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I bought a small HEPA air filter fan that was advertised as being a quiet model. But it's not quiet enough for me. I bought another "quiet" fan, but the new one is also too loud for my liking.

Could just I buy one of those extension cords with a dimmer switch (designed for use with room lamps), plug in my air filter, and safely use the dimmer to control the fan speed (and therefore volume) of the air filter?

Safety is my priority. I don't want to start an electrical fire. If it's not safe, I'll buy a different fan.

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Can't answer on safety but at lower speeds it might not change the air in the room often enough to do you any good. Can't remember the brand but there are a couple of models that are quiet and handle a lot of air. One even had this sensor to change the speed based on how dirty the air was. Pricey but had good reviews. In the mean time I'd consider maybe just using a timer or manually turning it on/off, high/low depending if you're in the room or not. –  OrganicLawnDIY Feb 28 at 20:03

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Light dimmers are designed to drive loads that are largely resistive in nature like light bulbs. They are generally not compatible with loads that are inductive. Most, if not all, AC motors are inductive type loads.

That said, whether a dimmer switch will work safely with your fan or not depends entirely upon the type of motor on the fan.

There are some types of AC motors that can work to gain speed control via a dimmer switch type of circuit. These motors are often found in AC corded power tools such as electric drills and jig saws. Just as often these tools come with a variable speed capability via a proportional squeeze trigger. These tool speed controls work just like a common lamp dimmer. The motor type involved is generally the type with brushes.

Common low cost fans do not use brushed type motors. The usage life time would be just too short and non-brushed low power motors are often way cheaper to manufacture.

With this information in hand you may be able to evaluate your fan and decide if it is suitable for use with a dimmer. I suspect that it is likely not compatible.

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Despite it being highly inefficient for inductive loads, the shaded pole motor fans in my wood heating stove extraction fan system uses a triac type dimmer switch for speed control. This is a factory unit. –  Fiasco Labs Feb 28 at 5:10
    
@FiascoLabs - Do you experience that triac control of the shaded pole motor only gives a limited range of speed control? That has been my experience with trying it with that type of motor. I guess if the motor burns up in your application you can just throw it in the fire! –  Michael Karas Feb 28 at 6:18
    
Yep, it wasn't the best solution. - The bearings have always given out before the windings do so I end up throwing them in the fire every 10 years. Needed, ball bearing fan motors. The heat extraction efficiency pays off in the long run though. –  Fiasco Labs Mar 1 at 7:34

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