I am going largely by hearsay on this...
Two reliable rabbis have told me that if you want to heat kosher food in a non-kosher oven, you can double wrap the food in foil and heat it in the oven, and your food remains kosher.
Can this method be used for chametz food? In other words, say the oven has been koshered for Pesach. Someone wants to keep it Kosher for Pesach. But, he has chametz food that he wants to heat before Pesach. Can he double wrap the chametz food in foil and heat it and still leave the oven Kosher for Pesach?
Regarding this, does chametz have any rules that are different than kosher food the rest of the year?