What device do low voltage thermostats directly control?

Low voltage thermostats are designed to control the operation of heating and cooling systems indirectly through various devices. The correct answer highlights that these thermostats directly interact with solenoids, relays, and contactors, which serve as intermediaries.

When a low voltage thermostat detects a temperature change that deviates from the set point, it sends a low voltage signal to the relay or contactor. This device then activates higher voltage systems such as the compressor or the heating element in a furnace. Given that relays and contactors are essential for handling the voltage and current required to operate other components, their role is crucial in the HVAC system's operation. This direct control of lower voltage devices allows the thermostat to manage larger appliances safely and efficiently.

The other options represent different types of components found within HVAC systems. Fans are typically controlled through the relays or contactors that the thermostat activates. Compressors can be part of the system but are not directly controlled by the thermostat itself; instead, they rely on the relays or contactors as intermediaries. Heat exchangers are components that transfer heat but are not controlled directly by thermostats in the same way that solenoids, relays, and contactors are.

Understanding this hierarchy and control mechanism is