Any of the LEGS Below when connected from RH/RC to the Units Below will activate the service.
Below is a Wire Diagram that shows the path of the HVAC system in a standard setup. It shows what happens inside the thermostat.
Example of this is RH/RC to G will turn on the fan.
Typically bonded together on most systems. If its not INVESTIGATE the path as in some systems such as radiant systems the circuits are not bonded powered on their own bonding them back without research can cause device failure.
RH- 24VAC LEG1 Voltage for the HEAT relay.
RC - 24VAC LEG2 Voltage for the Cool Relay.
W1 - Stage 1 Heating.
W2 - Stage 2 Heating. (Runs on top of W1, To activate secondary efforts.)
Y1 - Stage 1 Cooling.
Y2 - Stage 2 Cooling (Runs on top of Y1, To activate secondary efforts.)
The Condenser that runs outside. Needs to be connected between Y and C to Activate Seen in this diagram below.
G - Blower Fan.
E - Emergency Heat.
C - Alternate pole of the 24V System.
RH/RC is the theoretical Positive terminal of the AC Circuit. As the C is the Negative. But since we are dealing with AC Systems, Polarity is hard to quantify as fact in the industry.
We get into the murky waters with humidity. as not all thermostats act the same way. But Typically its a contact of its own or its a thermostat of its own. Here is a generic logic of the wiring system.
If you are using a drip membrane system. it is wise to use a 2nd transformer for the coil. They typically look like this in your system and is most common.