Abstract: A structure and method which substantially reduce the number of run-in substrates that have to be used in a high temperature (550° C. or greater) processing environment is presented. A barrier to conductive heat transfer is provided between a process gas distribution faceplate and its process chamber support. This allows the gas distribution faceplate to thermally float and substantially reduces the temperature transients in the faceplate, which can cause thermal (temperature) transients when wafer processing is begun. The present configuration uses a thermal separation assembly to substantially block conductive heat transfer to the cold processing chamber, by using a Vespel gasket or stainless steel washers and thereby reduces the thermal gradient experienced by the gas distribution faceplate. As a result of the improved thermal uniformity, the number of run-in wafer that need to be used is reduced by 80 to 95%.