Ductless Portable Air Conditioners. A "bypass" humidifier.
Is a variation of the flow-through humidifier that is mounted to the air return duct and connects to the hot air supply off the furnace by using a humidifier supply takeoff duct. The supply takeoff bypass duct diverts some heated air to the humidifier by taking advantage of a natural pressure differential between the supply and return sides of the furnace. The warm air absorbs moisture from the evaporator pad inside the humidifier and returns to the warm air stream through the cold air return duct and furnace.
Use a rotating drum styled foam pad or similar media that picks up water from a float controlled reservoir for evaporation. These types of humidifiers are prone to failure due to the foam pad becoming encrusted with sediment / mineral buildup and the reservoir having water stagnation and bacteria and mold growth. The foam pads should be cleaned monthly and replaced annually.
Ductless AC Units. Flow-Through Humidifiers.
Use a specially coated metal or plastic screen that has a small amount of water constantly dripping across it while the furnace is running. These screens need to be replaced each season. The problem with these humidifiers is that they tend to consume high amounts of water since the water drips across the media and then is drained away.
Are the most expensive and most efficient humidifiers available. They contain self generating steam heaters and are very effective. These humidifiers however require water to have a certain range of hardness and quality. Some homes may require filtration options to alleviate problems with water quality and hardness. Homes having extreme hard water problems should consider using a whole house water softener as the main filtration device.
Mini Split AC in the USA 