A hydrogen fuel gas boiler (2,2") comprising a mixer (4,4") to mix together a hydrogen fuel gas and an oxidiser, and a burner (6,6") to burn a mixture of the hydrogen fuel gas and the oxidiser from the mixer; wherein the mixer (4,4") comprises at least one venturi (8,8",10) and a single mixing flow pathway (12,12") extending from the at least one venturi (8,8",10), the mixing flow pathway (12,12") being configured to provide fluid communication between the at least one venturi (8,8",10) and the burner (6,6"); wherein the or each venturi (8,8",10) comprise a mixing chamber (14,14"), a hydrogen fuel gas inlet (16,16") to provide fluid communication between the mixing chamber (14,14") and a source of the hydrogen fuel gas (18,18"), an oxidiser inlet (20,20") to provide fluid communication between the mixing chamber (14,14") and a source of the oxidiser (22,22"), and a venturi outlet (24,24") to provide fluid communication between the mixing chamber (14,14") and the mixing flow pathway (12,12"); wherein the venturi outlet (24,24") of the or each venturi (8,8",10) opens into the single mixing flow pathway (12,12"); characterised in that a portion of a fluid communication pathway between the mixing chamber (14,14") of the or each venturi (8,8",10) and the source of the hydrogen fuel gas (18,18") is a flame arrester; in that the hydrogen fuel gas inlet (16,16") of the or each venturi (8,8",10) is said portion of the fluid communication pathway and as such is the flame arrester and has a hydraulic diameter of less than or approximately equal to the Maximum Experimental Safe Gap (MESG) for the hydrogen fuel gas and gaseous oxidiser mixture; in that said portion of the fluid communication pathway has a thermal effusivity of at least 8 000 J·s -0.5 ·m -2 ·K -1; and in that the Maximum Experimental Safe Gap (MESG) is the minimum Maximum Experimental Safe Gap (MESG) for the hydrogen fuel gas and gaseous oxidiser mixture at any equivalence ratio. wherein the mixer (4,4") comprises at least one venturi (8,8",10) and a single mixing flow pathway (12,12") extending from the at least one venturi (8,8",10), the mixing flow pathway (12,12") being configured to provide fluid communication between the at least one venturi (8,8",10) and the burner (6,6"); wherein the or each venturi (8,8",10) comprise a mixing chamber (14,14"), a hydrogen fuel gas inlet (16,16") to provide fluid communication between the mixing chamber (14,14") and a source of the hydrogen fuel gas (18,18"), an oxidiser inlet (20,20") to provide fluid communication between the mixing chamber (14,14") and a source of the oxidiser (22,22"), and a venturi outlet (24,24") to provide fluid communication between the mixing chamber (14,14") and the mixing flow pathway (12,12"); wherein the venturi outlet (24,24") of the or each venturi (8,8",10) opens into the single mixing flow pathway (12,12"); characterised in that a portion of a fluid communication pathway between the mixing chamber (14,14") of the or each venturi (8,8",10) and the source of the hydrogen fuel gas (18,18") is a flame arrester; in that the hydrogen fuel gas inlet (16,16") of the or each venturi (8,8",10) is said portion of the fluid communication pathway and as such is the flame arrester and has a hydraulic diameter of less than or approximately equal to the Maximum Experimental Safe Gap (MESG) for the hydrogen fuel gas and gaseous oxidiser mixture; in that said portion of the fluid communication pathway has a thermal effusivity of at least 8 000 J·s -0.5 ·m -2 ·K -1; and in that the Maximum Experimental Safe Gap (MESG) is the minimum Maximum Experimental Safe Gap (MESG) for the hydrogen fuel gas and gaseous oxidiser mixture at any equivalence ratio.