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Flow Hood

A flow hood is a device that measures the air flow rate into or out of a register while the air handler is operating. The flow hood channels the air through a short fabric duct containing thermal anemometers to measure the flow of air entering or exiting the register. During normal air handler operation, the flow hood is held over each supply and return register and the flow rate for that register is recorded. These measurements are used to check the total air flow for the house and room-to-room air flow balance. Exhaust flow rates at bathroom and kitchen fans can also be measured using the flow hood to ensure they are working properly. The difference between the sum of the flow rates at each supply or return register and the total flow rate at the air handler can also provide estimates of the supply and return duct leakage.

Photo of a man using a a flow hood to measure flow from a ceiling in a home.

Flow hood in use at a field test site
Lieko Earle


  • Flow hood readings must be adjusted when taken at high altitudes.
  • Accuracy is not very good (5-10% in most cases).


  1. Hancock, E.; Norton, P. and Hendron, R. Building America System Performance Test Performance Test Practices: Part 2, Air Exchange Measurements. National Renewable Energy Laboratory, August 2002. NREL/TP-550-30270