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 pan) when the water supply is turned on. 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