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. Spray mist Spray mist type uses a pipe, usually a small plastic one, to bring water directly to an electrically controlled valve (atomizer-this forces the water through a tiny orifice causing it to break up into tiny particles) in the humidifier. Water mist is sprayed directly into the supply air, and the mist is carried into the premises by the air flow. Advantages include: Simpler than bypass types to install, requiring a single cut hole for installation, no additional ducting. Uses little electricity. Small, compact unit which fits where other types cannot. (Approximately square.) Because it does not require bypass ducting it does not undermine the pressure separation (and therefore, blower efficiency) of the return and supply ducts. Does not require use of moisture pads (on-going expense). Highly efficient usage of water. Does not generate waste water, and does not require separate connection to a drain. Requires little maintenance. Periodic cleaning of nozzle may be required in hard water environments. Lack of a pan of potentially stagnant water to serve as a breeding ground for mold as with a drum-style humidifier. Disadvantages include: Spray nozzle can become clogged in hard water situations, necessitating the use of water filter, periodic cleaning of nozzle, or nozzle replacement. Disperses any minerals in the water into the airstream. Additional types There are many different types of Plant and Home Humidifiers which can help you to maintain humidity level of your home as required. Additional types include non-bypass flow-through (fan augmented), steam, impeller or centrifugal atomizer, and under duct designs. Problems The U.S. EPA provides detailed information about health risks as well as recommended maintenance procedures. If the tap water contains a lot of minerals (also known as "hard water") then the ultrasonic or impeller humidifiers will produce a "white dust" (calcium is the most common mineral