A high temperature fuel cell system comprising: a fuel cell stack, a compressor and a valve, the fuel cell stack comprising at least one fuel cell, each fuel cell comprising an electrolyte, an anode and a cathode, the compressor being arranged to supply at least a portion of the oxidant to the cathode of the at least one fuel cell, a fuel supply being arranged to supply to the anode of the at least one fuel cell, the fuel cell being arranged to supply a portion of the unused fuel from the anode of the at least one fuel cell to a burner, an oxidant supply arranged to supply the burner, the burner being arranged to supply the burner exhaust gases to a first inlet of a heat exchanger to the valve, the at least a portion of the oxidant from the compressor and the unused oxidant from the cathode of the at least one fuel cell being arranged to be supplied to a second inlet of the heat exchanger to preheat the oxidant supplied to the cathode of the at least one fuel cell, the heat exchanger comprising a temperature sensor configured to measure the temperature at the second inlet of the heat exchanger, the heat exchanger being arranged to supply the at least a portion of the oxidant from the compressor and the unused oxidant from the cathode of the at least one fuel cell from a second outlet of the heat exchanger to the cathode of the at least one fuel cell; and a controller, configured to determine a burner temperature of the high temperature fuel cell comprising a fuel cell stack and a burner, the fuel cell stack comprising at least one fuel cell, the controller configured to compare the burner temperature with a predetermined burner temperature set point to identify a burner temperature difference between the measured burner temperature and the predetermined burner temperature set point, and to control an amount of oxidant supplied to the burner by controlling an oxidant valve to decrease or increase the amount of oxidant supplied to the burner thereby to reduce the burner temperature difference and control a fuel cell stack inlet temperature. a fuel cell stack, a compressor and a valve, the fuel cell stack comprising at least one fuel cell, each fuel cell comprising an electrolyte, an anode and a cathode, the compressor being arranged to supply at least a portion of the oxidant to the cathode of the at least one fuel cell, a fuel supply being arranged to supply to the anode of the at least one fuel cell, the fuel cell being arranged to supply a portion of the unused fuel from the anode of the at least one fuel cell to a burner, an oxidant supply arranged to supply the burner, the burner being arranged to supply the burner exhaust gases to a first inlet of a heat exchanger to the valve, the at least a portion of the oxidant from the compressor and the unused oxidant from the cathode of the at least one fuel cell being arranged to be supplied to a second inlet of the heat exchanger to preheat the oxidant supplied to the cathode of the at least one fuel cell, the heat exchanger comprising a temperature sensor configured to measure the temperature at the second inlet of the heat exchanger, the heat exchanger being arranged to supply the at least a portion of the oxidant from the compressor and the unused oxidant from the cathode of the at least one fuel cell from a second outlet of the heat exchanger to the cathode of the at least one fuel cell; and a controller, configured to determine a burner temperature of the high temperature fuel cell comprising a fuel cell stack and a burner, the fuel cell stack comprising at least one fuel cell, the controller configured to compare the burner temperature with a predetermined burner temperature set point to identify a burner temperature difference between the measured burner temperature and the predetermined burner temperature set point, and to control an amount of oxidant supplied to the burner by controlling an oxidant valve to decrease or increase the amount of oxidant supplied to the burner thereby to reduce the burner temperature difference and control a fuel cell stack inlet temperature.