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Powered Flow Hood

A powered flow hood is a type of flow hood, primarily used for airflow measurement at a supply or return register. When properly used, it is able to provide a measure of airflow without altering the register pressure and flow as much as non-powered flow hoods can. Powered flow hoods are comprised of a standard flow hood plus a fan, pressure gauge, and controller. The fan speed is set to ensure the register pressure is the same as the ambient room pressure. In that way, there is no back-pressure from the flow measurement component (orifice plate or nozzle) on the duct system.

Only powered flow hoods should be used when a register is located near a duct branch or tee. The backpressure from a standard flow hood will reduce the measured flow on each side of that branch during the measurement, with the result being a strong underestimate of the volumetric flow.

Diagram shows differences in pressure taking flow measurements using a standard flow hood. On both sides, higher pressure results in lower flow.

Flow Measurement near a tee using a standard flow hood. Measuring on either side of a tee
results in underestimates of the typical flow through these registers.

Flow measurement near a tee using a powered flow hood. Powered flow hood creates ambient pressure to allow true flow and flow measurements.

Flow Measurement near a tee with a powered flow hood. Measuring on either side of a tee does not overly change the typical flow through these registers, resulting in more accurate measurements.

Some powered flow hoods have an automatic fan speed controller, others have a manual dial. Powered flow hoods have been found to be much more accurate than standard flow hoods [1], and are the preferred method.

Powered flow hoods can also be connected directly to an air handler, or to ductwork. Equipment user manuals, such as this one [2], provide full details.

A blower door is a type of powered flow hood. Because it has different considerations and is used to measure different phenomena, it is described in its own section.


  1. Wray, C. et al, (2002) Accuracy of Flow Hoods in Residential Applications, Lawrence Berkeley National Laboratory Report LBNL-49697.
  2. Minneapolis Duct Blaster Operation Manual