After installing of HEPA filter in dry heat depyrogenation oven we made the initial sterilization (burning of high tempereture HEPA filter according to procedure recommended from filter supplier), after that three normal sterilization cycles and than the validation was performed. Validation of the filter passed and we starte with one more sterilization cycle without anything inside the oven.
After half an hour the operators felt the smell of burning and the smoke was going out from the oven. When we opened the oven we fouung that there were oil particles inside the chamber and that the filter media was brown.
It is imortant to note taht we installed completely the same filter (ordered, produced and shipped together), performed completely the same validation and more than 250 sterilization cycles - filter media was white.
Please try to explain what could cause this happen.