A planar heater (16) is mounted in the base of a liquid heating vessel by bayonet means. The bayonet means are formed in part (14) on the vessel and in part on a clamping ring (24) which clamps the heater (16) against a sealing surface on the vessel. In another embodiment (Fig. 8), the heater is provided with a peripheral flange having bayonet fittings which engage with a clamping plate so as to clamp a portion of the vessel wall therebetween.