The present invention relates to an apparatus and to a method for filling a solution bag for dialysis with a liquid, wherein the apparatus comprises a needle for introduction into a filling line or filling opening of the bag, wherein the needle is a double wall needle having a preferably thermally conductive outer wall and having an inner wall, with a thermal insulation being present between the outer wall and the inner wall; and with a heating element being provided that is configured to heat the outer side of the outer wall to a temperature above the temperature of the inner side of the inner wall.