AprilAire introduced the first evaporative humidifier for forced air furnace systems back in 1954. Since that time, AprilAire continues to lead the industry in whole home humidification systems. Our evaporative humidifiers work by piping water into the distribution tray located at the top of the humidifier. The water is uniformly distributed across the width of the tray and through a scientifically designed system of outlets. It flows by gravity over the Water Panel evaporator. Dry, hot air from the forced air furnace moves through the moisture-laden Water Panel evaporator. Natural evaporation takes place, meaning the water is turned to vapor, and the humidified air is circulated throughout your home. This AprilAire 4788 Humidifier Maintenance Kit contains all components for step-by-step maintenance of your AprilAire Whole Home Humidifier, models 400, 400A, 400M.