An anemometer is a precision instrument used by HVAC professionals to measure the velocity or speed of airflow at a register or inside a duct. By knowing the air speed and the square footage of the duct, a technician can calculate the exact CFM (Cubic Feet per Minute) entering a room. This tool is indispensable for "balancing" a system to ensure that every room in a house gets the correct amount of conditioned air. Digital anemometers can often also measure temperature and humidity, providing a complete picture of a vent's performance. It is a key tool for diagnosing why a specific room in a house is "always too warm."