No, it does not mean something warm, it means (in American colloquial speech) that the source of heat was at about the level of the writer's bottom; a convenient way to warm up when they were thoroughly chilled.
The previous sentence says that there is a vent 'in the middle of the wall'. In most adults that comes about as high as their waist or, at the same height, their butt.
It is a common pattern in informal English to describe a height as high as some other known thing, particularly part of one's anatomy. Ankle-high, knee-high, chest-high, or even the inarticulate yay-high where you gesture with your hand the approximate height.
'Butt-high'is not a common usage mostly because 'butt' is a tiny bit taboo. 'Buttocks' is the more formal way to say it, but outside of Forrest Gump, is probably only used by doctors and those who want to sound snooty. There are a number of synonyms, al with their own levels of tabooness or minced -oath sounding. Rear, rear-end, seat, derriere, ass. Of these, 'ass' is the most taboo (in the adult direction, but there's a lot worse), and 'rear' the least.