I may be wrong, but I really don't think there's any such thing as negative or positive heat. Heat is one of the forms of energy, so it can never pop up out of nowhere, and it can never disappear. Like all energy, heat just moves from place to place. If it flows out of one thing, then it flows into something else. So I don't think it's ever correct to say that the heat that's over there, or the heat that's moving from here to there, is positive or negative.