Roger Mills presented the following explanation :
Three-port valves are horribly unreliable and are probably the single cause
of the greatest number of CH woes reported here. Maybe time to replace it
with 2 x 2-port valves and convert to S-Plan.

On that point I certainly agree. The actuators barely make it out of
the guarantee period before they fail. Mine average 18 months to 2
years, which is why I now keep a spare set up ready to swap out.

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