Heating or cooling system comprising a) at least one heat/cold generator (7, 21, 70), and b) at least one system subunit with a heat/cold exchanger which is used as a system separation, a mixing device (13, 13a, 61) which has two inlets (E1, E2 or E3) and one outlet, and a pump (15, 65), aa) the outlet of the mixing device (13, 13a, 61) being connected via an unbranched inlet section to a mixer-side inlet of the heat/cold exchanger, bb) the pump (15, 65) being provided in the unbranched inlet section or in an unbranched outlet section of the heat/cold exchanger, and cc) the mixing device (13, 13a, 61) comprising a controllable 4-way mixer valve with an outer first inlet, a middle second inlet and an outer third inlet, it being possible internally for either the first inlet and the second inlet or the second inlet and the third inlet to be open at the same time, characterized in that dd) one of the outer inlets (E1, E3) of the 4-way mixer valve is closed externally, and ee) a mixer-side outlet of the heat/cold exchanger is connected via the outlet section in an unbranched manner either to eea) a return line of the at least one heat/cold generator (7, 21, 70) or of another heat/cold source (7, 30, 66, 80, 82) of the heating or cooling system, or eeb) a feed line of a heat/cold sink (10, 30) of the heating or cooling system. a) at least one heat/cold generator (7, 21, 70), and b) at least one system subunit with a heat/cold exchanger which is used as a system separation, a mixing device (13, 13a, 61) which has two inlets (E1, E2 or E3) and one outlet, and a pump (15, 65), aa) the outlet of the mixing device (13, 13a, 61) being connected via an unbranched inlet section to a mixer-side inlet of the heat/cold exchanger, bb) the pump (15, 65) being provided in the unbranched inlet section or in an unbranched outlet section of the heat/cold exchanger, and cc) the mixing device (13, 13a, 61) comprising a controllable 4-way mixer valve with an outer first inlet, a middle second inlet and an outer third inlet, it being possible internally for either the first inlet and the second inlet or the second inlet and the third inlet to be open at the same time, characterized in that dd) one of the outer inlets (E1, E3) of the 4-way mixer valve is closed externally, and ee) a mixer-side outlet of the heat/cold exchanger is connected via the outlet section in an unbranched manner either to eea) a return line of the at least one heat/cold generator (7, 21, 70) or of another heat/cold source (7, 30, 66, 80, 82) of the heating or cooling system, or eeb) a feed line of a heat/cold sink (10, 30) of the heating or cooling system. aa) the outlet of the mixing device (13, 13a, 61) being connected via an unbranched inlet section to a mixer-side inlet of the heat/cold exchanger, bb) the pump (15, 65) being provided in the unbranched inlet section or in an unbranched outlet section of the heat/cold exchanger, and cc) the mixing device (13, 13a, 61) comprising a controllable 4-way mixer valve with an outer first inlet, a middle second inlet and an outer third inlet, it being possible internally for either the first inlet and the second inlet or the second inlet and the third inlet to be open at the same time, dd) one of the outer inlets (E1, E3) of the 4-way mixer valve is closed externally, and ee) a mixer-side outlet of the heat/cold exchanger is connected via the outlet section in an unbranched manner either to eea) a return line of the at least one heat/cold generator (7, 21, 70) or of another heat/cold source (7, 30, 66, 80, 82) of the heating or cooling system, or eeb) a feed line of a heat/cold sink (10, 30) of the heating or cooling system.