Central heating diverter valves


I've got a Honeywell diverter valve next to the hot water cylinder, it is a three position valve. (hot water, central heating and both) The problem I am having is that it sticks in the 'both' position, so even after the cylinder stat is satisfied it continues to heat the domestic hot water too hot.
I've removed the motor head from the valve body (two screws) and I can easily turn the valve shaft between finger and thumb, also the actuator seems to work OK when it's not coupled to the valve. It seems that the thing relies on a spring for movement in one direction.
The only think I can think of is to spray WD40 up it, but it's not made any difference. Are then any 'tricks of the trade' to persuade these fickle things to work as they should?
Cheers Julian.
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Julian wrote:

There should eb a spring in there somewhere but sometimes crud gets trapped in the valve or the valve deteriorates allowing some of the hot water to get past.
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Julian wrote:

the design hasn't changed at all. Firstly are you sure your cylinder stat is switching off? It may click but is it actuall switching electrically? It's very unlikely but it could be a cause.
The spring in the valve actuator is balanced against the motor in half power mode which is where/why it stops in the half way position. Can you turn the valve easily when the pump is running? You can buy new motors and what have you for the actuators but IIRC when I worked for Potterton the standard fix was to swap the actuators.
--
Malc

"AFB Mr Tracey."
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Thanks for the reply, yes the cyclinder stat is switching off electrically, if I switch power off totally to the heating system the valve remains stuck , if you give the valve body a tap with a knuckle (and it really is a slight tap) you can hear and see the valve closing - motive power is the spring in this case.

That's a good thought, I haven't tried turning the valve with the pump running, I'll give it a go in the morning and report back.
You can buy new motors and what have

I was kind of expecting this to be suggested :-) I'll have to partly drain the system - but if it comes to it I'll get stuck in!
Thanks
Julian.
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Julian wrote:

No you won't. The actuator is the electrical motory bit that you already have taken off, haven't you? If you haven't it's just held on with 4 screws if I recall correctly. Unless your valve's at an odd angle.
--
Malc

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A very reasonable assumption, but not entirely true.
In order to move to the middle position, the synchronous motor is supplied with AC. When it gets to the middle position, it gets switched to a DC supply which locks the rotor and holds it in place. Now the trouble is that the DC supply creates residual magnetism in the yolk. When the DC supply is removed, this residual magnetism remains, and will keep the valve in the middle position, although just touching it will often overcome this and cause the spring to return it. In order that it returns by itself, a tiny AC current is fed through the synchronous motor in order to degauss the yolk. This is way below the level required to operate the motor. It is derived through a resistor in the unit, typically something 100k or more. It might be that this resistor has gone open circuit, or the associated microswitch has stopped working.
--
Andrew Gabriel

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Thanks, on the strength of the above I went to the local Plumb Centre and got a new motor head this AM, it was behind the counter so they must sell loads of them. I fitted it, swapped the wires and can report that it now works fine! I've never had much luck trying to repair these little modern electric devices, so didn't even try - I just payed 40 for a new one.
Anyhow I'm well chuffed because I replaced the stupid electronic heating time switch (that requires a degree in electronic engineering and the instruction book + about two new swear words every time you try to change the programme) with a lovely new Horstman mechanical device that can be set in about 10 seconds! Also, the boiler pilot light had a load of rusty debris from the cast iron heat exchanger sitting on it, this kept the flame away from the thermocouple and caused it to fail at least once a day. After I hoovered all the crap off the burners up that now works as advertised. I'm guessing that a plumber would have charged a bit to do all these jobs.
Julian.
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