The Aprilaire 8022 3-wire to 4-wire Adapter Thermostat Accessory allows you to combine any two thermostat equipment terminals with the diode pair (not 24V hot or common). They are then wired to the Aprilaire 8022 3-wire to 4-wire Adapter Thermostat Accessory through a single wire avoiding the need to run a new thermostat cable.