What does a thermostat control in an HVAC heat pump system?

In an HVAC heat pump system, the thermostat primarily serves to control the temperature setting for both heating and cooling operations. This means that the thermostat monitors the indoor air temperature and compares it with the setpoint that the user has chosen. If the temperature deviates from this setpoint, the thermostat signals the heat pump to either activate or deactivate heating or cooling to maintain the desired indoor climate.

By adjusting the temperature setting, users can effectively manage their comfort levels throughout the year. During the heating season, the thermostat communicates to the heat pump to absorb heat from the outside air and distribute it indoors, while in cooling mode, it encourages the opposite process—removing heat from inside and releasing it outside.

The other choices represent functions that are either secondary or not the direct responsibility of the thermostat. The thermostat does not solely control electrical flow to the compressor, the speed of circulating fans is usually managed by different components or settings within the system based on the thermostat's signals, and it does not have any role in regulating refrigerant levels, which is determined by the system's design and maintenance. Thus, the overarching function of the thermostat in a heat pump system centers around setting and maintaining the desired temperature for optimal comfort.