I have a potterton boiler, potterton time switch with HW and CH having separate controls. The boiler recently started to fire occasionally when the timer was off and the pump runs even though the timer is off. Only resolution is to switch off at power socket. Called a plumber who removed the electric valve to the central heating which I am sure is not the problem as it only controls whether the water goes round the heating circuit. I replaced it but think the problem may lie in the frost stat. Any ideas on what the likely problem might be ?

Snowy Owl, Edinburgh, August 2008

Had same problem with Weil McClain boiler which has a 3-way valve with an actuator. It's the valve sticking down in the heat position. Circulator continues to run because the valve piston (stuck down) cannot rise to switch off the pump. Raise it manually and you will hear the pump go off. Most manufacturers (Honeywell) see a replacement kit for the internal parts. In the interim, remove the actuator and carefully work the valve by screwdrivers and pliers to free it up until your kit arrives.

Dan from Highland MD, March 2017

Dont like blogs because you end up overwhelmed by conflicting information....... however, last night my house turned into a sauna....
the stat was turned down as the wood-burner was on, at 11pm my wife complained that we were getting too hot so I checked the time clock - all off - so why was the boiler running full tilt and taking no input from the room stat or the time clock?
so at rugby this morning I spoke to my friendly heating engineer and he said oh that will be a stuck motorized valve. I thought like so many other people that this could not be right as there shouldnt be any power getting to the boiler. So I felt like the controller on the boiler was faulty. I removed the time clock completely and the boiler was still running.
Then i read this blog. I couldnt understand how you could get power to the burner and the pump if the controller was switched off. Anyways I followed your bloggers advice, I allowed the boiler to come on with the stat and clock turned off. I tapped the motorized valve once and hey presto boiler went off. So Thank you all for your input
New valve tomorrow and no call-out charge for an engineer.
It would appear that the boiler and valves have a direct feed, and all the controller and room stat do is turn off the hot water valve and the heating valve. the valve when it is open feeds power to the pump. likewise, it also feeds power to the burner via the boiler stat. so when the valve sticks open it acts as a switch. The pump will continue to run and the burner will kick in as soon as the boiler water temp drops.
Thanks everybody

Jonathan Mercer, March 2016

Spot on regards dodgy micro switch in valve, one tap and pump stopped.

Ted, November 2014

Mike the plumber u r a star :0))) had this problem followed your grey wires and can now sleep:0))) thank you

Acs101, April 2013

To Mike the Plumber, thanks for the advice on the grey/orange wire. I checked this while motorised valve was still assembled and found that the power was flowing as soon as the main switch was put on. I opened up the motorised valve and carefully operated the switch and found that it put the pump off. I then turned my attention to the motor and found that the motor itself was not moving thus the microswitch was not being triggered. Ordered a new sun valve as a result, thanks again Philip

Philip, April 2011

I have just had this problem with a Potterton Netaheat 12/22. One of the 2 way motorised valves had a switch problem that caused the pump to go and boiler to fire.
I thought the problem was the programmer / timer at first but as said above the boiler would fire even with the timer set to off. Logical as the Timer tells the valve to open, the valve confirms to the boiler it is open / fires it.
My valve was working but as in opening and closing but the switch was saying - I'm open switch the boiler on all the time.

Steve, September 2009

Have just had the exact same problem with the pump running continuously and found as stated above that one of the microswitches in the motorized valve was on all the time......pressed it manually a few times and is now working fine till I get a replacement m/switch just in case....thanks to whoever posted that reply it's saved me a call out

martin, November 2008

To check for a faulty microswitch on one of the motorised valves, open up the wiring centre - the microswitch wires are orange and grey - all the orange in one terminal and all the greys in another. Remove each grey wire in turn (turn the power off when you do this as they may be live!). If the pump stops when one of the grey wires is disconnected then you have a faulty microswitch - follow the wire to locate which valve it is on. Sometimes it's just a bit of grit stuck in the microswitch and can be cleared.

mike the plumber, September 2008

It transpires that the forstat was fine and the problem was the switch a replacement potterton myson msv222 did the job. Thanks for all your suggestions.

Snowy Owl, August 2008

Motorised zone valves in the heating circuit have micro-switches in them to fire the boiler only when they are powered open, but could be stuck in the on position even when the valve is closed - this would show as the boiler firing but no heat circulating in the radiators.

So if the boiler is firing but no heat is getting to the radiators (ie the valve is still closed) then you probably have a faulty microswitch - replacements available from Maplin for pennies.

However if the heating system appears to be working but the system is coming on even when it is not needed, then something else is triggering it - disconnect the live feed to the frost-stat when the boiler is firing, and if it then stops - bingo!

Ford Macfarlane, August 2008

Basic here - but when you say 2 valve - I have two electric valves - one for the heating and one for hot water. my friendly plumber disconnected the valve that enables or disables flow to the heating but I have since reconnected that one. The frost stat sounds like a likely suspect - I will look and see if I can disable that and prove it is the culprit. Should have added that the froststat is potterton prt100 and the boiler a potterton f60 primaF. I also have a switch on the return to the boiler which I guess is an overheat switch set a 60. Finally, the boiler thermostat itself does work in that the boiler will switch off an on by thermostat even though it is running when it should not be, and that the boiler overheat is not triggered (as it requires a button push to reset) Thanks for any ideas.

snowy owl, August 2008

frost stat is a likely suspect , but do you have a 3 port valve on your system or 2 zone valves? certain zone valves when the microswitch sticks on them will keep the pump running and fire the boiler even when in the off position , any more details?

kieran, August 2008