I think there's a lot of variance in the way people use HN (I don't upvote stories much either, though plenty must, given the vote counts we see), but dang has had plenty of time (i.e., nearly 8 years in the job) to observe what combination of upvotes, comments and other behaviour may indicate a possible flamewar.

I’m sure some heuristic that combines comments per hour, average thread length, and ratio of comments to upvotes could do a pretty good job detecting flame wars in a community the size of HN.