Furnace Humidifier: Definition, Cost, Pros and Cons

A furnace humidifier is an appliance attached to your furnace system that introduces moisture into the air passing through the ductwork with the purpose of raising the humidity in your home to a comfortable level.

The average cost of a furnace humidifier installed in your home is $500 to $1,500 depending on the type of built in humidifier it is. The four main whole house humidifier types are evaporative bypass humidifiers, evaporative flow through humidifiers, mister humidifiers and steam humidifiers.

What is a Furnace Humidifier?

The definition of a furnace humidifier is a mechanical unit attached to your furnace’s warm air supply and fed by a water supply line. Most models require a hole to be cut into the warm air supply over which the humidifier is mounted.

As your furnace heats, and as air is circulating through the furnace and ductwork, water is released. The air absorbs moisture, and the humid air is delivered to the rooms of your home. The result is higher relative humidity in your home, which makes the air more comfortable for your skin and for breathing.

Furnace Humidifier

How Does a Furnace Humidifier Work?

A furnace humidifier attached to your warm air supply ductwork works by adding moisture to the air flowing out of the warm air supply. This can be accomplished through evaporation, steam or atomization of water. All require having a small-diameter water line running to the whole house humidifier.

The level of humidity produced by a humidifier is controlled by a humidistat. Most units have a built in humidistat attached directly to the humidifier though some are mounted separately.

Humidistats regulate the amount of water released by the humidity level of the air flowing through the furnace. The humidistat can be set to the relative humidity you prefer. If the relative humidity is below the humidistat setting, the humidifier will turn on when your furnace is in heat mode and provide the required amount of moisture to raise the humidity to satisfy the humidistat.

What Are the Types of Furnace Humidifiers?

Here are the three types of furnace humidifiers with an overview of how they work.

Evaporative Bypass Humidifiers: A bypass humidifier attaches to the outside of the warm air supply duct, and air is directed into it to evaporate moisture before being directed back to the warm air supply ductwork.

As the name implies, bypass humidifiers contain a damper that is opened during the heating season that causes some of the heated air to bypass the main flow of air and flow through the humidifier. A pad or rotating drum in the evaporative bypass humidifier absorbs or holds water from the water supply line. When the humidistat calls for higher humidity, the water valve is opened to allow water to flow to the pad where it is evaporated.

The damper is closed or turned to “summer” mode when the humidifier is not needed during other seasons of the year.

A fan is included in some bypass furnace humidifier models to improve the amount of airflow through the bypass, which leads to more water being evaporated into the air.

Bypass humidifiers require easy access to the warm air supply and the space necessary to mount the unit to the outside of the ductwork.

Flow Through Humidifiers: Flow through humidifiers attach to the warm air supply side of the furnace. They are fed by a water supply line. Most have a humidistat and dial for you to set to your desired humidity.

The name refers to water flowing through the unit rather than a reference to airflow.

A metal or plastic screen is located inside the humidifier cover and is exposed on the duct side of the unit. Water flows over the screen, which is shaped to hold it as heated air passes by. The result is that water evaporates off of the screen and into the supply ducts leading to the rooms in your home.

Water that is not evaporated flows through a drain to a floor drain, which is a mandatory requirement for using a flow through, or flow-through, humidifier.

Misting Humidifiers: This newer humidifier type serves homes up to 1,500 to 2,000 square feet. Water from the supply line is misted into the warm air supply where it evaporates and is carried to the air grates throughout your home.  

Most misting humidifiers have a built-in temperature sensor and switch. They will not operate if the temperature of the air flowing through the warm air supply is not high enough to produce evaporation.

A humidistat can be installed on a wall near the furnace or attached to the cold air return duct to monitor the humidity level of air entering the furnace.

Steam Humidifiers: A steam humidifier heats water from the supply line to boiling using electric current to electrodes in a heating canister. The steam that is created flows through a hose and into the air flowing through the warm air supply ductwork. A humidistat is set to control the amount of moisture added to the air.

These whole house humidifiers are considered a premium design and are especially useful in very large homes. Steam humidifiers are also called atomizing humidifiers.

Steam humidifiers are stand-alone humidifiers. They do not attach to the furnace. Instead, a small hole is cut into the warm air supply, and a hose is connected through which the steam flows from the humidifier into the air flowing through the supply ductwork.

How Much Does It Cost to Add a Humidifier to Your Furnace?

The cost to add a humidifier to your furnace is $500 to $1,500 or more based on the type of humidifier, its features and who does the installation.

Here are furnace humidifier types and costs.

Evaporative bypass humidifier cost is $165 to $255 retail. The installed cost of a bypass humidifier is $335 to $600. The average cost of an installed system is $525.

Flow through humidifier cost is $200 to $475 retail. The installed cost of a flow-through humidifier is $390 to $845. Average cost is $745.

Misting humidifier cost is $135 to $315 retail. Installed cost of a misting humidifier is $285 to $550 with an average cost of $435.

Steam humidifier cost is $450 to $1,650 retail. The installed cost of a steam humidifier is $615 to $2,030. The average installed cost is $1,820.

What are the Pros and Cons of a Furnace Humidifier?

The pros of a furnace humidifier are air in your home that is more humid and potentially lower heating bills.

Here are the details of the pros of a furnace whole house humidifier.

A higher humidity level is automatically controlled for you: Air that is too dry makes people and pets uncomfortable. Skin, eye and breathing passage irritation is reduced or eliminated through better air humidity. Better humidity is also good for wood furniture and other household materials. The US Environmental Protection Agency Mold Course Chapter 2 recommends an indoor humidity level between 30% and 50% which might only be possible in winter with the use of a whole house humidifier. Steam whole house humidifiers are the best at raising the humidity level when winter temperatures are below freezing.

Your home may use less energy to heat: Moist air feels warmer. Therefore, you can set your thermostat a few degrees lower and feel just as comfortable when the air humidity is higher.

The cons of a furnace humidifier start with the initial cost of the unit plus installation, unless you are handy and can install it yourself. They also include the following potential problems.

Ongoing maintenance can be expensive: Most humidifiers require cleaning on a monthly basis and replacement of the drum pad or the screen at least once per heating season. Any humidifier that uses a pad or screen that must be replaced will have higher maintenance costs.

Mold and mildew caused by excess humidity is possible: The EPA warns that humidity levels above 60% can produce an environment for mold and mildew. These harmful agents can cause breathing problems and illness for people and pets plus cause staining on walls, the ceiling and other materials in your home. Caution is needed when using a steam humidifier because they produce the most moisture in a short period of time.

Water is wasted: This is a major problem with pass through humidifiers. Most of the water that enters them goes through them and down the drain. This is a costly problem if you use metered water.

How to Install a Humidifier on a Furnace?

The basic steps to furnace humidifier installation are the same for all humidifier types.

The steps are to:

  1. Cut a hole in the warm air supply duct. See the humidifier installation guide for the right size and shape.
  2. Mount the humidifier into the hole, and secure the unit to the furnace sheet metal.
  3. Cut a hole for the humidistat base, and secure it to the sheet metal.
  4. Wire the solenoid, which activates the water release to the humidifier, to the furnace control board with the attached 24V and common wires.
  5. Install the humidistat cover that includes the humidity control dial.
  6. Install the saddle valve, also known as a water tap, into a cold water line.
  7. If the unit is a pass-through humidifier, attach the drain hose and run it to the floor drain.

Where Should a Humidifier Be Installed on a Furnace?

The best practice is to install a humidifier on the warm air supply plenum of the furnace ductwork.

When installation on the warm air supply is impossible due to space restrictions, most humidifier types can also be installed on the cold air return side of the furnace. Mist humidifiers however must always be installed on the warm air side. If there isn’t room there, then you will need to choose a different humidifier type – an evaporative bypass, pass-through or steam humidifier.

Do All Furnaces Have a Humidifier?

No, not all furnaces have a humidifier. In fact, most do not. Humidifiers are not installed at the factory. They are installed after the furnace is installed.

Furnaces in cold climates are most likely to have a humidifier. Homes in warm climates, especially in warm and humid regions, have no need for a humidifier.

A furnace humidifier would benefit you if you experience clothing static cling, dry skin or eyes, throat dryness and irritation or similar discomfort in the winter due to dry indoor air.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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