I just held my hand on the pump. I would guess it is 100* or what I am perceiving as "warm". I could hold my hand on it indefinitely and it wouldn't be at all uncomfortable. I went to grab my digital temp gun but it's dead and I can't find a 9v. Much cooler than any other pump I've ever run (multiple different Reeflo, Iwakis, pan world, etc.). Since I am running it externally it wouldn't be fair to compare it to an internal pump that is bathed in a cooling medium, but I would not be at all concerned to run it internally. I just prefer the plumbing style with the unions I have.
Hope that helps,
Ed