Circuit breakers and gas insulated switchgear (GIS) filled with SF6 (sulphur hexafluoride) represent the state of the art in high voltage switching equipment. Nevertheless the discussion on climate change forces the search for possible SF6 substitutes resulting in first CO2 (carbon dioxide) filled circuit breakers and circuit breaker prototypes available. Therefore the optimization of the switching chamber for the thermal and thermodynamic properties of CO2 is in the focus of this contribution. For this purpose nozzle systems with two heating channels supplied from two heating volumes are investigated. Their thermal interruption capability is determined at varying filling pressures from p = 0.35 MPa to 1 MPa and current amplitudes of Î > 20 kA. Here the maximum thermal interruption capability is found for an absolute filling pressure of p = 1 MPa. The experimental results are compared to those of CFD (computational fluid dynamics) simulations and first design criteria for switching chambers with two heating channels are determined.