There are several types of in-duct humidifiers. Which you choose might be determined by preference or by necessity. Steam and drum humidifiers can work best in certain situations.
Bypass humidifiers are usually mounted on the air-return duct, connecting to the hot air supply using a supply-takeoff duct. The supply takeoff diverts heated air to the humidifier using natural pressure differences between the supply and return sides. The warm air picks up moisture from an evaporator pad and returns to the furnace. Some bypass humidifiers are mounted directly to the hot air duct.
Powered flow-through humidifiers work the same way as bypass but include a fan to blow air across the moistened pad for increased water evaporation. They can generally produce a gallon more humidity each day compared to a bypass model. The electricity required for the fan equals that of a 25-watt light bulb. Powered flow-through systems don't need a bypass duct, meaning they can be installed in smaller spaces. They are perfect for homes built on slabs or with HVAC systems in a closet.
There are some undeniable benefits to whole-home humidification, but unfortunately, there is often a trade-off. Better humidifying means extra power is needed, and saving water means more frequent cleaning.