Circulation pump continuously running

My circulation pump is continuously running. The HW and CH is otherwise working ie coming on and off as it should, but the pump stays on, and it never used to do this. The boiler is not staying on with the pump. It is operating as it should do when called for by HW and CH. I have had problems with the actuators before and have tested the wires from these. Grey always on, orange wire only live when the respective actuator is calling for heat. I think this is correct. Brown wire never seems live!

Also, not sure if related, but for the past few days my HW has not been very hot. It comes on very early morning. Could it be the pump has been circulating that hot water round the system (with the boiler off) thus cooling it?

I've heard on here about a pump overrun switch, but don't know if I have one. There is a froststat, which seems to be working ok, ie if I turn it up the boiler fires up, then down it turns off. The live wire to the pump is connected to a yellow wire but I can't see where it runs to next without tearing my house apart!

System below.

- Gas fired boiler Glowworm Ultimate 60FF
- Gravity fed
- Two 2 port Drayton ZA5 actuators, one downstairs (CH), one in the loft by the cylinder (HW)
- CH and HW working fine

Thanks in advance.

as you have two motorised valves then you have a fully pumped open vent system, dont use the term Gravity it means something completely different, with your system the pump should be wired into the boiler the terminals should be maked P and PN for pump live and pump neutral, the boiler does indeed have a pump over run function, what this does is that when any demand for heating ceases, the boiler will keep the pump running for a short period to dissipate the heat from the boiler then the pump should stop , some are done by temperature some are a fixed time, sorry I cant remember which yours is, sounds like this is your problem

Thanks for that Ian. I am just reading the boiler troubleshooting guide now and realised the term gravity seems to conflict with what I have. I have just opened the boiler cover to check the PCB and as soon as I powered it back on there was a loud buzzing noise from the PCB. I've had this before and replaced only a couple of years ago. These things don't seem to last long! The troubleshooting flowchart suggests I remove the wire from terminal SL on the boiler and the pump should stop after a few minutes. Mine continues to run, so this suggests (as does the horrible buzzing) the PCB is faulty.

Cheers for your super quick response.

yes does sound like the pcb but as I am not there to test I cant say for certain, but with your description it does sound like the pcb