Duct ventilation fan

A duct ventilation fan comprising:

• a fan casing having an intake port and an exhaust port;
• an impeller housed in the fan casing and configured to rotate about a rotation axis and to discharge air in a radial direction;
• and
• a duct connection part having a duct connection port communicating with the exhaust port of the fan casing,
• the fan casing including
• a suction side flat plate disposed opposite to the impeller and having the intake port,
• a side wall having a scroll shape ranging from a winding start portion to a winding end portion,
• a diffuser having a cylindrical shape connecting the winding start portion and the winding end portion to the exhaust port, and
• a tongue portion connecting the winding start portion and the diffuser,
• the diffuser including
• a first wall extending from an end of the side wall that is adjacent to the winding end portion to the exhaust port,
• a second wall extending to the exhaust port along a same plane as the suction side flat plate,
• a third wall opposite to the second wall, and
• a tapered surface disposed at a corner between the first wall and the second wall and decreasing an air passage cross-sectional area of the diffuser along a flow direction of the air in the diffuser,
• the duct connection part including
• a shutter configured to open and close the duct connection port, and
• a shutter rotation shaft on which the shutter is rotatably mounted,
• when a tapered end that is a side of the tapered surface that is adjacent to the exhaust port, the shutter rotation shaft, and an outline of the exhaust port are projected on a virtual plane orthogonal to a flow direction of the air flowing through the duct connection port, an extension line of the shutter rotation shaft intersecting the third wall and the first wall,
• on the virtual plane, the shutter rotation shaft intersecting an extension line of the tapered end.