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 as condenser or gas cooler, at least one expansion member and at least one further heat exchanger 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 as condenser or gas cooler, at least one expansion member and at least one further heat exchanger 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, <EOS>