Lighting – Why would a light fixture be incompatible with dimmer switches

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I'm considering buying the Aero Pure AP80-RVL, which is a recessed bathroom light/exhaust fan combo. It's important to me to be able to use a dimmer switch for the light, but the installation instructions for this unit specifically say not to use one.

Use an on/off switch to operate this fan. See “Connect Wiring” for details.
The humidity control and fan can be operated separately. Can be used with a dual “stacked” switch for fan / light control. Do not use
a dimmer switch to operate the humidity control or light.

I checked the "Connect Wiring" section for further detail, as mentioned in the quote above, but there is no further information about dimmer switches. Doing a text search on the rest of the installation manual yields no further references to dimmer switches either.

I understand why you can't install a dimmer switch for the fan/humidity control, but for light fixtures I always assumed it was just the bulb and whether or not the dimmer switch was compatible with the bulb (in the case of an LED bulb) that determined if you could install a dimmer switch. This leads me to the following two questions:

  1. Why would a light fixture itself not be compatible with a dimmer switch or why would a manufacturer say not to use a dimmer?
  2. Would I run any risks such as damaging the unit, causing a fire, etc. if I went against the installation instructions and put a dimmer on the light anyway?

Best Answer

The best explanation I can think of is that many compact fluorescent(CFL) and LED bulbs are not compatible with dimmers. The reason why is

non-dimmable CFLs cannot and should not ever be used with dimmer switches.

and

To put it in layman's terms, most modern dimmer switches essentially function the same way as if you were to turn a light on and off really, really fast (about 100 times per second). (snip)

basically the problem is this: CFLs have less resistance than incandescent bulbs, which means that the floodgates are opened when they are exposed to the electrical fluctuations that a dimmer sends, and they actually can consume up to 5x the current as when they're not connected to a dimmer (and that's when the dimmer is set to max). This overheats the bulb and can actually cause it to light on fire. Bad news.

The fixture has a GU24 base (non-standard) which is going to further limit your selection of bulbs (as in your local store may only carry non-dimmable bulbs). So rather than accept liability for a potential fire hazard, they're just going to tell you not to use a dimmer at all for the light.

In theory you should be able to hook a CFL/LED friendly dimmer to the light only and install a dimmable GU24 bulb, but beware that any liability for doing so lies solely with you.

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