The Aprilaire 45 Water Panel is a direct replacement for the Aprilaire 400 series whole home humidifiers (400, 400A, and 400M). The Aprilaire 45 water panel is engineered for one full season of use before it needs replaced. For those who only humidify during the winter season, it is common practice to replace the humidifier water panel after the season is over so the wet pad does not sit and grow mold and bacteria. It is also good practice so your humidifier is ready to go once you need it again for the following winter season. The OEM Aprilaire 45 water panel measures 10″ x 12-3/4″ x 1-3/4″ and there are two included in each box. Water panel is also be referred to as a humidifier pad, humidifier wick, or even a humidifier filter – they are all the same thing.