How a Furnace-Mounted Home Humidifier Works

How a furnace-mounted home humidifier works
Bob Formisano
  • 01 of 10

    Anatomy of a Flow-Through Furnace Humidifier

    Diagram of a humidifier's air duct system

    Although all furnace-mounted humidifiers work under the same principles, two basic types are the most common.

    The humidifier may be a flow-through type, which is shown here and is the subject of this article, where the water flows through the humidifier and drains out. Or it may be a reservoir type, which uses a reservoir of water and a rotating drum to introduce the moisture necessary for humidified air. (A third type is known as a "steam humidifier" is not discussed in this tutorial.)

    With flow-through humidifiers, you don't have to worry about possible bacteria growing in the reservoir's standing water, which can cause illness or humidifier fever.

    Flow-through humidifiers are often mounted to the air return duct and connect to the hot-air supply off the furnace through a humidifier supply takeoff duct, as shown in this photo. The supply takeoff bypass duct diverts some heated air to the humidifier by taking advantage of a natural pressure differential between the supply and return sides of the furnace. The warm air absorbs moisture from the evaporator pad inside the humidifier before returning to the warm air stream through the cold air return duct and furnace.

    Some models may be mounted directly to the hot air supply duct and do not use a supply takeoff duct at all.

    How a Humidifier Works

    All furnace-mounted home humidifiers work by introducing moisture from water and evaporating it into a warm air stream coming from the furnace.

    Now let's take a look at humidifier components.

    Continue to 2 of 10 below.
  • 02 of 10

    Humidifier Components

    Diagrammed humidifier components

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

    • Water (to create humidity).
    • Water collection medium (e.g., evaporator pad).
    • Blowing air (to cause evaporation).
    • Water control valve (solenoid) or float.
    • Humidistat (controls humidity setting).

    In the photo above, you can see most all the components of a flow-through furnace-mounted humidifier. In summary, here is how it all works together:

    • Water tap/supply line: A water supply line for the humidifier is tapped into an existing water pipe source. A common way to do this is with an ordinary saddle valve. 
    • Water inlet orifice: The orifice reduces the water flow to the humidifier inlet valve for the economy.
    • 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. The solenoid is usually low voltage, powered by a transformer mounted to the furnace.
    • Water feed tube: Distributes water to the distribution trough under the top cover, and feeds water to the evaporator pad.
    • Evaporator pad (media): 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, where it then flows 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, then a damper is required.
    Continue to 3 of 10 below.
  • 03 of 10

    Water Tap

    Humidifier water supply tap

    First things first: the humidifier needs water. This will be provided by tapping into an existing cold-water supply line close to the humidifier, often with a device known as a saddle valve. Once tapped, the flexible copper water line will run to the humidifier.

    Continue to 4 of 10 below.
  • 04 of 10

    Water Inlet Valve Assembly (Solenoid)

    Humidifier solenoid valve assembly

    Since water can't just run to the humidifier all the time, the water-supply flow needs to be controlled. This is done by a water inlet orifice and an inlet valve assembly controlled by an electric solenoid. The solenoid is controlled by a humidistat (sort of like a thermostat for humidity).

    When the humidistat says to increase humidity, the solenoid opens and supplies the water feed tube. When the humidistat says the humidity level is good, or when the furnace shuts down, the solenoid shuts off, stopping water flow to the humidifier.

    Through months of seasonal non-use, these parts can become seized or clogged and prevent the humidifier from working properly. Let's take a detailed look at the water inlet valve assembly in the next section.

    Continue to 5 of 10 below.
  • 05 of 10

    Water Inlet Valve Assembly Detail

    Humidifier solenoid valve assembly

    Here 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.

    Continue to 6 of 10 below.
  • 06 of 10

    Inlet Feed Tube and Trough

    Humidifier water feed tube

    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 the evaporator panel.

    Continue to 7 of 10 below.
  • 07 of 10

    Humidifier Evaporator Pad or Panel

    Humidifier evaporator pad

    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 also collects any mineral deposits from the water. After a season of use, the metal pad will be coated with white powder from the minerals in the water. Although these pads can sometimes be cleaned, it is best to have them replaced every heating season.

    Continue to 8 of 10 below.
  • 08 of 10

    Humidifier Drain

    Humidifier drain leads to floor or sink drain

    With flow-through type humidifiers, any unevaporated water from the evaporator pad runs off into the drain pan and flows through to a household drain.

    This is a key feature of flow-through humidifiers since unlike reservoir humidifiers (ultrasonic or impeller/drum types) where water stands for a period of time, flow-through humidifiers will not allow bacteria to develop.

    Continue to 9 of 10 below.
  • 09 of 10


    Humidifier humidistat viewed up-close

    The humidistat is like a thermostat. You set a thermostat for a specific temperature, and when that temperature is reached, the furnace turns off. Same with a humidistat. You set it for the desired humidity level (usually 30% to 45%), and when the humidifier reaches that level, the water supply is turned off by the solenoid water valve assembly.

    Continue to 10 of 10 below.
  • 10 of 10

    Manufacturers and Parts

    There are several manufacturers of furnace mounted humidifiers. GeneralAire makes the one featured in this tutorial.