CH pumping over

Ok I have trawled most of the posts on this subject and they all suggest one thing - sludged up system. But I am not convinced as there are some weird things happening to my system

It is an ideal w2000 boiler with 2 position motorised valves for CH and HW.
The pump is a Grundfoss Selectric. Gravity and pumped I guess.

Ok so faced with a expansion tank pumping water into the tank like crazy I identified that the CH motorised switch was seized so replaced the whole thing. All seemd ok but noticed even though the rwb 20 prog controller was off the pump was continuously running and the only way to turn off the boiler was to switch it off at the mains or turn the boiler thermostat down to zero.

I replaced with RWB 29 which is the Siemens direct (and expensive) replacement. No joy. Well, the room thermostat and immersion tank thermostat have both been replaced and the motorised valves whirr when the thermostats kick in but still the boiler and pump runs continuously.

There is a frost controller which I also replaced and even when the connecting wires for this were open circuit the pump and boiler still ran independantly. The overflow feed into the expansion tank is connected to the pumped flow into the hot water tank and the only way to stop the pumping over is to manually switch the motorised valve to allow permanent flow into the tank. Of course the water is now hot enough to melt Titanium... Pump is set at minimum flow, boiler at 4 (max 6)

So last ditch before I call the experts
1. Does the boiler have a second thermostat that keeps the pump running until the water gets to a lower temp then stops the pump/shuts system down independently of the rwb29? In which case if so it's faulty
2. Could it be the pump is not getting enough flow and the boiler is overheating then pumping over due to gravity / syphon effect? In which case is there a sticky non return valve that is allowing syphoning to take place?

I am reasonably competetent with DIY and I have checked all the obvious stuff. But why is it all still running in spite of the RWB29 being permanently in the off position - clearly the pump and boiler are getting a feed from somewhere - I thought it was the frost thermostat but I have replaced it too and it makes no difference whether I switch it in or out.

HELP !! IF YOU ARE IN THE WALTON ON THAMES AREA AND CAN HELP LET ME KNOW!! I will pay you of course. The boiler was last serviced in 2003 and is a cast iron job. Any suggestions welcome!! I am about to kick the cat.... Rich

You have a wiring issue or knackered zone valve and by the sounds of it a pipe configuration issue aswell.
Live 230 miles away so cant help there sorry.

Well I'm only a couple of junctions up the A3 but .. your problem is a stuck microswitch on a motorised valve. Presumably the HW one you didn't change, unless you got the wiring wrong.

Thanks for the replies so far - I am going to swap the motorised valve (I have a used spare) but was I correct in my thoughts - is there a secondary circuit that has to be completed in the zone valve when it is open / shut position that also switches the pump/boiler on/off respectively ? That would explain why the HW/CH controller has no effect.
It's constantly heating up water and it has no place to go except the overflow tank

I took a picture and drew out the wiring on the motorised switch before I swapped it so I am 99% sure it's wired back ok.

ANY STUFF ON MOTORISED VALVE OPERATION/WIRING ANYONE?

Chris - I may be need of some expert help if this doesn't help!
Best wishes
Rich

Perhaps you turned a by-pass valve off, maybe a red handled wheel valve.

Not altogether surprising it pumps over if both valves are shut. Not ideal but doesn't need mending itf it's ok wth no fault.
Before you swap the valve/head, just lift the orange wire out of the junction box.

Normal S plan wiring is here:

ChrisR's skanky version here: (except that your pump and boiler are connected together, to the orange wires from the valves' switches)

Well it's fixed I think

I opened up my spare motorised valve, looked at the circuit diagram and my own theory and hey there is the switch. It latches in and out when the thermostat tells it to and it is this that is controlling the boiler and pump. I then took off the switch on the immersion motorised valve and hey presto it was permanently live at the junction box (tested it with all power off before removal with my ohmmeter and it showed permanently on) Freed it off and it all seems fine, switching on and off as I moved the tank thermostat.

Thanks very very much for the help chaps - especially A3 2 junctions Chris (last posting). If you are interested we will be doing an extension in the new year and need a good plumber to help us with the new rad installations etc. So just leave me a message if ur interested 07767 212553
Thanks again
Rich
Rich