In various representative aspects, a security alarm system for protecting an HVAC unit from theft. The system includes an alarm; a control box; a switch connector comprising two terminals, as well as an open state and a closed state; and a fastener that holds the switch connector in the close position while connected to an HVAC unit. The conductors connect the switch connector to the control box, forming a circuit. The control box can apply an energy potential across the circuit, detect an energy flow across the circuit, and trigger the alarm if the energy flow stops. Normally, the terminals do not touch each other, and the switch connector is in its open state. When they touch each other, the switch connector goes to its closed state. If the fastener is removed from the HVAC unit, the switch connector returns to the open state and triggers the alarm.