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Exhaust and Ventilation Fans

Exhaust and ventilation fans move air out of or into a space. Airflow and sound are rated to standard test methods, and the fan must be chosen at the static pressure it will actually see in the duct system.

Ref HV-0026AMCA 210HVI 916

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A fan moves air against resistance. The two main families are:

Centrifugal fans: air enters the center of a wheel and leaves at right angles. Forward-curved wheels are compact and common in small blowers. Backward-inclined wheels handle higher pressure efficiently.
Axial fans: air moves parallel to the shaft. Propeller fans move lots of air at low pressure. Tubeaxial and vaneaxial fans are mounted in a tube and handle more pressure.

Common products include bath and kitchen exhaust fans, inline duct fans, roof exhausters, wall propeller fans, and make-up air units.

How to identify

Look at the wheel. A drum of many small blades is a forward-curved centrifugal wheel. Fewer, larger blades that curve away from rotation are backward-inclined. A propeller is axial. The nameplate shows model, voltage, motor and speed. Rating labels may show a certified ratings seal.

Fan types compared

Type	Pressure capability	Typical use	Notes
Propeller (axial)	Low	Wall exhaust, warehouse ventilation	Large airflow, little duct
Tubeaxial or vaneaxial	Low to medium	Inline industrial exhaust	Directional flow in a tube
Forward-curved centrifugal	Low to medium	Bath fans, small blowers	Compact, quiet at low speed
Backward-inclined centrifugal	Medium to high	Roof and utility exhaust, commercial	Efficient, non-overloading motor
Mixed flow inline	Medium	Inline duct exhaust	Between axial and centrifugal

Ratings and fan laws

Airflow ratings for commercial fans are tested to AMCA 210. Residential exhaust fans are tested to HVI 916 for airflow and are listed with an airflow at a stated static pressure and a sound rating in sones. Lower sones mean quieter.

For the same fan in the same system, the fan laws hold:

Airflow changes in direct proportion to speed.
Static pressure changes with the square of speed.
Power changes with the cube of speed.

Key points

Fan performance must be read from the fan curve at the system's static pressure. Free-air ratings overstate real airflow.
Ventilation airflow requirements come from the mechanical code and ASHRAE 62.1 or 62.2, not from the fan maker.
Exhaust needs make-up air. A tight building starves the fan.
Kitchen grease exhaust and hazardous exhaust have separate listing and code rules.

Common mistakes

Picking a bath fan from its zero-pressure rating.
Running long, small or crushed duct that cuts airflow.
Doubling speed and expecting the same motor to cope. Power rises eight times.
Venting exhaust into an attic instead of outdoors.

Governing standards: AMCA 210 (fan airflow testing); AMCA 211 and 311 (certified ratings program); HVI 916 (residential fan airflow); International Mechanical Code; ASHRAE 62.1 and 62.2.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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