimed success after telling my plumber to replace the pump.

Well it hasn't worked, so I was wrong !!

However, the problem is so intermittent, that it has never arisen
when a plumber has been here, so I am going to have to continue to
try and diagnose it myself.

I have bought a multimeter, opened the box where the wire from the
pump goes, and attached it to the pump wires which I am presuming are
connected to the PCB - it certainly looks like a PCB. The meter reads
anything between 240-250V when things are working, and it seemed to be
working fine so I risked a shower. This went fine until I turned the
water off and back on again when it ran cold.

I had a look at the multimeter and it read zero volts. Also the
overheat sensor had activated its cut-out which requires pressing a
button to reset it.

I fired up the heating and it worked fine, and finished off my shower.

It's working fine as I write.

I am 99% certain that something in the control box, (PCB??), is
cutting the power to the pump, then the non running water overheats,
and the sensor does its job and cuts the burner.

I am wondering if there is anything in the safety system of a boiler
which would stop the pump before cutting off the gas. I cant think of
a reason why his would happen as it actually causes the overheat.

Could this be anything other than a problem with the PCB?

Could be the air pressure switch.

Andrew