A vertical batch furnace assembly for processing wafers comprising a cassette handling space, a wafer handling space, and an internal wall separating the cassette handling space and the wafer handling space. The cassette handling space is provided with a cassette storage configured to store a plurality of wafer cassettes provided with a plurality of wafers. The cassette handling space is also provided with a cassette handler configured to transfer wafer cassettes between the cassette storage and a wafer transfer position. The wafer handling space is provided with a wafer handler configured to transfer wafers between a wafer cassette in the wafer transfer position and a wafer boat in a wafer boat transfer position. The internal wall is provided with a wafer transfer opening adjacent the wafer transfer position for a wafer cassette from or to which wafers are to be transferred. The cassette storage comprises two cassette storage carousels.