Fast FOUP swapping with a FOUP handler

A vertical batch furnace assembly for processing wafers having a cassette handling space, a wafer handling space, and a first wall and separating the cassette handling space from the wafer handling space. The first wall has at least one wafer transfer opening in front of which, at a side of the first wall which is directed to the cassette handling space, a wafer transfer position for a wafer cassette is provided. The cassette handling space comprises a cassette storage having a plurality of cassette storage positions and a cassette handler configured to transfer wafer cassettes between the cassette storage positions and the wafer transfer position. The cassette handler has a first cassette handler arm and a second cassette handler arm.