humidifier is a device, primarily an electrical appliance, that increases humidity (moisture) in a single room or an entire building. In the home, point-of-use humidifiers are commonly used to humidify a single room, while whole-house or furnace humidifiers, which connect to a home’s HVAC system, provide humidity to the entire house. Medical ventilators often include humidifiers for increased patient comfort. Large humidifiers are used in commercial, institutional, or industrial contexts, often as part of a larger HVAC system.




For buildings with a forced-air furnace, a humidifier may be installed into the furnace. They can also protect wooden objects, antiques and other furnishings which may be sensitive to damage from overly dry air. In colder months, they may provide modest energy savings, since as humidity increases, occupants may feel warm at a lower temperature.

Bypass humidifiers are connected between the heated and cold air return ducts, using the pressure difference between these ducts to cause some heated air to make a bypass through the humidifier and return to the furnace.

Any humidifiers should usually be disabled during the summer months if air conditioning is used; air conditioners partially function to reducing indoor humidity, and having a humidifier continue to operate will waste significant amounts of energy.

Disc wheels

A disc wheel style (bypass) is very similar in design to the drum style humidifiers; this type of furnace humidifier replaces the foam drumming with a number of plastic discs with small grooves on both sides. This allows for a very large evaporative surface area, without requiring a great deal of space. Unlike the drum style humidifiers, the disc wheel does not need regular replacement.

Advantages include:

  • Very low maintenance (basin of humidifier should be cleaned out periodically, unless an automatic flushing device is installed)
  • No regular replacement of parts necessary
  • Higher output due to large evaporative surface area
  • Can be installed in hard water situations
  • Maintains efficiency throughout lifespans
Disadvantages include:

  • Higher price
  • Water evaporation even when humidification is not required (due to the pan of water which remains exposed to a high velocity air stream)

Bypass flow-through

Bypass flow-through style (bypass – also known as “biscuit style” or many other, similar variant names) uses a pipe to bring water directly to an electrically controlled valve at the top of the humidifier. Air passes through an aluminum “biscuit” (often called a pad; the term “biscuit” emphasizes the solid rather than foamy form) which is similar to a piece of extremely coarse steel wool. The “biscuit” has a coating of a matte ceramic, resulting in an extremely large surface area within a small space. When the hygrostat calls for humidity, the valve is opened and causes a spray of water onto the “biscuit”. Hot air is passed through the “biscuit”, causing the water to evaporate from the pad and be carried into the building.

Advantages include:

  • Reduced maintenance (new “biscuit” is needed only when clogged with dust or mineral deposits, typically once per year)
  • Lack of a pan of potentially stagnant water to serve as a breeding ground for mold as with a drum-style humidifier
  • No incidental humidification caused by a constantly replenished pan of water in a high velocity air stream
  • Reduced requirement for expensive air filters
  • Uses little electricity
Disadvantages include:

  • A somewhat higher purchase price
  • Manufacturer and model-specific replacement biscuits (versus the relatively generic drum-style pads) may be more expensive and difficult to find
  • For most models, a portion of the water supplied to the unit is not evaporated. This can generate a considerable amount of waste water containing residual minerals, which does require connection to a drain. There is a limited selection of drain-less models that recirculate water, but mineral buildup must then be removed manually on a periodic basis.