Abstract: An indexing valve for a regenerative thermal oxidizer (RTO). A valve housing includes a diverter assembly further comprising: a pipe shaft, the pipe shaft disposed through the valve housing; a diverter blade subassembly connected to the pipe shaft between the pipe shaft ends for rotating within the valve housing, the rotation of which is supported by a bearing assembly at the base; the diverter blade subassembly further comprising a pair of opposing diverter plates, each of the diverter plates curved, wherein a large-volume inlet plenum is defined between the diverter plates for receiving fluid flow from one of the ports. To aid in sealing, the corner section ends within the housing extend beyond a width of each corner and are chamfered to limit wear of the seal. Fluid flow is either atmospheric pressure or negative pressure relative to said valve housing to further aid in sealing.