For buildings with a forced-air furnace, a humidifier may be built right 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 substantial energy savings, since as humidity increases, occupants feel warm at a lower temperature. These units are typically called bypass humidifiers (because they 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). The humidifier should usually be disabled during the summer months if air conditioning is used; air conditioners partially function by reducing indoor humidity. There are three basic styles:

• Drum style: A pipe brings water directly to a reservoir (a pan) attached to the furnace. The level of water in the pan is controlled by a float valve, similar to a small toilet tank float. The wick is typically a foam pad mounted on a drum and attached to a small motor; hot air enters the drum at one end and is forced to leave through the sides of the drum. When the hygrostat calls for humidity, the motor is turned on causing the drum to rotate slowly through the pan of water and preventing the foam pad from drying out.
Advantages include:
• Low cost
• Inexpensive maintenance (drum-style pads are cheap and readily available)
Disadvantages include:
• Requirement for frequent (approximately monthly) inspections of cleanliness and pad condition
• Water evaporation even when humidification is not required (due to the pan of water which remains exposed to a high velocity air stream)
• Mold growth in the pan full of water (this problem is exacerbated by the large quantity of air, inevitably carrying mold spores, passing through the humidifier whether in use or not).
For the latter reason especially, drum-style humidifiers should always be turned off at the water supply during summer (air conditioning) months, and should always be used with high quality furnace air filters (MERV ratings as high as possible to ensure small numbers of mold spores reaching the humidifier's pan) when the water supply is turned on.

• Disc Wheel Style: Very similar in design to the drum style humidifiers, this type of furnace humidifier replaces the foam drum 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 replacing.
Advantages include:
• Very low maintenance (basin of humidifier should be cleaned out periodically, unless 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 lifespan
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)

• Flow-Through Style (also known as "biscuit style" or many other, similar variant names): A pipe brings water directly to an electrically-controlled valve at the top of the humidifier. Air passes through an aluminum "biscuit" (often called a pad; using the term "biscuit" to emphasize 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 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 no electricity
Disadvantages include:
• A somewhat higher purchase price
• Manufacturer and model-specific replacement biscuits versus the relatively generic drum-style pads
• A portion of the water supplied to the unit is not evaporated. This can generate a considerable amount of waste water, and does require connection to a drain.