Andrew Gabriel brought next idea :
Original posting not shown up here, but when I installed my
heating, I used the 8-pin circular bulgin connector, which used
to be (and maybe still is) common on disco lighting and the
like. I drilled out a blank patress plate to take the socket,
and attached the plug to the actuator. Two of the spare pins in
the plug are looped and effectively in series with the thermostat
for the zone on the 'A' port (upstairs heating in my case) so
that if the plug is removed and hence the actuator is stuck on
the spring return 'B' zone (downstairs heating in my case), the
other zone cannot demand heat which cannot be satisfied.

I enquired of the supplier what the differences were between the four
types of actuator in the spring return range. He suggested that one of
them was a plug in type, but did not seem able to supply a matching
socket.

As indicated the actuator gives less problems since I added an heat
deflector. I suspect the usual jamming leading uo to complete failure
is a result of the grease lubricant on the actuator drying out due to
heat.

I also noticed the original actuator (as fitted when the heating was
installed 18 years ago) was not fitted with what I assume to be an
internal heatsink for the motor as fitted to later models of the
Sunvic.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org