A rack for holding flat panels containing PCM while the flat panels are conditioned and a method of conditioning the flat panels. The rack includes a frame structure defining an interior through which a conditioning air flow is directed. The interior includes a plurality of vertically spaced shelves for supporting the flat panels in a vertically spaced apart arrangement. The shelves are configured to hold the flat panels with at least a majority of their upper and lower surfaces in contact with the conditioning air flow. A conditioning method includes arranging the flat panels in a spaced apart array in the rack to create air flow channels between adjacent surfaces of the flat panels. Air is circulated between and in contact with the flat panels at a first temperature sufficient to bring the temperature of the PCM in each flat panel at least closer to a required temperature range.