Powered Flow Through Humidifier The most popular type of humidifiers are the Flow-through type. Called "flow-through" because a portion of the air from the supply side of the furnace is detoured to force it across a wetted humidifier pad. The humidified air is then routed back to the return side of the furnace, blended with air from the cold air returns, heated by the furnace and delivered to the conditioned space. The most remarkable feature of all flow-through models is the patented PerfectFloTM water distribution tray. The tray allows for speedy and complete humidifier pad water saturation and efficient humidification. The unique tray also ensures fast delivery of water to the humidifier pad assuring that the pad is wet at the start of the blower cycle and wets the entire humidifier pad evenly providing optimal evaporation. Pros: Like bypass flow-through, powered flow-through are hard-water tolerant and minimal maintenance - just change the pad. The fan inside a powered unit blows heated supply-side air across the humidifier pad so its water efficiency is higher. Powered models can typically produce about one gallon more humidity per day using the same size humidifier pad as a bypass model. The additional electricity needed to run the fan is minimal - about equal to a 25 watt light bulb - and could be more than offset by the energy-savings possible with lower thermostat setting when the air is kept at an optimal humidity level. In addition, powered flow-through humidifiers do not require a bypass duct, so they can be installed in tighter spaces. In fact, there is a specific market niche for powered models - geographic areas like the Southeast and Southwest where homes are built on slabs and where HVAC systems are installed in closets, with minimal room for the mechanicals. Cons: Because it includes a separate fan, a powered flow-through humidifier costs more to purchase and operate than an unpowered model.