Types of Furnace Mounted Humidifiers:
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 as shown here and the subject of this tutorial, 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 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 may be mounted to the air return duct and connect to the hot air supply off the furnace by using 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 and returns 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 not use a supply takeoff duct at all.
How a Humidifier Works:
All home furnace mounted humidifiers work under a principle of 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.