Help with Central Heating problem

I'm hoping someone can help with a Central Heating problem I'm having. Any advice would be much appreciated. This is my first home (been in 3 months) and I'm totally new to plumbing, etc (I can wire a plug, replace sockets/switches, etc but have never worked with water/gas).
I have a Potterton Prima F boiler with an EP3001 controller, a PRT100 thermostat downstairs and in the airing cupboard with the cylinder a Potterton MSV222 valve, a Myson valve (can't see a model number), a Myson Compact CP53 pump and a Potterton PWB1 apparently controlling it all. A diagram can be found at http://www.communitymoneyadvice.com / ch.jpg showing all the bits in the airing cupboard and how they are connected. The labels 1, 2 and 3 I found written on paper tags tied to the relevant pipes (presumably by the builders). The devices P, A and B are the pump and the two valves.
Yesterday I had to switch the heating on for longer, as the house was particularly cold with the snow earlier in the day. Today I find that the heating won't switch off! The only way I can stop the heating is to cut the power to the entire system.
Following earlier posts in this group I have turned off the boiler (by turning the dial on the front to 0) and disconnected the EP3001 programmer. That stops the boiler heating any more water, but the heating still continues to run as I can hear water moving through the radiators. I've confirmed the thermostat downstairs works as I can hear it click when I move it. I don't think I have a frost stat as the boiler is in the kitchen and there's no pipework or suchlike outside the house.
I then visually inspected the valves (as far as you can in a cramped cupboard with little direct light). The Potterton MSV222 has a lever on the side which I can move (with a little resistance) and it then springs back to the original position. The Myson also has a lever, but this only slides between positions marked AUTO and MANUAL without any resistance and doesn't spring to or fro. I also notice a small amount of water appears to have leaked from the bottom of the Myson valve which makes me reluctant to do anything with the power on to the system (I don't want to electrocute myself!)
Can anyone offer advice? It would really help me if someone could explain what each bit of the system does and which valve does what. I think, looking at diagrams the house builders left (the house was built in 1995 but I moved in in November), that I have an S plan system?
I presume the next step would be to examine the valves? If so, which valve does what? Can I open them up without frying myself or flooding the house? I don't even know if the system is open or closed as I cannot find any sign of either a feed tank or a pressure gauge. There's a cold water tank in the loft but not obvious sign of a pipe from that to the airing cupboard.
Any suggestions much appreciated...
Regards,
Andrew Dancy
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On 9 Feb, 22:08, snipped-for-privacy@gmail.com wrote:

Strongly suspect the Myson valve may have seized in the open position and is placing a continuous call for heat on the boiler and pump. Take a look at S Plan on Honeywells site and study the circuit diagram IIRC the powerhead can be removed from the valve without draining. Do so and try turning the shaft of the valve - it should be fairly easy. If it is then your gearmotor in the head needs changing (about a tenner), if not you need to free it off
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Thanks for that. I've had the top of the Myson off and I can turn the shaft underneath by hand if the system is off (a quarter turn only - just enough to move it from open to closed). If I turn the system on then the valve won't budge by hand (presumably cause of water pressure on it?). It's currently in the Open position. With the powerhead off I can move the valve to Closed by hand and then turn the system on, but of course the powerhead still thinks it's open as the powerhead itself is still in the open position and the control wires come from the powerhead and not the valve itself. I can imagine that it would not be a good idea to have the valve closed but the system thinking it's open and still pumping, so I've left the valve as it was (open) and the system off for now.
I did notice that a) there's a lot of rust on the valve itself, b) a fair bit of liquid came out of the powerhead when I took it off and c) the operating light on the powerhead does light up when I put the system on, so electrically the powerhead is still functioning to some degree.
I also discovered (poking around in the back of the airing cupboard) that the original occupants of the house must at some point have replaced one of the powerheads as I found a box from Potterton dated 1997 (too late to be the builders - they were in 1995/6) that originally contained a powerhead and valve, and still contains a brand new valve. I'm assuming that the occupants must have replaced an original Myson powerhead with a new Potterton one but not bothered to change the valve. That would explain why I've got one Potterton and one Myson. Something just to keep, or should I be thinking about using it to replace the rusty existing valve (which presumably will involve draining down the system?)
So, is the next step to have the powerhead open, or should I just get a new one?
Many thanks for your help on this.
Regards,
Andrew
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In an earlier contribution to this discussion,

Sounds like you need a new actuator (power head).
Have a look at http://content.honeywell.com/uk/homes/systems.htm and scroll down to S-Plan to see how your system is supposed to work.
Basically, you have two motorised valves - one for hot water and one for central heating. The HW valve is controlled by the programmer and cylinder stat. The CH valve is controlled by the programmer and room stat. Inside the actuator of each valve is a set of contacts which are electrically isolated from the motor circuit. These contacts 'make' when the valve is open, and are used to switch on the boiler and pump.
If a valve (or just the actuator) remains in the open position even when the programmer and stat are not telling it to open, the boiler and pump will continue to run. This is what's happening in your case. If the actuator doesn't return to the closed position even when it's removed from the wet part of the valve, it sounds like the return spring is broken. [The motor only works in one direction, and a spring closes the valve when motor power is removed].
If the wet part of the valve is leaking, that will need replacing too - but that is a bigger job since it involves a partial drain down of the system. If it's only a small leak, and you can prevent the water getting into the actuator, and if the spindle of the valve moves fairly freely between open and closed, you can probably get away with replacing just the actuator for the time being - and leave the valve itself until some warmer weather arrives.
--
Cheers,
Roger
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