Abstract: A method of, and a system for promoting the flow of air from a lower location to an upper location, the method comprising: using heat from a heat source to provide a first heat store; and making a secondary use of heat from said heat source to power a heating device for heating an air conduit connecting said lower and upper locations; wherein said secondary use of heat from said heat source is controlled on the basis of an indicator of whether or not there is an excess of heat in the heat store.