Adapter for a flue gas outlet with a first end (2), a second end (4), and a central passageway (6) extending there between. The second end (4) has at least two interconnected cylinder shaped walls (8, 10, 12) having different internal diameters (d1, d2, d3), each cylinder shaped wall (8,10,12) being arranged for sealed engagement with a corresponding pipe to be inserted into the adapter (1). An internal diameter of the at least two cylinder shaped walls (8,10,12) decreases with increased insertion depth (h) as measured from the second end (4) toward the first end (2). The at least two cylinder shaped walls (8,10,12) are eccentrically disposed and mutually tangent in a longitudinal direction of the adapter (1). The adapter (1) further has a pipe securing member (14) circumferentially disposed at the second end (4).