Abstract: A heating unit for heat-transfer fluid for a central heating installation including a tubular external wall, a first end wall and a second end wall forming a substantially cylindrical space; a first tubular internal partition located substantially concentrically within the tubular external wall and forming a first annular space between the tubular external wall and the first internal partition; a second internal partition having a diameter smaller than the first internal partition located substantially concentrically within the first internal partition and forming a second annular space between the first internal partition and the second internal partition; a central pipe located inside the second internal partition, the second annular space being in communication with the central pipe adjacent to the first end wall and in communication with the first annular space adjacent to the second end wall; an inlet orifice opening out of the first annular space adjacent to the first end wall; an outlet orifice in the

Abstract: The invention concerns a heating unit comprising a tubular wall (1) and first and second end walls (2, 3). A first annular space (6) is located between the tubular wall (1) and a first tubular partition (4). A second annular space (7) is located between the first partition (4) and a second tubular partition (5) inside the first partition (4). An immersion heater (25) is mounted in the central channel (8) formed by the second partition (5). The second space (7) communicates with the central channel (8) near the first end wall (2) and with the first space (6) near the second end wall (3). An intake orifice (13) emerges into the first space (6) near the first end wall (2) and an outlet (14), in the second end wall (3) emerges into the central channel (8).