Is it Safe?

Yes, when using up-to-date hardware all components of the kits have been shown to be extremely safe. Just because you can, doesn’t mean you should though. Leave that 20-year old box fan in the attic and get a new one to make sure everything is certified and safe. Below we discuss some of the safety concerns related to installation of DIY air purifiers.

Fan overheating & Risk of Fire

One valid concern about modifying a box fan by adding one or more filters to the back is whether this can strain the motor and cause it to overheat.  The way three-speed fans are wired, the fan blades spin at a constant speed, and adding a filter just reduces air flow without causing the fan to work harder or draw more power (lots of experts have demonstrated this).  However, airflow through the fan also helps cool the motor, so one would expect the fan to run hotter with a filter attached.  How much hotter?

To quantify this effect, researchers at Chemical Insights, an Institute of Underwriters Laboratory, Inc. (the organization that provides UL certifications of electrical and fire safety of consumer products), measured the temperatures of all major components of many box fans when air filters are added to the back compared with a bare fan.  In their report, they note:

“Across all fan/filter test scenarios, the most notable temperature increases were observed at the motor and motor windings. However, all measured temperatures fell below the maximum acceptable thresholds defined by the market safety standard for electric fans (UL 507). Exterior surfaces that can come into direct contact with people remained below 36 °C, which is below the first-degree burn potential of 47.5 °C. Further, an extreme scenario where both the front and back of the fan were blocked for an extended period of 7 hours did not pose any observable fire hazards. And after an initial rise, temperature remained steady throughout the test.”

Read the full report.

More safety recommendations are provided by the Oregon Health Authority, which can be accessed on this site.