What It Is
A humidistat is a device used to measure and control relative humidity. It can be set for a desired humidity level, with the humidistat signaling to the humidifer to turn off the water supply once that level is attained. Humidistats, in the home, are frequently part of an air conditioning or central heating system and are usually found near the wall-mounted thermostat. While the main purpose of a humidistat-controlled environment is to achieve a level of comfort, they can also be effective in preventing indoor mold outbreaks during particularly hot, moist weather.
How It Works (General Terms)
When used in conjunction with an air-conditioning system, a humdistat will "cycle" the conditioner on and off in response to internal humidity level instead of responding to internal temperature levels. Progress-Energy.com provides an example of how a humidistat set at 70 percent relative humidity operates, if it's functioning properly. When the relative indoor humidity reaches 70 percent or above, the air conditioner will "cycle" on, even if the thermostat setting does not call for cooling.
How It Works (Technical Terms)
A typical household humidistat includes a sensing element, made of a material that is sensitive to air moisture, and a relay amplifier. Increases or decreases in indoor humidity strengthen or weaken the electrical resistance occurring between the metal conductors of the sensing element. These variations are in turn gauged by the relay amplifier.