To prevent the carry-over of contaminated air to the fresh air in rotary heat regenerators, a small part of the incoming fresh air is used to purge the heat transfer matrix during its transfer from the hot to the cold zone. This purging causes a loss in the effectiveness of heat transfer in the regenerator. A simple numerical scheme employing heat transfer theory for elemental heat exchangers has been developed. This scheme was obtained by dividing the gas stream and the ‘stream’ of matrix material into a number of flow channels, and the effect of fresh air purging has been calculated over a wide range of parameters. The results are presented numerically, graphically and in the form of approximate algebraic expressions.