After a brief discussion of typical heat transfer problems which arise in unwanted fires, a discussion is given of heat transfer from two-dimensional gravity currents of hot gas which are found in unwanted fires. High rates of heat transfer are observed between these currents and the ceiling and this heat loss strongly affects the motion of the current. The high rates of heat transfer are shown to be the result of a gravitational instability in the cool boundary layer which forms between the current and the ceiling.