DIY Doctor UK DIY Forums

For the past couple of weeks my central heating pump has not shut down for several hours after the heating has turned off.

As the pump is in a cupboard in my bedroom this is causing me sleepless nights.

After doing some research I thought the problem might be a faulty overrun thermostat however as far as I can tell the pump is not wired to an overrun thermostat. How can I tell if it is wired to an overrun thermostat? If it is not wired to an overrun thermostate is there a fault that could result in the pump continuing to run after the boiler shuts down?

thanks for your help

There are many central heating systems as well as combi boilers normal condensation boilers will normally run to one of the Honeywell Plans. C, W, S or Y and with some of these systems the pump is controlled by the motorised valve so first step is to find what system you have. If you have a motorised valve how many ports will help to start with. Honeywells web site does show the four methods. Of course it does not have to follow one of these plans but most do.

thanks eric - I will have a look tonight and see what more I can find out

PUMP OVER RUN CAN OCCUR IF YOU HAVE A FAULTY ROOM STAT , CYLINDER STAT OR PROGRAMMER WHICH IS DEMANDING THE PUMP TO OPERATE. IF YOUR BOILER HAS A PUMP OVER RUN STAT THAT MAY ALSO BE AT FAULT. WHAT IS THE BOILER?

PS. FORGOT MID POSITION/ZONE VALVES. iF MID POSITION VALVE STUCK IT CAN CREATE DEMAND CIRCUIT ALSO. dOES YOUR HOT WATER GET TOO HOT? cHECK CABLING NOT ROUTED OVER HOT PIPEWORK,ETC. :?

pump is acting in response to demand for hot water or heat. check to see if the mid position valve is stuck. Do you have a frost stat on the system?

Thanks for the replies - I have had another look at the system and the mid position valve does appear to be stuck. When the heating is on it goes right over to the H (presumably meaning heating) but when the water is on it doesn't go all the way across to the W (presumably water) but most of the way.

I am however pretty sure it has been like this for a long time though as for at least a year the radiators have heated up a bit when the hot water has come on. It is however only the last couple of weeks that the pump has been running half the night (even when the hot water and heating are turned off).

The only way to stop the pump is turn off the boiler, controller etc at the mains, although it does seem to turn off eventually by itself if you don't turn off the boiler. Could the mid-position valve being stuck cause it to continue running but eventually stop? The valve and the controller are both acl lifestyle and the controller is model LP722 - I can't tell if the pump is controlled via the valve or not.

I did initally think it might be a pump overrun thermostat as this seems to be a common cause of this problem but no idea how to find out whether the boiler has one or not - it is a Baxi Solo 2 PF but I don't have the instruction manual.

thanks again for the help

I doubt you have an over run thermostat, you need to test your wiring centre to find out where the problem is. You will need a multi meter to do this.
A wiring diagram from honeywell will help although the terminals numbers that are in the book are not always used exactly. You need to determin where each wire goes. no. 10 should be live out to boiler and pump so this means one should not fire up with out the other. Unless there is a overrun device between them which if there is follow the wire to find it.. but I doubt it. you need to follow the wiring through to see what is passing the live through to the pump but not the boiler. Checking all the stats are switching the right things on and off. It is quite difficult and and requres a good understanding of how the wiring and system works. If your pump is ciculating cold water round your system it will encourage corrosion problems so I suggest you let a professinal look at it sooner rather than later.

thanks for the advice podgeplumb(!)

I will be getting a professional in as the boiler has started making funny noises now as well - might just be able to get British Gas to fix it under my homecare contract whereas not covered if just the pump

Simon