A technician is checking the output of a 5 kW supplemental heater used with a heat pump. What stage of the thermostat controls the heating element?

The correct answer is auxiliary heating. In a heat pump system, supplemental electric heaters, such as a 5 kW heater, are typically controlled by the auxiliary heating stage of the thermostat. This stage is activated when the heat pump alone cannot meet the heating demand of the space, often during extremely low outdoor temperatures or when the system is recovering from a defrost cycle.

Auxiliary heating is used to provide additional heat and help maintain comfortable indoor temperatures, complementing the heat pump. Since the supplemental heater is designed to support the primary heating system, it fits correctly within the auxiliary heating designation.

The other options refer to specific stages in a heat pump's operation. First stage heating typically corresponds to the main heat pump operation, while second stage heating is usually associated with a backup heat source coming on when additional heating is required, but typically from a different source or a higher output level than the first stage. Emergency heating is often a manual override that activates the heating elements directly, indicating a failure in the heat pump system, rather than operating in conjunction with it as auxiliary heating does.