Two-Transformer HVAC systems (RC & RH wiring)Updated a year ago

When an HVAC system has separate 24V transformers (one for heating, one for cooling) this will be indicated when there are two independent R-wires and not using a RH/RC jumper. The thermostat is powered from the heating transformer on RH and C. The cooling transformer R-wire gets connected to RC. 

To ensure that the two 24V sources are separated requires removing Link 2 from the back of the thermostat. See Single or Two Transformer Link1/Link2 configurations article for more information.

Special consideration is needed when a heat pump is being used (instead of an AC system). W1-O/B will require the use of an isolation relay as O/B output is supplied with 24V from the heating transformer.