Bypass and power humidifiers make a big difference in the air quality of homes. They are installed as "whole home" humidifier systems, and filter humidified air to every room in the house. These systems transform a house that is dry into one where the air is better to breathe and rooms feel more comfortable. A bypass or power humidifier can provide relief for those who suffer from allergies and asthma, and for homeowners in areas where winters are long and the indoor air quality is dry.
The bypass humidifier is the most commonly used system in residences. Bypass humidifiers are composed of a cabinet, a water reservoir, a rotating drum that has a motor and a float unit. The components connect to the home's furnace, and leverage the pressure differentials in the plenum mechanics of the furnace to move air, moisten air and distribute it into the rooms of the home. In a bypass system, warm air is forced into the cabinet, then is forced to flow over the drum, which is made of sponge or foam. The motor moves the drum over the water reservoir, and the float unit controls the speed of the drum as it moves. These processes and mechanisms generate warm, moist air that is fed to the duct channels of the home's HVAC system to humidify the air that is vented into rooms.
As the name suggests, a power humidifier has "power." These systems have the same basic components as a bypass system (cabinet, reservoir, drum, motor and float), and one more component -- a fan. The fan is what gives a power humidifier an advantage over bypass humidifiers. The fan drives air across the float at a higher rate of speed so the unit can produce as much as a gallon or more of humidity every day, with a minimal increase in energy usage. The fan uses about the same amount of electricity as a 25-watt lightbulb. Power humidifiers are a good choice for homes without basements, homes built on concrete slabs, and those where the furnace and HVAC system are located in a closet or garage.
Bypass systems are the most affordable type of "whole home" humidifier. They can adapt to various water types, such as hard or soft water, and have low maintenance requirements. Changing the filter annually and checking the drain tube to get rid of any buildup of mineral deposits are about the only major tasks required. Some types of units can be mounted directly to a wall and connected to air return ducts to make installation even easier. A bypass system can quite easily be upgraded and converted into a power humidifier.
Power humidifiers are also space savers because they don't require use of a bypass duct. Because these systems produce more humidity, they use water more efficiently. Unlike a bypass system, which depends on the furnace to operate, a power system operates independent of the furnace and can be used when the furnace is not running. Another advantage is that power systems do not generate standing water, as is the case with bypass systems.