Every inline duct fan gets a score from 0 to 100% built from six factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Fans are grouped by duct size (4 in, 6 in, and 8 in and up), and value and airflow are scored against reference points for each duct size, since a bigger fan moves more air and costs more.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per 100 CFM of rated airflow | 25% |
| Efficiency | rated airflow per watt at full speed | 25% |
| Airflow | rated airflow against others of the same duct size | 15% |
| Noise | rated noise at full speed | 15% |
| Controls | included speed controller, automatic control, app and timer | 10% |
| Warranty | warranty length | 10% |
Where the numbers come from
Airflow, power draw, noise, motor type, controls and warranty come from the maker’s spec page, manual or product copy, including the maker’s own copy on a retail listing. Where the maker’s own figures conflict, the maker’s spec page or manual is used and the difference is noted on the product page. Airflow is the maker’s free-air CFM rating, with no duct or filter attached. Prices are checked at Amazon and the maker’s own store, and the lowest price at a store the page links to is used.
The factors
Value is the current price divided by the rated airflow, in dollars per 100 CFM, on a log scale, so halving the price always counts the same. For 4 in (and 5 in) fans, $60 per 100 CFM scores zero and $25 full marks. For 6 in fans, $40 scores zero and $15 full marks. For 8, 10 and 12 in fans, $30 scores zero and $10 full marks.
Efficiency is the rated airflow divided by the power draw at full speed, in CFM per watt: 3 CFM/W scores zero and 12 CFM/W full marks. Most makers give the maximum draw. Some give a figure labeled average; that figure is used, but the factor is marked estimated, since an average draw flatters the result.
Airflow is the rated free-air CFM, scored against the duct size: 120 to 240 CFM for 4 in, 150 to 300 CFM for 5 in, 250 to 450 CFM for 6 in, 500 to 900 CFM for 8 in, 800 to 1,300 CFM for 10 in, and 1,100 to 1,800 CFM for 12 in, from zero to full marks.
Noise is the maker’s noise rating at full speed: 60 dBA scores zero and 25 dBA full marks. Makers rarely state the measuring distance, so small differences mean little.
Controls is 40% for an included speed controller, 30% for temperature or humidity control that adjusts the fan on its own, 15% for a phone app and 15% for a timer or cycle modes. A fan sold without any controller scores zero here.
Warranty is the maker’s warranty: 1 year scores zero and 5 years full marks.
What isn’t scored
Static pressure is listed where the maker publishes it but isn’t scored, since only a few makers publish it and they measure it in different ways. A power figure given at a lower speed, RPM, housing material, IP rating, voltage and release date are listed but not scored.
Missing data
If the full-speed noise rating isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated. Power draw and warranty are treated differently, because leaving them out would favour the makers that publish least: a fan whose maker doesn’t publish its power draw scores zero for efficiency, and a fan whose maker doesn’t publish a warranty scores zero for warranty, the same as a one-year warranty. Both are marked estimated on the product page. A controller, temperature control, app or timer the maker doesn’t mention counts as absent.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.