Potterton Profile 80 - Pump runs continually

[VFXED]

I've recently moved to a new home, where the boiler is an old but functioning Potterton Profile 80. Perfectly OK boiler, the only problem being that the pump runs continually when the timer is calling for Central Heating, regardless of whether the boiler is firing or not. Is this correct? I would have expected it to shut off a few minutes after the burner has extinguished.

The timer was replaced on moving in, as was the room thermostat with a Programmable one. I've chased through the wiring and all seems fine, with no errors on the installers part. The pump is wired correctly into the boiler, and the motorised valve is functioning properly.

Is this problem likely to caused by a faulty PCB, or a damaged Over Run thermostat sticking in the 'ON' position?

 

[JohnD]

The pump should run all the time the room stat (or the cylinder stat) is calling for heat, even if the boiler is not firing. This boiler has a big iron heat exchanger which takes time to heat up and cool down. Start by verifying that the stats are both wired and working correctly.

Old boilers are sometimes wired oddly, the stats should control the 3-port motorised valve (if you have one) which makes the boiler fire.

If no problem there, your boiler may have a pump overrun to circulate water through it after the firing has stopped.

You can get a useful wiring diagram from the makers of the motorised valve, it will probably be a Honeywell or a Drayton. Look carefully for its model nymber.

The 28mm valves have one more wire than the 22mm valves, so it makes a difference.

I think you are going to need a multimeter, lots of paper, and some tinkering to work out what it is doing.