The low-voltage output between the transformer secondary and thermostat side is:

• A. 6 V
• B. 12 V
• C. 24 V
• D. 48 V

Answer: (C)

In residential HVAC, the thermostat is powered by a low-voltage control circuit supplied by a step-down transformer. The secondary of that transformer delivers 24 volts AC to the thermostat, with one side typically called R (hot) and the other C (common). When the thermostat calls for heat or cooling, it closes switches that connect R to W, Y, G, etc., energizing the corresponding relays or valves in the furnace or air handler. That’s why the standard control voltage is 24 V AC—the voltage is high enough to reliably drive relays and control circuits, yet low enough to be safe and work with small thermostat wires. Voltages like 6 V, 12 V, or 48 V aren’t used for typical thermostat controls for these systems, so 24 V is the standard.