<p>A heater for a room heating system is provided. The heater comprises a heating element and a housing for the heating element. The heater includes an inlet for a thermal distribution liquid into the housing and an outlet for the liquid from the housing. The housing is arranged to contain the liquid as it follows a flow path from the inlet to the outlet over the heating element. The housing is adapted to be mounted in a predetermined orientation to provide a top side and a bottom side of the housing. A first thermal cut off is positioned substantially at or below the top of the heating element when the housing is in the predetermined orientation. The heater is a backup heater in a room heating system mostly powered by a heat pump.</p>
