What Is a Humidistat and Do You Need One?

Digital display of electronic hygrometer sitting on a table.

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A humidistat is a device that reads the humidity in a space and automatically turns a humidifier or dehumidifier on or off based on whether the room is meeting the desired humidity setting.

A humidistat or hygrostat (used interchangeably) works like a home's thermostat, which is commonly connected to an air conditioning or heating system. Much like a thermostat monitors the room temperature and automatically kicks on the air conditioner or heater to meet your temperature preferences, you need a humidistat to do the same with moisture.

Humidistats are commonly installed in stand-alone humidifiers, dehumidifiers, or air conditioning systems. A whole-home humidifier is connected to your HVAC system, evenly distributing moisture throughout your home using the home's ducting. This option is the most expensive, requiring a separate cold water connection and space for the humidification unit, but it keeps humidity levels the most consistent.

How Does a Humidistat Work?

A humidistat is a humidity switch. It has a sensing element made of alternate metal conductors on a flat plate. A humidistat senses the electrical resistance that varies with changing humidity levels and relays the signal to turn a device on or off when needed. Humidity sensors have an average lifespan of around 10 years.

Humidistat vs. Hygrometer

There's a critical difference between a humidistat and a hygrometer. A hygrometer calculates the humidity or water vapor level in a space. A humidistat is like a hygrometer-plus. It can read or calculate the moisture level in the air and can work interchangeably with another device. The humidistat signals a humidifier, dehumidifier, or air conditioner to turn on or off once a preset humidity level is attained, keeping it at your desired level.

A dehumidifier removes excess humidity. A humidifier adds moisture to a room. A humidistat does not remove or add humidity; it only senses and signals other devices to turn on and off. Factors that influence the need for these devices are your local climate, weather conditions, temperature, heating or cooling effects on air comfort, and other conditions specific to the home, such as excessively wet or damp rooms or environments.

Humidistat Benefits

It's helpful to have a humidistat for several reasons—comfort, mildew control, maximizing your room's HVAC efficiency, and air quality. According to the EPA, 30 to 50% indoor humidity is ideal.

  • Comfort level: Rooms that are too humid are uncomfortable and often feel sticky and hotter. Humidity holds in more heat in the air. You also feel hotter because more moisture in the air stops you from releasing your moisture (in the way of sweat), making you feel muggy.
  • Mildew and mold control: Humidistats help prevent mildew from forming. Certain rooms in a home are more prone to developing mildew, such as a bathroom. In a bathroom, a humidistat can be connected to the ventilation fan in the bathroom, automatically cycling off once the humidity level drops to an acceptable level. Similarly, an attic humidistat works with a fan, while a basement humidistat works with a dehumidifier. Both keep temperatures manageable and mildew in check.
  • Save on air conditioning and heating: Too much humidity in the room while operating an air conditioner makes an air conditioner work harder, requiring more energy to cool a space. Using a humidistat that maintains a proper moisture level in your home can save on heating and cooling bills.
  • Improve indoor air quality: Too much or little humidity in a room can affect air quality. Too much moisture may encourage bacteria, viruses, fungi, cockroaches, and dust mites that can lead to illnesses. Insufficient humidity in the air can cause dry lips, nose, throat, and skin.

How to Set Your Humidistat

Each home, region, and family has different needs. Ultimately, you will need to experiment with different settings that work best for you and in your home.

External temperatures can affect the settings for your humidistat. In the winter, when it gets colder, you need less humidity (closer to 30%), and vice versa in spring, summer, and fall (closer to 50%).

When you leave the house for an extended period, set your home temperature to 78 degrees Fahrenheit. Leave your humidistat set at 58%. At that temperature, the levels will remain consistent while away.

Humidistat Placement

Where you place your humidistat and how you set it are equally important for controlling your home's humidity levels. Not all homes need a humidistat. Some homes may have damp basements that require constant monitoring to keep mold and mildew in check. Your humidistat should be positioned in the space where you need air vapor measured.

When an HVAC professional sets up a whole-home humidistat system connected to your home's heating and cooling system, a humidistat probe is usually placed in the airflow of a duct; usually, it's in the return vent before the humidifier, which is an excellent way to measure whole-home humidity.

Using a Humidistat and Hygrometer Together

A disadvantage of using a humidistat is it is limited by its design. A humidistat can only measure humidity in one specific area—where it is situated. Ten feet away, the moisture reading might be different than at the unit.

For example, it's safe to assume that when a humidifier operates, the air right around it will be more humid than the air a few feet away. And the humidistat confirms it, turning off when the air around it reaches a specific humidity level. Unfortunately, the moisture level at the humidistat does not accurately reflect the entire room's moisture level.

Since a humidistat is limited by usually being fixed in one spot, not accurately reflecting the humidity of an entire room, get a supplemental, portable hygrometer that can measure humidity levels throughout a room.

Place a hygrometer at the other end of the room to confirm a humidistat's efficiency and whether you should adjust the room's humidity levels slightly higher or lower to cover the entire space. Getting an accurate humidity reading is a helpful feature. You can reduce the risk of over-humidifying the room. Combining a companion hygrometer with a humidistat allows you to manage a room's moisture level best and adjust accordingly.

Article Sources
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  1. Why and where mold grows. Environmental Protection Agency.

  2. Moisture Control. United States Environmental Protection Agency.

  3. Jones, ER, Cedeño Laurent, JG, Young, AS, Coull, BA, Spengler, JD, Allen, JG. Indoor humidity levels and associations with reported symptoms in office buildingsIndoor Air, vol. 32,  no. 1, 2022. doi:10.1111/ina.12961