What should a technician do first when troubleshooting a typical residential HVAC system?

When troubleshooting a typical residential HVAC system, starting by turning the thermostat fan switch to the on position helps to ensure that the system's blower is running. This action allows the technician to determine whether air is flowing through the ducts, which is essential for diagnosing issues related to airflow, airflow restrictions, or other operational problems.

By manually activating the fan, the technician can observe if air is circulating throughout the system, which is crucial before delving deeper into specific component diagnostics. This step also assists in assessing any noise or issues related to the blower motor itself, providing insight into whether the HVAC system is operational at a basic level.

Other actions, such as turning the unit disconnect off or toggling the thermostat to the heat position, could potentially prevent air circulation or escalate a problem if the unit is experiencing a significant fault. Activating the fan switch allows for an immediate assessment of one of the most critical aspects of HVAC operation—air movement.