How a Furnace-Mounted Home Humidifier Works

Diagram of a humidifier's air duct system

A furnace-mounted, or whole-house, humidifier connects to a forced-air heating system to deliver moist air throughout the house. There are a few different types of furnace humidifiers. A flow-through humidifier uses freshwater that flows through the system and drains away. A reservoir type has a reservoir of water that moistens a rotating drum. A steam humidifier heats water to create steam that is injected into the furnace airflow.

Of the three, flow-through humidifiers are often preferred because they are more reliable and usually more hygienic than reservoir types, and they are less expensive and use less electricity than steam units.

A flow-through humidifier typically is mounted to the furnace's cold-air return duct and connects to the hot-air supply off 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.

Some flow-through humidifiers are mounted directly to the hot air supply duct and do not use a supply takeoff duct.

Furnace 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 (causes evaporation)
  • Water control valve (solenoid) or float
  • Humidistat (controls humidity setting)

Many of the components of a flow-through humidifier are shown here. 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: 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, 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.

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. The solenoid is controlled by a humidistat (sort of like a 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, stopping water flow to the humidifier.

The solenoid is usually low-voltage and is powered by a transformer mounted to the furnace.

Humidifier solenoid valve assembly

Water Inlet Valve Assembly Close-Up

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.

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, it is best to replace them at the beginning of every heating season.

Humidifier evaporator pad

Humidifier Drain

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

This is a key distinguishing feature of flow-through humidifiers. Unlike reservoir humidifiers where water stands for a period of time, flow-through humidifiers do not have any standing or recycled water that can host bacteria—a common problem with reservoir types.

Humidifier drain leads to floor or sink drain

Furnace Humidifier Humidistat

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 (typically 30 to 45%), and when the humidifier reaches that level, the water supply is turned off by the solenoid water valve assembly.

Humidifier humidistat viewed up-close