Motorised Valve sticking

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I have a Glowworm boiler, and no changes made recently.

A few days ago my heating started coming on when set to hot water only regardless of the room thermostat setting.

After reading posts on here it sounded like my motorised valve was sticking/bust. So I think I've found it and attached a picture of it (some clown has installed a wall over it! - not me BTW).

When I switch to heating only the lever moves to the right, on hot water only the lever only moves to the middle and the rads stay hot. If I force it over by hand the rads go off as they should.

My question is - is this my problem and can it be replaced without draining down the system (there's no drain valves in this house!!)? Some posts have suggested replacing just part of it (the motor?) without draining down, but this depends on which unit it is. I don't know which one mine is, perhaps someone could tell me from the photo. There are only two pipes entering it as far as I can tell (unless the third enters from the back, which I doubt).

Any help much appreciated.

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thats a drayton zone valve.
you should have another one somewhere. near the cylinder etc maybe.

looks like its not fully closing.
 
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you will have to look down under the midle of the valve if its a 3 port there will be another pipe going into it thereits a drayton by the look of it
on the back of it as you look at it there is a buton press it and the actuator lifts off
you need to find which modle it is, info on top of actuator
 
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its a drayton ZA5 zone valve it only has two positions A,B
mid position valve has three positions W,M,H.
 
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Got the cover off, which was hard enough because of this wall in the way! There's only two pipes going into it. You are of course right it is a Drayton, and the Actuator Code is ZA5.

Does this help tell if I can replace the motor part without draining down, if so how. I'm trying to do this without having to rip out the whole stud wall that's built over it!!!
 
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press the button on the back in the middle and the actuator lifts off
it direct behind the lever marked AB
 
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Got it off, couldn't get to the back as it's installed so close to the wall, but the switch follows through to the front and is accessible underneath.

The motor works fine all the way back ond forth while off, so that suggests to me it's the valve itself I guess. I will try squirting with WD40 and moving by hand first to see if it loosens up.
 
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Yes I meant with pliers, my fingers aren't that strong :)

It turns easily fully both ways with a 2" spanner, and the motor works both ways when off, but stops half way when fitted.

It certainly seems the fault is here, but now what?
 
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Already got the lid off. The spring seems fine, it's attached to the plastic lug at one end and something else at the other. It expands and collapses fine when the motor operates. It's only when the motor is fitted to the valve that it doesn't close all the way.
 

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