NOVELTY - A fuel cell system includes a fuel cell block (2) and a catalytic reactor (3) converting hydrogen and oxygen to water. One line (8) transfers hydrogen-containing residual gas (RH2) directly from a fuel cell block to the reactor. Another line (9) transfers an oxygen-containing residual gas (AL) directly from the fuel cell block to the reactor. A heat exchanger (11) transfers reactor heat to a fuel cell system coolant, especially to coolant for the fuel cell block.    USE - Used as a fuel cell system with reduced residual gas output, used in vehicular or domestic applications.    ADVANTAGE - Some residual gases always leave a fuel cell stack. The above fuel cell burns these residues, rather than facing the risks of venting, i.e. possible fire or explosion. The fuel cell burns off the gases, supplying them directly through closed lines to the e.g. platinum-based catalytic combustor, hence minimizing hold-up and avoiding potentially-dangerous releases completely. This allows the fuel cell to be operated safely in an enclosed space accessible by an operator. Heat generated is put to good use. It is especially valued during low power modes, for maintaining cell temperature and electrical efficiency. This is important at start-up especially. It can also be used as a source of hot water. The fuel cell suitability in both domestic power and vehicular applications is enhanced.    DETAILED DESCRIPTION - Coolant is recirculated. The heat exchanger (11) transfers heat from the reactor to a hot water circuit (15). The heat exchanger is integral to the reactor. Residual gas flow from the fuel cell block to the reactor is controlled. The reactor is a tube bundle heat exchanger. Either the inner or the outer walls of the tubes are coated with a catalyst promoting reaction of hydrogen with oxygen to form water. The reactor is alternatively a fine pore body, in which the pore surface area is coated with catalyst.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the fuel cell system.    fuel cell block (2)    catalytic reactor (3)    line transferring hydrogen-containing residual gas (8)    line transferring oxygen-containing residual gas (9)    heat exchanger (11)    hot water circuit (15)    oxygen-containing residual gas (AL)    hydrogen-containing residual gas (RH2)