ACL mid-position value

13 Feb 2011
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United Kingdom
My central heating failed a couple of weeks ago, and the problem was an ACL mid-position valve (MA1/679) leaking water up the spindle into the electronics. The valve has now been replaced, but has introduced a strange problem.

The mid-position valve gets stuck in the mid-position under certain repeatable conditions.

If both the CH and HW are on (with the valve in its mid-position), and the CH turns off, then the valve will move to the HW position (correct operation)

However, if both the CH and HW are on (with the valve in its mid-position), and the HW turns off, then the valve will not move to the CH position (fault).

My first thought was a faulty valve, however, if the valve has been left in the HW position, and the CH comes on, then the valve will move from the HW position, through the mid-position, to the CH position (correct operation).

I've come up with 2 possibilities: the new valve is faulty or the Drayton Lifestyle control unit got damaged when the original valve short circuited.

I'd be very interested in opinions on the problem.
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might be your cylinder stat, programmer or wiring problem. Sounds like its not getting an "off live" signal to the valve in the situation you mention. Needs checking over with a multimeter.
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Diagrams proved invaluable, in diagnosing the fault. The problem was faulty electronics in the new mid-point valve. All now fixed.

Thanks very much for the help.

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