A little help diagnosing an air conditioning problem because if you see your thermostats blank then you know that you are either missing 24volt power or you are missing power from the air handler altogether hooking up common wire in digital hvac thermostat first with the power off i connect the optional common wire up to the thermostat. Additionally see thermostat wiring color code chart to help with how to wire a thermostat how to wire a thermostat thermostat wiring diagram this thermostat wiring diagram is a split system these split systems include an air conditioner with a gas furnace oil furnace or electric furnace the wires going to the condensing unit are shown . Single stage heat pumps are easy to connect to thermostats if you follow this simple guide 24 volt thermostat wiring for single stage heat pump systems by timothy thiele updated 09 17 18 pin this powers up the condenser and the blower fan cooling your home . Thermostat wiring colors code always follow the thermostat manufacturers instructions whenever changing the thermostat typically a thermostat wire pull is made to the air handler on split systems this wire is then spliced for the separate wire pull which is made to the condenser it depends on what you have for an hvac system but . Heat pump thermostat wiring a typical wire color and terminal diagram as shown in the diagram you will need to power up the thermostat and the 24v ac power is connected to the r and c terminals the color of wire r is usually red and c is black c is known as the common terminal these two connections will ensure that there is power to the thermostat that you are operating