Why would a light fixture be incompatible with dimmer switches?

Question

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?

Answer 1

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.

Answer 2

My guess is that it's a boilerplate CYA statement due to the variety of bulb options listed. A user could inadvertently attempt to dim a CFL or other non-dimmable bulb and exceed the heat specs of the unit.

Assuming that the light circuitry is completely isolated from the fan, etc., and you use dimmable bulbs, I'd probably go ahead with your plan.

Answer 3

Product testing, listing, and instructions

The first six paragraphs of NEC include 110.2: Equipment must be approved, and 110.3 it must be installed according to its labeling and instructions. What's up with that?

The approver is generally Underwriters Laboratories or other nationally recognized testing lab. The manufacturer submits the product, labeling and instructions. The lab tests it according to the labeling and instructions.

If you go, as we graciously say, "off-label" -- you are in the wild west. Nobody knows what will happen if you use the device that way. That means

• It may not work.
• The company won't be responsible for that usage, i.e. void the warranty.
• If trouble follows and a safety inspector connects it with the misapplication, homeowner/fire insurance may refuse coverage and leave you to finance the consequences yourself.

In this case, I could not find any prohibition on dimmers in the item's marketing and instructions, which seem to describe a fixture made for incandescents, and taking an Edison socket LED only as an afterthought. Note that the fixture I describe cannot comply with modern building codes, unless it is used with a motion sensor. So I suspect you are dealing with a mod of that fixture which is either socketed for LED only, or has a built-in hardwired LED. In those cases, the prohibition on dimmers is reasonable.

Dimmers are not magic. They are not resistor or variac based. They do fairly bizarre wave-shaping to make the dimmer as cheap as possible to build with 1970s silicon tech, exploiting a unique behavior of incandescents. Now they are building CFL and LED bulbs which have a microcontroller which analyzes that 70‘s waveform and decodes it into a dimming command for the CFL or LED. So now we are stuck with that dimming method for good. If only we had found another system, like the 0-10V signal used by commercial dimming.