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?


Richard, I haven';t been follwing this thread much...I am a little
confused. On my boiler for example, if the pump stops, so does the
boiler. They are, in fact, wired together.

That is, if anything wants 'heat' the combinations of time switches and
thermostats move motorised valves to open a flow path for the hot boiler
water. The motorised valves have little switches on them, and once they
are in the open position, they switch the boiler and pump on together.
The boiler fires up, and stays up until the return water temp is good,
and then the boiler cuts out as it should, but the pump stays running
beyond the boiler cut out, moving the hot water around till it cools
down, and the boiler fires up again...But this is an indirectly heated
hot water system...


Now I assume you have a combi boiler, in which case it probably has
direct heated hot water? In which case I am at a loss to know why the
pump is running at all when you call for hot water? What I assume
happens in a combi, is that there is a flow switch that activates when
hot water alone is needed, the boiler fires up, and when the flow stops
(you turn off the tap) the bloody thing shuts down. I am no combi expert
tho.

If that IS how they work, then it would seem that your flow switch is
sticking. So the boiler does NOT shut down when water flow is stopped.
Then if the CH is on as well, it doesn't matter, because the extra heat
diverts to the CH circuit, but if CH is off, the boiler is overheating
the water and the safety cutout shuts it down.

As I say, I am no combi expert, and hope that others will confirm/refute
what I have said.

Ther is confusion in my mind as to what sort of system you have, and
what the pump is actually for.