Which control operates the first stage of secondary heat in a heat pump?

The first stage of secondary heat in a heat pump is typically operated by the indoor thermostat's second heating stage. This control is designed to activate supplemental or secondary heating elements when the heat pump alone cannot meet the heating demand.

When the primary heating source (the heat pump) is not sufficient to maintain the desired indoor temperature, the thermostat senses this need and engages the secondary heating system to provide extra warmth. This secondary heat can come from various sources, such as electric resistance heaters or other auxiliary heating systems specifically linked to the thermostat's controls.

In contrast, the primary compressor control manages the operation of the heat pump itself, while the defrost control timer serves a different function related to the defrosting of outdoor coils in colder weather. The outdoor thermostat control, if present, would manage operating conditions based on the outdoor temperature but does not directly control the first stage of secondary heat.