What types of humidifiers are there?
There are several types of humidifiers. The least expensive consists of a tray with evaporative pads. Some of these units are designed so the tray sits inside the plenum, immediately over the heat exchanger, while on others the tray sits outside the plenum beside the furnace.
These generally low-quality humidifiers allow no control over humidity. When the furnace is on, they are humidifying; when the furnace is off, they are not. They add little humidity to the home. The units that sit inside the plenum are particularly troublesome, as they tend to leak and drip onto the heat exchanger, causing pre-mature rusting of the heat exchanger and failure of the furnace.
Drum-type humidifiers are perhaps the most common. A small electric motor turns a foam or fabric mesh-covered drum through a tray of water. Water level in the tray is maintained by a float valve and a tube connected to the supply plumbing piping. These humidifiers are typically mounted on the return air plenum of the furnace with a bypass duct to the supply plenum. They can also be mounted on the underside of a horizontal duct. A deflector attached to the inside of the duct directs air through the humidifier.
The furnace fan blows air across the wet drum, picking up moisture. These units have a humidistat usually mounted beside the thermostat in the living space, or on the return duct close to the humidifier. The recommended humidity levels should be maintained.
Some drum (rotary) humidifiers have drains and overflows, which help protect against damage due to leakage.
Spray humidifiers use a tray, a series of nozzles and a pump to break the water up into a mist that can be carried away by the air. These need drains to carry excess water away. Some use heating coils to generate steam that is absorbed into the air.
Trickle humidifiers use a solenoid (electrically operated) valve to control water flow through an expanded metal mesh. As the water trickles down the mesh, it evaporates into the air passing over it. These are possibly the best, most reliable type of humidifier.