A reversible housing for a humidifier of the type for attachment to the ducts of a forced air heating system. The housing includes a sheet forming a back wall, a pair of side walls and a top wall integrally hinged to the back wall and pivotable through one-half turn with respect to the latter so as to take two operative positions both protruding forwardly of the back wall depending which one of the back wall faces is disposed flat against the duct. The back wall has a central air passage opening bordered with flanges protruding from each of the back wall faces to connect with a duct opening. One of the side walls has a second air passage with flanges protruding from each of its two faces to selectively connect with a by-pass duct to the left or to the right of the housing. An angular cover and a water tray completes the housing and fit the sheet in either one of its two operative positions. A screwdriver is only required to secure the sheet in either one of its operative positions. A deodorizer body is releasably mounted within the humidifier drum.