The invention relates to a control mechanism (1) for a gas boiler (2) for combusting combustion gas, the mechanism (1) comprising a flame sensor (3) for detecting the presence of a flame, an ionization electrode (4) for detecting an ionization current, and a control unit (5) to control the operation of the boiler (2), said control unit (5) being connected to the flame sensor (3) and the ionization electrode (4) to acquire a flame detection signal and ionization detection signal, respectively, wherein the control unit (5) is configured to set an operating mode of the boiler (2) based on at least one of the acquired flame detection signal or the acquired ionization detection signal, wherein the control unit (5) is configured to set a first operating mode if the combustion gas comprises hydrogen with a concentration that corresponds to or is higher than a predetermined value, in particular 95 mol%, and the control unit (5) is configured to set a second operating mode if the combustion gas comprises carbons, in particular hydrocarbon, and hydrogen, wherein the concentration of hydrogen is lower than a predetermined value, in particular 95 mol%.