The difference between a thermostat and a humidistat is that the former regulates the temperature of an indoor space, while the latter measures and maintains relative humidity levels. While thermostats are attached to heating, ventilation and cooling systems, humidistats are connected to humidifiers or air cleaner units.
Humidistats are commonly also referred to as hygrostats and humidity control settings. They are valuable in maintaining the humidity in arid climates. The advantage of a humidistat is that it continuously monitors humidity levels, reducing the risk of over-humidifying, which can cause excessive moisture problems, such as mold or mildew. The counterpart of a humidistat is a dehumidistat, which similarly monitors humidity levels but activates a dehumidifier to remove moisture from the air if it exceeds an already set level.
A thermostat is considered a temperature sensitive switch. It monitors the indoor climate and activates once the interior exceeds the threshold set by a user. These units are generally divided into two categories: low voltage and line voltage thermostats. The former transforms incoming household voltage from 120 volts to between 12 and 24 volts.
There are four main types of low voltage thermostats: mercury contact, mechanical contact, digital and electronic models. The first two types are older styles, relying on contacting metallic components to note temperature and activate the furnace. They are not as efficient as contemporary digital and electronic units.