A central or whole-house humidifier is generally mounted on a forced air furnace (gas or electric) or heat pump. While the actual path of air flow may differ depending on the type of model, the general idea is to take warm air from the furnace or heat pump, pass it through the humidifier unit where it will increase in humidity by contacting the humidifier's evaporative surface, and to then pass the air back into the central heating ducts for distribution of the warm, more humid air, to the home. The most frequent installation configuration is of a by-pass humidifier. The humidifier is generally mounted on the cold air return plennum. A portion of the warm air from the furnace's hot air supply duct is directed into a flexible duct, usually about 6 inches in diameter, which is connected to the humidifier on the cold air return duct. The air travels through this flexible duct, into the humidifier, back into the cold air return duct and into the furnace where the furnace fan re-distributes it to the home. Simply and effective...

Since humid air will tend to release its humidity when it comes into contact with cold objects, it is generally not recommended that a central furnace humidifier he used in conjunction with a central air conditioner. When the humid air reaches the air conditioner coils (which are very cold), it could cause excessive condensation. On the bright side, very few homes require humidification during the summer months or air conditioning season anyway since warm summer air generally already has a high capacity to hold moisture. Cold winter air, even when the relative humidity level is high, holds limited actual mositure.