Fuel cell system, its use and method of its operation

In a fuel cell system, for example HTPEM fuel cells. a valve system is employed by selectively guiding exhaust gas from the burner either to the reformer for heating the reformer, especially during normal operation, or to a heat exchanger while bypassing the reformer in startup situations in order to heat the fuel cell stack before starting heating the reformer. Optionally, a compact burner/reformer unit is provided where the heat from the burner is reaching the reformer by conduction of heat through the intermediate walls.