I had a cup with about an ounce of whey protein and water. I filled it up with hot water ( from the tap, but possibly yad soledes, it was uncomfortable to leave my hand under it) and poured it down the fleishig sink/drain. Do I have to kasher the sink? Thank you!
Since what you did is considered pouring from a klei sheni (i.e. the cup) and their is also some doubt if the water was yad soledes, therefore it is unnecessary to kasher the sink. (If you want to be machmir (stringent) you can pour boiling hot water from the electric urn on to the sink, providing it is made from metal and not porcelain).
Y:D 105:2, Shach 5