A heat regenerator of the type comprising, within a wall of refractory material, a space containing a pile of bricks (checker brick) and a burner stack separated therefrom by a separating wall, the cylindrical wall of the heat regenerator and the separating wall consisting of more than one layer of refractory material, in which at least for part of the walls and in particular for the separating wall between checker bricks and burner stack, at least one of the layers of refractory material is subdivided by a vertically extending expansion joint to avoid cracks, damage and gas leaks through the concerning wall or walls. Preferably there is a number of such joints in the separating wall, extending at least over the height of the lower half of the burner stack. The facing edges of the layer at the joint have grooves with sealing bricks engaging therein or tongues and grooves engaging sealingly, with a compressible filler of ceramic wool or felt sealing the joint in cold condition of the regenerator.