As I suspected, the actuator is sticking.
The original is a Danfoss HSA 3, which is sticking.
I have a brand new spare which looks in all respects identical, a Sunvic SD 2701. It was bought as a spare head for our system, before it was all replaced a couple of years ago.
Can anyone confirm whether this is compatible please?
Yes. Fully compatible. Check it out here:
To answer my own question, yes it is. I eventually managed to find a site which confirmed they were identical.
It also connects with exactly the same colours, except for the Danfoss's brown/white becomes the white on the Sunvic unit.
Whilst swapping it out, I took the opportunity to save myself a few minutes for the next time - a simple way to make it a 'plug-in' swap so far as the wiring is concerned.
Basically you permanently fit a 4 way 5amp connector strip on the ends of the actuator wires, in the correct order. Fit the same to the wire ends of the spare actuator and a third 4 way strip to the 4 actuator wires in the 8 way junction box - adding 4x short bits of solid 2.5mm copper into these latter connectors to act as a male connector.
The actuator's connection strip then just pushes straight onto the
2.5mm and you tighten the four screws.Then when you need to swap actuators you just undo the four terminal screws, drop the faulty unit off and fix the replacement in its place. No messing about with the main connector block or making notes as to which wire came from where, just plug it in and away you go.
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