Abstract: A space heater is disclosed, comprising a housing, a heat source within the housing and a plurality of openings to allow the convection of air through the housing, in which: the heat source comprises a pair of panels defining between them a closed compartment, a liquid within and partially filling the compartment and an electrical heating element extending within the compartment, substantially parallel to the panels and in thermal contact with the liquid; the compartment is sub-divided into a lower liquid chamber and an upper expansion chamber and the cross-sectional area of the liquid chamber where it meets the expansion chamber is less than that of the expansion chamber; and the openings in the housing allow radiant heat from the heat source to escape from the housing. In further embodiments, the lower liquid chamber contains liquid at room temperature, but the upper expansion chamber does not.