Tom's Life Bits

Red to the thermostat RH or thermostat RC terminal with a jumper wire between thermostat RH and thermostat RC. Or Red to the thermostat R terminal which is shared with both the heating and cooling. It has an internal jumper built in to the sub-base. The red wire is the source hot wire from the transformer. All other wires, except the common wire, controls a specific relay or contactor that energizes the fan, heating, or cooling depending on the selection. The following is the common wiring colors but your system may not be common and different colors could have been used.

Green to the thermostat G terminal. This is the color that controls the fan or the relay that control of the blower.

Yellow to the thermostat Y terminal. This is for control of the air conditioning.

White to the thermostat W terminal. This is for control of the heating. (this is actually blue on Downing Place downstairs)

The Common wire must also be present to use the Honeywell WiFi thermostats. Basically this means the thermostat gets power from the C or common 24 VAC and either of Rh or RC which are the L1 side of the 24VAC.

Sensing whether the thermostat calls for AC is accomplished between the Y terminal and the C or common. The relay or voltage sensor must hook to these two leads.