When the thermostat is set to cool the system energizes the circuits that start the compressor the outdoor reversing valve and the indoor fan.

The thermostat sends power from the red terminal to activate the system and then closes the circuit between the red and yellow terminals to start the compressor which circulates refrigerant and removes heat from the home. At the same time the red to green circuit is closed to run the indoor blower fan so conditioned air moves through the ductwork and reaches living spaces efficiently. For heat pump systems the red to orange circuit is also driven to switch the reversing valve into cooling orientation so the outdoor unit rejects heat rather than absorbing it. Proper wiring requires secure connections and the correct gauge wire to prevent voltage drop so use consistent color coded wire and screw down terminals firmly without over tightening. Always allow a short delay of thirty to sixty seconds before restarting the compressor after a shutdown to protect the compressor from short cycling and follow manufacturer specified time delay settings when available. If you are adjusting settings or replacing a thermostat record existing wiring connections and photograph the terminal block so you can restore original connections accurately.

Common mistakes include miswiring the reversing valve so the system runs in the wrong mode which can be fixed by swapping the orange and white connections as appropriate to match heat pump wiring diagrams. Another frequent issue is neglecting the compressor short cycle protection which can lead to premature compressor failure so always respect the minimum off time and use thermostats with built in delay features or add an external time delay relay. A third concern is forgetting to run the fan independently which reduces airflow and can cause coil icing; set the fan to automatic or run it on manual to verify proper airflow and inspect filters and evaporator coil for restriction or frost.

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