I have a Goodman CTK03AA thermostat controlling an Amana AVPTC series Air Handler and ASZC18 series heat pump. The airhandler has an overflow catch tray under it with a NC water sensor switch.

The installer wired the sensor in series with the +24VAC powering the thermostat. I understand that if the tray fills, the power to the thermostat will be interrupted, the display will go dark and the system will shutdown. So in the world of "brute force" control, this wiring will be functional, but not very pretty.

Since there is no "Y" or "R" control available as in a legacy control system, wondering if interrupting the 24VAC to the thermostat was the best way to signal a pan full condition for this system? It would seem that with all the "bells & whistles" built into the electronics, there should be a connection on the control board that would stop the compressor and throw an alarm on the CTK03.