Impeller pump having - a pump chamber (22, 122, 222) with an inlet into the pump chamber and with an outlet out of the pump chamber, - an impeller in the pump chamber (22, 122, 222), wherein the impeller has a direction of rotation for pumping fluid from the inlet to the outlet, and wherein the pump chamber is formed in a ring-shaped fashion around the impeller, - an auxiliary outlet out of the pump chamber together with an auxiliary outlet flap (44, 144, 244), wherein the auxiliary outlet flap (44, 144, 244): - has a closed position and at least one open position, - is designed to be movable between the closed position and the at least one open position, - in the closed position, closes off the auxiliary outlet and, in each of the open positions, at least partially unblocks or opens the auxiliary outlet, - has an actuating means (159, 244) and is subjected to force loading by the actuating means such that the auxiliary outlet flap (44, 144, 244) is moved automatically from the closed position into one of the open positions if the auxiliary outlet flap is free from fluid flow in the direction of rotation of the impeller for pumping fluid from the inlet to the outlet, - in the closed position, forms a continuous continuation of the profile of the pump chamber or of a wall (24, 29, 129, 229) of the pump chamber in the region around the auxiliary outlet, - has an inner side (46, 146, 246) which, in the closed position, points toward the auxiliary outlet, characterized in that - the auxiliary outlet is arranged in an end surface (29, 129, 229) of the pump chamber in an axial direction along an axis of rotation of the impeller, wherein the end surface is a base surface (29, 129, 229) or a cover surface of the pump chamber, - an outer side (45, 145, 245) opposite to the inner side (46, 146, 246) of the auxiliary outlet flap (44, 144, 244) extends, in the closed position, in a continuous fashion in said end surface (29, 129, 229), - the impeller has two cover surfaces (32, 33), and the end surface (29, 129, 229) with the auxiliary outlet runs approximately in a plane of one of said cover surfaces, - the auxiliary outlet flap (44, 144, 244), in an open position with the maximum degree of opening, is situated close to the impeller, and the inner side of the auxiliary outlet flap (44, 144, 244) is at least on the axial level of and radially outside the other cover surface of the impeller, such that fluid flows out of the impeller directly outward in a radial direction into the auxiliary outlet flap (44, 144, 244) and the auxiliary outlet. - a pump chamber (22, 122, 222) with an inlet into the pump chamber and with an outlet out of the pump chamber, - an impeller in the pump chamber (22, 122, 222), - an auxiliary outlet out of the pump chamber together with an auxiliary outlet flap (44, 144, 244), - has a closed position and at least one open position, - is designed to be movable between the closed position and the at least one open position, - in the closed position, closes off the auxiliary outlet and, in each of the open positions, at least partially unblocks or opens the auxiliary outlet, - has an actuating means (159, 244) and is subjected to force loading by the actuating means such that the auxiliary outlet flap (44, 144, 244) is moved automatically from the closed position into one of the open positions if the auxiliary outlet flap is free from fluid flow in the direction of rotation of the impeller for pumping fluid from the inlet to the outlet, - in the closed position, forms a continuous continuation of the profile of the pump chamber or of a wall (24, 29, 129, 229) of the pump chamber in the region around the auxiliary outlet, - has an inner side (46, 146, 246) which, in the closed position, points toward the auxiliary outlet, - the auxiliary outlet is arranged in an end surface (29, 129, 229) of the pump chamber in an axial direction along an axis of rotation of the impeller, wherein the end surface is a base surface (29, 129, 229) or a cover surface of the pump chamber, - an outer side (45, 145, 245) opposite to the inner side (46, 146, 246) of the auxiliary outlet flap (44, 144, 244) extends, in the closed position, in a continuous fashion in said end surface (29, 129, 229), - the impeller has two cover surfaces (32, 33), and the end surface (29, 129, 229) with the auxiliary outlet runs approximately in a plane of one of said cover surfaces, - the auxiliary outlet flap (44, 144, 244), in an open position with the maximum degree of opening, is situated close to the impeller, and the inner side of the auxiliary outlet flap (44, 144, 244) is at least on the axial level of and radially outside the other cover surface of the impeller, such that fluid flows out of the impeller directly outward in a radial direction into the auxiliary outlet flap (44, 144, 244) and the auxiliary outlet.