To prevent disruptions in downstream processes due to non-uniform flows arising from the switching operation in two interchangeable regenerator systems, both the feed of a cold fluid to the cooled regenerator and the feed of a hot fluid to the heated regenerator are interrupted, the hot fluid is then introduced into the cooled regenerator while the cold fluid is first conducted through a heat accumulator connected in parallel with the regenerators; a portion of the fluid heated in the heat accumulator is fed into the heated regenerator to purge residual hot fluid therefrom and subsequently the cold fluid is passed in its entirety through the heated regenerator. Particular application is in preventing disruption in the flow, composition and temperature of air of combustion preheated by flue gas.