By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
Question: May one cook parve food in a fleishig pot and eat it together with milchigs? For example, may one use hot water cooked in a fleishig pot for a milchig coffee?
Answer: Parve food that has been cooked in a fleishig pot may be eaten together with milchigs, as long as the pot was not ben-yomo (the pot had not been used for hot meat, or hot meat residue, within the previous 24 hours). However, this should not be done l’chatchilah (with the intention of eating that food with milchigs), unless there is no other pot available.
Some poskim suggest that a different halachah applies to water. For example, water cooked in a non-ben-yomo fleishig pot should not be used to make a milchig coffee. This is because water is readily available, and pouring it out is of no consequence. However, if there is no other pot available, one may boil water l’chatchilah in a not ben-yomo fleishig pot with the intention of using it with milchigs.
Since a kettle is often kept on a blech or hotplate next to fleishigs on Shabbos, it may inadvertently become halachically fleishig; one should therefore preferably be careful not to use water from a kettle that has become fleshig to make a milchig coffee, tea, or to warm up milk in a baby bottle—unless, twenty four hours have passed and there is no other utensil available for boiling water.