Reply To: Switching a server room split AC off and on from Hermetry with a 16 A plug relay

I am new to the IoT side but HVAC I know a bit from our buildings, and we had the same discussion with our installer.

Leave the mains alone. Most wall splits of that size have a connector on the indoor unit board for an external on/off contact (installers use it for key card switches in hotels and for fire panel shutdown). Some brands need a small interface adapter for it, the installer will know the part number for your model. A closed contact means run, open means stop, and the unit does its normal start and stop sequence, so the compressor is protected and it restarts after a power cut on its own logic.

So the plug relay does not switch the AC load any more, it only switches that low voltage contact. Your ESP8266 firmware and the plug.set command stay exactly as they are; I would rename it to something like ac.enable so nobody thinks it cuts power.

On the Hermetry side what we did:

• the command is Basic tier in the profile’s Commands tab, otherwise the board will not send it
• Switch card, Current state bound to a key the unit really reports (we read the indoor unit’s run status contact, ac.running), not to the command
• a rule: room.temp rises above 27 and stays so for 5 minutes, then Send a command ac.enable with on true, then Tell people

The control card page explains the tier and current value parts: Control cards. Reading back the real state is the part that saved us, a switch that shows “on” because you clicked it is worse than no switch.