Wall Switch Dimmers Generating too Much Heat?


Wall Switch Dimmers Generating too Much Heat?

Over the past month, I have installed many wall dimmer switches. My concern is that they are running warm to hot. They seem to be fine when one to two lamps are running off one switch, but once I get closer to the maximum load rating of the switch, they seem to be very hot to the touch.

We often hear from customers who may leave a light on for 3 or 4 hours and when they go to turn off the switch they are startled by the heat felt from beneath the trim plate. Should you be concerned? Let's talk about this common issue.

Is it normal for wall dimmer module to get warm to the touch?

All the dimmers we sell use an electrical component called a triac. A triac is a semiconductor, like a transistor, which can vary the amount of electricity it can pass. As electricity flows through this component, heat is built up and must be dissipated somewhere, most commonly it is dumped into the metal mounting bracket. This is the metal plate on the front of the switch. This heat in turn is transferred to the switch cover plate.

In most cases, it is not noticeable when the dimmer switch is driving a single bulb or a load of 100 watts or so. The heat does become more noticeable when the wattage of the load gets above 300. All single gang dimming wall switches must be able to dissipate one watt of internal power (heat) for every 100 watts of controlled load. At full load, this pushes the internal and external temperatures to the safe limits imposed by UL or ETL. This isn't to imply that higher wattage ratings are dangerous. As long as the total wattage is below the rating printed on the dimmer, you can rest assured that it is safe to use in your home.

All the wall switches we sell are approved by a nationally recognized testing service, typically Underwriter's Labs (UL) or ETL. During the development of our SwitchLinc dimmer for example, we stress tested it by connecting it to a 1500-watt halogen floodlight for 24-hours. This switch was rated for only 600-watts, but we wanted to make sure that it could handle much more safely. It was left on for 24 hours and was still driving the floodlight at the end of the test. The heat radiated by the switch was so intense that all the plastic had melted and deformed around the switch. While it could no longer be used as a regular switch because of the plastic deformation, the switch never smoked or caught on fire.

Using multiple dimmers in the same junction box

If you are using 2 or more dimmers within the same junction box, you must derate the wattage capacity of each switch. What the derating will be depends on the switch. Our most popular dimmer (SwitchLinc) is 600 watts but when next to another dimmer it must be derated to 400 watts. If it is in between two dimmers, the derating is doubled so the total wattage capacity is 200 watts while the dimmers on the outside are 400 watts each.

Minimizing the heat build up

The most common path for heat to escape the switch is by the metal bracket. That metal mounting bracket is in direct contact with the trim plate, hence the trim plate becomes hot as the heat tries to escape. Replacing a regular size plastic trim plate with a slightly larger metal trim plate will help the heat dissipate from the dimmer switch.

Most electrical boxes today are plastic and they do not conduct heat nearly as well as the older metal boxes. If the current box is plastic, replace it with metal, thereby improving the heat sink. Take care that the front of the metal box is flush with the finished wall, giving the dimmer a broader surface to make contact with to improve heat conduction. If you stick to using plastic wall boxes, we recommend getting the largest ones you can find. Besides making it easier to install the dimmer in a larger box, the larger air gaps inside the box will help spread out the heat.

Another important mounting consideration is the area behind your dimmer switch. If the dimmer switch is mounted on an exterior wall, remove as much insulation as possible around the junction box. Often times, removing the trim plate and pushing back the insulation with a putty knife will help the heat build-up. There is usually a small gap between the electrical junction box and the wallboard to get the knife between for working the insulation back.

Do you need a dimmer switch?

You may want to consider changing the switch if you don't need dimming. If you are using wall dimmer switches to control floodlights, task lighting, or lighting that normally runs at full brightness, dimming may not be needed and changing to a non-dimming switch (often referred to as a relay switch) can fix this problem instantly.

Here is a simple screw-in solution to help cut-down on the heat generated by a wall dimmer. Replace the existing bulbs with lower wattage models. For example, replacing six 75-watt bulbs that is controlled by one dimmer with 40-watt equivalents will bring down the total controlled load from 450 watts to 240. The old bulbs can always be used for other purposes around the house. Check the instructions included with your switch, some models need to work with bulbs that have a minimum rating of 60-watts.

Looking for a dimmer? Smarthome specializes in remote controllable "smart" dimmers that replace existing switches in your home but can be controlled from a computer, handheld remote and even an iPhone or iPod touch.

Paddle Style Dimmers
Toggle Style Dimmers

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