Unusual use of M8/M9 to control a compressor

[Doddy]

Couple of caveats -

You'll be back - I reckon within a couple of weeks - to complain that, whilst under UCCNC control someone switched the compressor on accidentally, and after UCCNC has quit that the compressor has been left operating. That's a more complicated problem!

You could use a C/O contact on the relay to gate the building wiring into a contact that, if M8 is active, and the building wiring "live" (i.e. it would hold the compressor on after M9) that it illuminates a lamp or sounds an alarm, or something. But that would require that you isolate that circuit from the compressor under M8 control.