What instrument would a technician use to measure the return airflow to a furnace?

The anemometer is the appropriate instrument for measuring return airflow to a furnace because it directly measures the velocity of air moving through a duct or vent. By determining airflow velocity, a technician can calculate the volume of air being returned to the furnace, which is crucial for assessing the system's efficiency and performance.

Understanding airflow is essential in HVAC systems, as it impacts heating efficiency, indoor air quality, and overall system functionality. Anemometers typically display velocity in either feet per minute or meters per second, allowing technicians to gauge whether the airflow is within the recommended parameters for optimal furnace operation. This measurement helps in diagnosing potential issues such as restrictions or leaks in the airflow system.

In contrast, other instruments mentioned do not serve this specific purpose; for example, a manometer measures gas pressure, a thermometer measures temperature, and a volt meter measures electrical voltage, none of which provide a direct measurement of air movement or airflow in ducts.