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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.

References:

  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