Use a flow hood to measure volumetric airflow at a supply or return outlet, then compare the reading with the system’s design and balancing data. A hood must seal or cover the outlet appropriately without distorting flow excessively. Swirl diffusers, small outlets and backpressure-sensitive grilles may need special correction or an independent measurement.
Check the hood seal before trusting the displayed flow
A gap between the hood skirt and the grille bypasses the measuring section; an oversized hood may change the terminal pressure balance. Repeat a reading after reseating the hood and compare with terminal type and airflow pattern. A hood measurement describes the terminal where it was taken, not the outdoor-air fraction of a central ventilation system. For another outlet-flow method, see vane anemometer measurement guidance.
Before you place the hood
Identify the terminal and its design airflow, the operating fan mode, filter condition and damper setting. Inspect the hood fabric, frame, straightener and sensor for damage. Choose a hood size suitable for the outlet; unsupported adapters or gaps can introduce errors. In cleanrooms or controlled-pressure spaces, follow site protocols before changing terminal conditions. For more on interpreting filter condition through pressure drop, see filter differential pressure guidance.
Measurement sequence
- Stabilize the HVAC system at the specified airflow setting.
- Seat the hood evenly over the whole terminal without blocking adjacent openings.
- Read volume flow in m³/h or CFM, allowing for the instrument’s averaging method.
- Repeat with the hood seated consistently. Note any large sensitivity to position or orientation.
- Compare against the design schedule, and document terminal ID and measurement method.
- If readings remain inconsistent, verify using an approved alternative such as a duct traverse or calibrated terminal method.
Worked example: deviation from design
Hypothetical: design flow is 300 m³/h, and the hood reads 270 m³/h. The deviation is (270 − 300) ÷ 300 × 100 = −10%. This only quantifies the difference; pass/fail depends on the project’s balancing tolerance and the uncertainty of the method. At 270 m³/h, equivalent flow is approximately 159 CFM.
What can make a flow hood read incorrectly?
| Issue | Effect | Check |
|---|---|---|
| Hood not seated | Air escapes or bypasses sensor | Reseat and repeat |
| Swirling outlet jet | Nonuniform flow at sensor grid | Use appropriate flow straightener or correction |
| Hood adds restriction | Actual terminal flow changes | Verify with balancing procedure |
| Supply versus return mode wrong | Sign or calculation may be incorrect | Check orientation and instrument settings |
Testo equipment suited to this job
The testo 420 is an airflow capture hood with an integrated flow straightener designed to reduce measurement issues at swirl outlets. The hood supports volume-flow measurement and reporting; whether a correction factor is necessary is determined by the outlet, measurement method and balancing requirements. ASHRAE guidance discusses hood limitations and comparison with Pitot tube traverses.
Do not confuse hood flow with outdoor-air ventilation
Measuring supply airflow at an outlet does not directly establish outdoor-air fraction, room air-change effectiveness or cleanroom containment. These questions may also require ventilation balance, pressure relationships and additional measurement methods. For a room-level ventilation check, see meeting-room CO2 ventilation checks.