System comprising: - a high-temperature electrolysis or co-electrolysis reactor comprising a stack of individual solid oxide (co-)electrolysis cells each comprising a cathode, an anode, and an electrolyte inserted between the anode and the cathode, the cells being electrically connected in series, the stack comprising two electrical terminals (30, 32) for supplying current to the cells and defining, on the cathodes, first flow chambers and, on the anodes, second flow chambers; - at least one first supply line, capable of supplying the inlet of the first chambers with a mixture of steam and hydrogen or with a mixture of steam, hydrogen and carbon dioxide (CO 2), at a flow rate D H; - at least one second supply line, capable of supplying the inlet of the second chambers with air or with oxygen or with a mixture of oxygen-containing gases, at a flow rate Do; - at least one first discharge line, capable of discharging the hydrogen produced at the outlet of the first chambers; - at least one second discharge line, capable of discharging the oxygen produced at the outlet of the second chambers; - a first heat exchanger, arranged between the second supply line and the second discharge line, so as to recover the heat from the gases leaving the second chambers and transfer it to the gases entering these same chambers in order to preheat said gases; - a bypass line of the second discharge line, arranged between the inlet and the outlet of the exchanger, so as to divert all or some of the gases from the outlet of the second chambers, the portion or all of the gases diverted not then flowing through the exchanger; - a flow control valve, capable of allowing through a flow rate of from 0 to 100%, arranged on the bypass line. - a high-temperature electrolysis or co-electrolysis reactor comprising a stack of individual solid oxide (co-)electrolysis cells each comprising a cathode, an anode, and an electrolyte inserted between the anode and the cathode, the cells being electrically connected in series, the stack comprising two electrical terminals (30, 32) for supplying current to the cells and defining, on the cathodes, first flow chambers and, on the anodes, second flow chambers; - at least one first supply line, capable of supplying the inlet of the first chambers with a mixture of steam and hydrogen or with a mixture of steam, hydrogen and carbon dioxide (CO 2), at a flow rate D H; - at least one second supply line, capable of supplying the inlet of the second chambers with air or with oxygen or with a mixture of oxygen-containing gases, at a flow rate Do; - at least one first discharge line, capable of discharging the hydrogen produced at the outlet of the first chambers; - at least one second discharge line, capable of discharging the oxygen produced at the outlet of the second chambers; - a first heat exchanger, arranged between the second supply line and the second discharge line, so as to recover the heat from the gases leaving the second chambers and transfer it to the gases entering these same chambers in order to preheat said gases; - a bypass line of the second discharge line, arranged between the inlet and the outlet of the exchanger, so as to divert all or some of the gases from the outlet of the second chambers, the portion or all of the gases diverted not then flowing through the exchanger; - a flow control valve, capable of allowing through a flow rate of from 0 to 100%, arranged on the bypass line.