How Electric Fog-Free Mirrors Work

Just like that dripping wet glass of ice-cold lemonade on a one-hundred degree, ninety-percent humidity day a bathroom mirror is the same. To reverse that process, or rather, keep it from happening in the first place you need to make sure that a surface is the same (or higher) temperature as the dew point. Dew point as defined by Wikipedia:


'The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. The measurement of the dew point is related to humidity. A higher dew point means there will be more moisture in the air.'*


By definition, a mirror would have to be warmed to a temperature warmer than the dew point to promote evaporation vs. condensation (also Wikipedia):


'At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, forming more liquid water. The condensed water is called dew when it forms on a solid surface, or frost if it freezes. The condensed water is called either fog or a cloud, depending on its altitude, when it forms in the air. If the temperature is below the dew point, the vapor is called supersaturated. This can happen if there are not enough particles in the air to act as condensation nuclei.'*


Along comes a mirror heater, also commonly known as mirror defoggers. There are many brands to choose from and they are manufactured throughout the world, but they all operate and are installed in a similar practice. A mirror defogger will normally be very thin, less than 1/8" and be made by either wires or a printed circuit laminated between two non-conductive pieces of material like silicon rubber or polymer. Out of the end will be two leads, these leads are neither neutral nor positive (it doesn't matter which side of the power supply is connected to either lead). 


On one side of the mirror defogger is an adhesive coating like a PSA, covered with a contact sheet. You peel the sheet and press it firmly and evenly onto the back of the mirror. You then connect the leads to a new junction box (hidden behind the mirror) and push the mirror back into place. The mirror defogger is turned on and off with the bathroom light or the bathroom fan.


When turned on the mirror heater, gently warms the mirror (usually to 100 degrees +/-). Relative humidity maximizes at 100%, and if the air temperature (not water temperature) is anything less than 100 degrees (temperature of the mirror) the RH will max out at that temperature. In other words, the air temperature will need to exceed the 100+ degrees of the mirror for the fog to form. Highly unlikely in a typical shower, no matter how many shower heads, or how hot the water is.


A mirror defogger is usually 'Tested By' or sometimes 'To' Underwriters Laboratories (UL) standards. This can be a big difference for your safety and overall peace of mind. Sometimes they are tested 'To' or 'By' other safety testing agencies both in the United States and overseas.  As long as they are Tested By Certified By, or Listed By you can have confidence that this electrical device being installed in your shower or over your vanity has met a rigorous safety testing plan as set by a certified testing engineer. All others, be highly suspect before you click 'Buy Now'!


A mirror defogger can be round, square, rectangle, big or small. It can be low-voltage (12 or 24 volts), or line-voltage (120 or 220 volts). The number of watts will vary based upon the safety rating of the maximum 'watt density' as permitted by certified safety testing, the country, and the operational need of the individual heater and its application.


Low-voltage versions, typically for a recreational vehicle or in wet environments like showers and spas should come with a Class-2 transformer sized for the correct volts and watts. Again for safety, the shower versions and the Class-2 transformer should be Listed or Certified (not just 'Tested To') together, along with the installation procedure.


We hope this helps your understanding of how a mirror defogger, i.e. mirror heater works. We've been at this since 1998 and have confidence in calling our selves experts but please reach out to any of the top American Made manufacturers for additional information. They are all very smart, have great products and have only your comfort and safety in mind. 


- The ClearMirror Guys, March 7, 2018