A humidifier for an HVAC system includes a replaceable humidifier pad, and a housing for holding the replaceable humidifier pad in an operating position during operation of the humidifier. The back side of the housing may define an air flow aperture that is in fluid communication with the replaceable humidifier pad when the replaceable humidifier pad is in its operating position. In some instances, the right side of the housing may define a first aperture that is sufficient in size to remove the replaceable humidifier pad from the housing, and to install a new replaceable humidifier pad. Likewise, the left side of the housing may define a second aperture that is sufficient in size to remove the replaceable humidifier pad from the housing, and to install a new replaceable humidifier pad. The humidifier may also include a first cover member for covering the first aperture when the replaceable humidifier pad is in its operating position, and a second cover member for covering the second aperture when the replaceable humidifier pad is in its operating position. In some cases, an aperture and cover may also be provided on the front and/or top sides of the housing so that the replaceable humidifier pad may be replaced from the front and/or top sides of the housing.