Abstract: An air preheater comprises flow conduits for heat transfer between a heating gas and combustion air which is to be pre-heated with heat pipes extending between the flow conduits for the two gas streams to provide heat transfer The fins on the heat pipes in at least the condensation zone of the flow conduit of the heating gas are provided with one or more serrations on the sides of the pipes facing away from the oncoming gas flow. The serrations assist in removing any liquid film which may accumulate on the fins during operation in the condensing mode. A corrosion resistant coating material such as glass may be provided on the condensing side of the finned heat pipes.