NOVELTY - The method involves forming a flow path for a supply flow from a fuel processor (2) to a burner (34). The flow path is opened when the pressure in the flow path exceeds the air flow pressure. The delivery of hydrocarbon fuel to the burner is reduced according to the supply flow from the flow path, to retain a desired supply of heat from the burner and enable a relatively smooth transition from start to running modes.    USE - To form smooth transition from start to running mode in fuel cell system. For system combining burner to generate heat and fuel cell stack to generate electricity. For vehicle technology.    ADVANTAGE - Effective operation of burner and fuel processor. Effective operation of system during start and running modes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for a method to operate a fuel cell system and for a method to operate a burner.    DESCRIPTION OF DRAWING(S) - The figure shows a fuel cell system (Drawing contains non-English language text).    Fuel processor (2,BRENNSTOFF-PROZESSOR)    Fuel flow (6,BRENNSTOFF)    Check valve (7)    Water flow (8)    Air flow (9)    Reactors (12,REAKTOR)    Fuel reforming reactor (14)    Undercurrent reactor, e.g. reactor for selective oxidation (16)    Hydrogen-rich reformate (20)    Fuel cell stack (22,BRENN-STOFF-ZELLE)    Oxidizing agent flow (24)    Exhaust from anode side of fuel cell (26)    Exhaust from cathode side of fuel cell (28)    Compressor (30,COMP)    Valves (31,32,47,50,V)    Burner (34,BRENNER)    Conductors (36)    Regulator (38,R)    Chamber (41)    Inlet end of chamber (42)    Injector (43,Inj)    Outlet end of chamber (44)    Hydrocarbon fuel (46)    Catalyst section (48)    Stop valve (140)    Noise insulator (142,SCHALL-DAEMPFER)    Controller (150,STEUERUNG)    Outlet of anode side (AAUS)    Inlet of anode side (AIN)    Outlet of cathode side (CAUS)    Inlet of cathode side (CIN)    Air flow (LUFT)    Mass flow sensor (MFM)    Pressure monitors (P4,P5)    Memory (SPEICHER)    Temperature monitor (T)