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