The bypass humidifier works in conjunction with the furnace fan. As soon as it is time for the humidifier to turn on, with the furnace fan running, water begins to rise through a tube into the distribution tray at the top of the unit. The water is evenly distributed across the entire width of the tray due to special holes and recesses. Under the influence of gravity, it flows down to the humidifying surface. The dry, warm air passes through the water-saturated humidifying surface, the water evaporates and the air is humidified with steam.