Circuit system for a fuel cell vehicle at least comprising: - a first circuit which carries a first fluid and which can be operated in heat pump mode and in which there is arranged, in the intended flow direction of the first fluid, at least one compressor, at least one heat exchanger (7, 9) as condenser or gas cooler, at least one expansion member and at least one further heat exchanger (17, 21) as evaporator, wherein at least one first heat exchanger, to which the first fluid can be supplied, is arranged downstream of the at least one expansion member and upstream of the at least one compressor, - a second circuit, which carries a second fluid, normally for cooling at least one traction battery, wherein the second circuit has arranged therein at least one delivery device for the second fluid, the first heat exchanger, to which the second fluid can be supplied for heat exchange with the first fluid, and a heat exchange connection designed for heat exchange of the second fluid with a traction battery, - a third circuit, which carries a third fluid, for cooling a fuel cell arrangement, wherein the third circuit has arranged therein at least one delivery device for the third fluid, a heat exchanger, to which the third fluid can be supplied for heat exchange with the second fluid, and a heat exchange connection designed for heat exchange of a fuel cell arrangement with the third fluid, characterized in that the circuit system further comprises a fourth circuit which carries a fourth fluid, and the fourth circuit has arranged therein at least one delivery device for the fourth fluid, at least one heat exchanger, to which the fourth fluid can be supplied, and/or convector for heating at least one interior region of a fuel cell vehicle and a heat exchanger, to which the fourth fluid can be supplied for heat exchange with the first fluid, wherein this heat exchanger, to which the first fluid can also be supplied, is arranged in the first circuit downstream of the at least one compressor and upstream of the at least one expansion member. - a first circuit which carries a first fluid and which can be operated in heat pump mode and in which there is arranged, in the intended flow direction of the first fluid, at least one compressor, at least one heat exchanger (7, 9) as condenser or gas cooler, at least one expansion member and at least one further heat exchanger (17, 21) as evaporator, wherein at least one first heat exchanger, to which the first fluid can be supplied, is arranged downstream of the at least one expansion member and upstream of the at least one compressor, - a second circuit, which carries a second fluid, normally for cooling at least one traction battery, wherein the second circuit has arranged therein at least one delivery device for the second fluid, the first heat exchanger, to which the second fluid can be supplied for heat exchange with the first fluid, and a heat exchange connection designed for heat exchange of the second fluid with a traction battery, - a third circuit, which carries a third fluid, for cooling a fuel cell arrangement, wherein the third circuit has arranged therein at least one delivery device for the third fluid, a heat exchanger, to which the third fluid can be supplied for heat exchange with the second fluid, and a heat exchange connection designed for heat exchange of a fuel cell arrangement with the third fluid,