Heat exchanger valve (8) with thermostat valve (12), a housing (9) with inlet (20) and outlet (21), a thermostat bellows displaceable spindle (30) with seal (42) which, by the expansion/contraction of the thermostat bellows and against the pressure of a surrounding spring (31), can close/open an opening (27) with a seating (27a) in a transverse wall (24) between the inlet (20) and the outlet (21), and a piston (13) controlled by differential pressure for the control of a medium's differential pressure across a pre-setting (11) and a thermostat valve (12) consisting of the seating (27a) and the seal (42) between the inlet (20) and the piston (13), the thermostat valve (12) being able to cut off the flow between the inlet and the outlet independently of the pre-setting (11). With the valve (8), only a predetermined maximum amount of the heat-carrying medium flows through the heat exchangers (5). If the differential pressure is increased when the valves on a pipe length are closed, the flow in the individual heat exchangers (5) is not increased.