Suprima 50 Responds to heat call but won`t stop!

Suprima 50 boiler; ACL 3 port valve; ACL lifestyle controller

Had to fit a new Synchron motor to the valve as and it now responds fine to initial calls again. The boiler fires up in response to CH calls and HW calls and drops off when CH call cancelled at the controller HOWEVER, when the boiler responds to a call for heat from CH (using the room thermostat), it fires up and stays on, even if the call from the stat and controller are cancelled. Boiler keeps going and driving up the heat!

BTW When this happens a HW call puts the MPV at W position and a manual shift of the valve to CH trips a switch push it all the way to CH position.

Is this a microswitch fault in the valve?

If the rads keep getting hot the valve must be open to CH

So it sounds like the valve is sticking.
Make of valve?

Thanks for the reply, Chris.

The valve is the ACL lifestyle 679H340-3OLO. I take your point, but with the power down, the valve drifts back nicely to the rest position (W). I assumed (perhaps wrongly) that his meant it was moving ok.

KM

To clarify the above (!?):
The valve is winding to heating position due to live on both signal wires. Yes?
This is tripping the boiler into activity (from orange signal wire if my reading is right).

In this situation, what is it that causes the boiler to stand down and ignore the signal from the valve, when the thermostat stops calling for heat?

This is the part of the cycle that is not happening.

KM

There's a couple of microswitches in the valve head. One of them is the auxilliary switch which makes Orange live for "CH only", by connecting it to the live on the white wire from the room stat. Unless I'm going daft...
Or ACL valves aren't the same as Honeywell which I know a bit better.

Found this diag which helped alot:

http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm

Checked the circuit in the valve head and its as we think.
I tried resetting the controller (just in case) but no change THEN
I wiggled the valve head assembly and bingo, it`s all working again!

I`m still not convinced the valve was sticking as it always fell beck to rest position on powering down.

I`m now thinking that the heat when the Synchron motor burnt out may have caused a dry joint on the circuit in the valve head.

For now I`m going to sit tight for a couple of days and see what happens.