Understand the principle before the equipment
Residential HVAC systems combine line-voltage power circuits with lower-voltage control circuits. Understanding the distinction helps explain how thermostats can command equipment without directly powering motors and compressors.
Voltage is electrical potential, current is the flow of charge, and resistance opposes current. Electrical power depends on voltage and current.
HVAC equipment also uses switches, relays, contactors, capacitors, transformers and protective devices to start and control components.