A heater (10) of the kind which includes a vessel (11) in which liquid is contained, and means (20) to heat the liquid in the vessel has an inlet (13) by which liquid can be fed into the vessel, an outlet (18) through which liquid may flow to a point of delivery (19), the inlet comprising a supply conduit (13) connected to a liquid supply, means (14 a ) to mix gas with liquid flowing along the supply conduit (13), a gas trap (25) to collect gas passing with the liquid from the supply conduit within the vessel, the gas trap including means (30) to release gas collected thereby into the vessel, at least when the liquid flow along the supply conduit (13) is stemmed, the released gas providing an expansion space (32) in the vessel into which liquid can expand during subsequent heating, the expansion space (32) being vented from the vessel.