Central heating pump problem

Upzeecreek

Just looking for some advice on a central heating pump problem. 
When all the timers are set to off the boiler pump continues to run ( for hours ). On the info panel on boiler it is saying "c" but not actually firing.( Although I noticed last night it did fire up for about 20 secs and then stop) It only has this problem very occasionally. It happened last night and the previous time was before Christmas( where it was happening 3 to 4 times a week)
Now British Gas have been out 3 times to this problem and changed 2 motorised valves. There is a 3rd valve ( 3 zone) but that has been replaced in the last few years. The problem is that the fault is never presenting when BG show up so I understand they are having to guess what the fault is. 
This problem seemed to start after we had a small leak after taking a radiator off wall to clean. Heating and water were all left off ( house empty, not yet moved in)  but when I returned to the house the next day pressure had dropped and the pump was running in the airing cupboard. Could this problem be caused by air getting into the system ( from when we had leak) or is that just a coincidence? 
Really greatful for any pointers as it's driving me insane!! Thanks

bris

It may be a faulty control. The pump runs because something is telling it to, have the controls checked.

The pump over run will kick in for temperature reasons but usually only for a short time.

Jeepers_Creepers

The previous leak and the possibility of air in the system is surely a red herring.

This pump is separate from the boiler, and not built-in? If so, the pump is either controlled by the boiler itself - so that it can 'overrun' for a while if the boiler deems it a good idea - or is effectively wired in parallel with the boiler, so when the boiler is told to go 'on' by the heating controls, the pump does so too.

It would be strange, therefore, for the pump to run independently of the boiler, unless the boiler was telling it to. If it was down to a faulty zone valve, then I'd expect the boiler to be coming on as well as the pump. (And it might, if 'C' suggests it is). 

When the pump runs when it shouldn't, do the rads become hot?

Make and model of boiler?

grumbler

This can happen, say, when heating HW cylinder - if its thermostat is set to a higher temperature than the boiler's own thermostat (or either is faulty). However, as the OP states that "all the timers are set to off", I'd suspect the controller in the fist place.

TadleyBaggie

Froststat?

Upzeecreek

Thanks for all the comments so far...so to answer questions ...
The make and model of boiler is Ideal logic heat 24.
The radiators  stay cold when pump stays on..
There is no frost stat on boiler as in utility. 
BG checked water temp thermostat and all fine there.
When the pump is staying on the boiler does fire up for about 15 seconds and then goes off..All timers are off. 
I didn't mention ,but it is an unvented system. 

grumbler

"Frost protection

If parts of the pipework run outside the house or if the

boiler will be left off for more than a day or so then a

frost thermostat should be wired into the system.

The frost thermostat should be sited in a cold place but

where it can sense heat from the system.

Note. If the boiler is installed in a garage it may be

necessary to fit a pipe thermostat, preferably on the

return pipework."

Are you sure there is no any frost thermostat in the system?

Upzeecreek

No, there is definitely no frost thermostat. Boiler is in utility room and all pipe work is situated within the house..

Jeepers_Creepers

How old is this boiler?

When I tried to look up the instructions for that model, I can't see anything that would make a 'c' show on the display. That's not a fault code, it isn't an 'on' icon - no idea what it implies.

Do you know if the pump is wired from the boiler - ie is controlled directly from the boiler - or is it fed from a wiring centre (where I would expect it to be wired in parallel with the control wire to the boiler, so that a zone valve tells them both to come.)?

What type of pump is it? I see the boiler PCB has terminals for 'pump PWM' which might mean it can be used with a specialised pump which is speed controlled. In which case it might be the boiler getting confused and telling the pump to do stuff it shouldn't...

Are all three zone valves in the same place? With the CH timer 'on', does turning each zone's stat up and down make the valves respond smoothly - a nice 'whirrrr and click' each time?

Jeepers_Creepers

You could also try phoning up Ideal, or emailing them - give a short but concise description of what's happening, and what's already been tried.

Upzeecreek

Boiler was installed in 2013 when house was new. 
When the programmer is on for heat, there is a "C" displayed which apparently means there is a call for heat. Then when the boiler actually fires ,the "c " remains but is lit blue. When no call for heat a "0" is displayed.
It's a grunfoss pump. All 3 zone valves are in airing cupboard near to the pump and they seem to be opening and closing normally. When the pump wouldn't shut down , I disconnected the upstairs valve and pump shut down straight away. British Gas then assumed it was the upstairs zone valve that was faulty. They changed that but the problem reappeared when the heating turned off that  evening.
Thanks for all the replies so far...very greatful