An expansion tank (28) comprising a passage (34) with a an inlet aperture (34a) for liquid replenishment of the expansion tank (28), a cover (32) which in a non-fitted state leaves the passage (34) clear and in a fitted state closes the passage (34), and a first expansion chamber (29) for receiving liquid which circulates in a first system (A). The expansion tank (28) includes a second expansion chamber (30) for receiving liquid which circulates in a second system (B) and said passage (34) divides, at a distance from the inlet aperture (34a), into a first branch (34b) which leads liquid to the first expansion chamber (29) and a second branch (34c) which leads liquid to the second expansion chamber (30).