How a Furnace-Mounted Home Humidifier Works

Flow-Through, Reservoir, and Steam Humidifiers for Your Home

How a Furnace-Mounted Home Humidifier Works

The Spruce / Candra Huff

A humidifier for a furnace, otherwise known as a whole-house humidifier, connects to the HVAC system to deliver moistened air throughout the house. There are several types of furnace humidifiers. A flow-through humidifier for furnaces uses freshwater that flows through the system and drains away. A reservoir type has a reservoir of water that moistens a rotating drum. And a steam humidifier heats water to create steam that is injected into the furnace airflow.

Of the three types of furnace humidifiers, the flow-through is often considered the best humidifier for furnaces. That's because it is more reliable, more hygienic and requires much less maintenance than reservoir types.

A flow-through humidifier typically is mounted to the furnace's cold-air return duct and connects to the hot-air supply of the furnace through a humidifier supply takeoff duct. The takeoff duct diverts some heated air into the humidifier, where it absorbs moisture from an evaporator pad before returning to the warm air stream through the cold-air return duct and furnace. However, some flow-through humidifiers are mounted directly to the hot air supply duct and do not use a supply takeoff duct.

How Much Do Furnace Humidifiers Cost?

How much it costs to add a humidifier to your furnace can vary widely. On average, a humidifier for the furnace costs between $398 and $753. Small humidifier models will be on the lower end of the price range while high-end large models can cost up to $2,500. Prices for the labor to install the humidifier also can vary by location, company, and model.

Specifically, a flow-through type of furnace humidifier costs around $200 to $950 on average for the unit plus installation. A reservoir/drum type costs $100 to $300 on average. And a steam model costs around $500 to $2,200.

Furnace Humidifier Components

Although there are some variations to the components of a humidifier for a furnace, they all require:

  • Water: Increases humidity as it evaporates into the air stream
  • Water collection medium: Usually an evaporator pad
  • Blowing air: Facilitates the evaporation of water
  • Water control valve (solenoid) or float: Controls the water flow
  • Humidistat: Controls the humidity level in the air

Many of the components of a flow-through humidifier are shown in the image below. This is how all the parts work together:

  • Water tap/supply line: A water supply line for the humidifier is tapped into an existing water pipe. 
  • Water inlet orifice: The orifice reduces the water flow to the humidifier inlet valve.
  • Water inlet valve: This valve allows water to flow to the humidifier based on demand. The valve is usually electrically operated by a solenoid controlled by the humidistat. 
  • Water feed tube: This distributes water to the distribution trough under the top cover and feeds water to the evaporator pad.
  • Evaporator pad: This is the water collection medium that holds the water briefly as it is evaporated to create humidified air.
  • Drain pan: Water flows through the evaporator pad and into the drain pan, from which it flows out into a household drain.
  • Air damper/air duct: Some models have an air duct from the hot-air side that supplies air to the humidifier mounted on the cold-air return. If the home has central air conditioning, a damper is required.
Diagrammed humidifier components

Humidifier Water Tap

Water is provided by tapping into an existing cold-water supply line close to the humidifier, often with a device known as a saddle valve. A flexible copper water line runs between the valve and the humidifier. Saddle valves are no longer allowed by code in some communities, so you might instead find a traditional ball valve or gate valve controlling this flexible supply line.

Humidifier water supply tap

Water Inlet Valve Assembly (Solenoid)

The flow of water to the humidifier is controlled by a water inlet orifice and an inlet valve assembly controlled by an electric solenoid—usually a low-voltage device powered by a transformer mounted to the furnace.

The solenoid is controlled by a humidistat, which operates as a kind of thermostat for humidity. When the humidistat calls for an increase in humidity, the solenoid opens and supplies the water feed tube. When the humidistat senses the humidity level is good or when the furnace shuts down, the solenoid shuts off and stops water flow to the humidifier.

Humidifier solenoid valve assembly

Water Inlet Valve Assembly (Detail)

Below, you can see the power wires running to the solenoid and valve assembly. The water inlet orifice is attached to the valve bottom at the lower water-supply connection. Coming out of the top of the valve is the water inlet feed tube, which runs to the top of the humidifier.

Humidifier solenoid valve assembly

Inlet Feed Tube and Trough

The water inlet feed tube runs from the water inlet valve to the top of the humidifier. It connects to the top cover (held in place by knurled nuts) and delivers water to a water distribution trough located directly under the top cover. The water distribution trough allows the water to flow evenly to all portions of the medium, which in this case is an evaporator panel.

Humidifier water feed tube

Humidifier Evaporator Pad or Panel

The evaporator pad goes by several names, including evaporator panel or water wick. The pad's purpose is to disperse water evenly, which facilitates evaporation, and to collect any mineral deposits from the water. After a season of use, the metal pad will be coated with white powder of minerals. Although these pads can sometimes be cleaned as part of your humidifier and furnace maintenance, it is best to replace them at the beginning of every heating season.

Humidifier evaporator pad

Humidifier Drain

With flow-through humidifiers, any remaining water from the evaporator pad runs off into a drain pan and flows through to a household drain. This is a key benefit of flow-through humidifiers. With reservoir humidifiers, water stands for a period of time. Consequently, these furnace humidifiers can cause mold growth. Flow-through humidifiers do not have standing or recycled water that can host mold.

Humidifier drain leads to floor or sink drain

Furnace Humidifier Humidistat

The humidistat controls humidity levels much like a thermostat controls air temperature. You set a thermostat for a specific temperature, and when that temperature is reached, the furnace turns off. A humidistat works on a similar principle: You set it for the desired humidity level (typically 30 to 50 percent), and when the humidifier reaches that level, the water supply is turned off by the solenoid water valve assembly.

Humidifier humidistat viewed up-close

Do You Need a Furnace Humidifier?

In many cases, it is good to put a humidifier on your furnace. Heat can dry out the air in a home, causing dry skin, static, and more. This is why it is typically recommended that you run your furnace humidifier through the months you're using heat. However, if you live in a warm, humid climate and don't often use heat, you might be fine using a portable humidifier as needed (if at all) instead of a whole-house humidifier.

Besides being an unnecessary expense, a major con to adding too much humidity to your home's air is it can encourage mold growth. So it's best to check your home's humidity level throughout the year to help you determine whether you need to supplement humidity.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Central Humidifiers. International Association of Certified Home Inspectors.

  2. Humidifiers and Indoor Allergies. American Academy of Allergy Asthma and Immunology.

  3. Why and Where Mold Grows. U.S. Environmental Protection Agency.