What instrument would a technician use to calculate return airflow to a furnace?

A technician would use an anemometer to calculate return airflow to a furnace because it is specifically designed to measure air velocity. This instrument provides readings in feet per minute (FPM) or meters per second (M/S), enabling the technician to assess how much air is returning to the furnace. Understanding the return airflow is crucial for ensuring the system operates efficiently, as sufficient airflow directly impacts the performance of the heating system.

Other instruments mentioned, such as a velometer, which also measures air velocity, may not be as commonly used or as versatile in practical HVAC applications. The draft gauge is primarily used for measuring pressure differences, such as the draft in a flue, rather than airflow rates. A megger, on the other hand, is used for testing insulation resistance in electrical systems and has no relevance to measuring airflow. Thus, in the context of calculating return airflow, the anemometer is the most suitable choice.