A humidifier including a humidifier housing with an adjustable backplane configured to fit to a variety of humidifier pad sizes. Different sizes of humidifier pads are available, and the choice of humidifier pad may depend on the capacity of the HVAC system. The backplane of this disclosure may be adjusted by moving to one position to accommodate a larger humidifier pad or to a second position for a smaller humidifier pad. The backplane of the humidifier housing is configured to mount over an opening of an air duct of an HVAC system such that air is directed along an air path defined by the humidifier housing, through the humidifier pad positioned in front of the air duct and into the air stream of the HVAC system. Water flows through inlet tubing to a water distributor, which may direct the water to the top of the humidifier pad.