Like Peachey says, a furnace is extremely hot. In the US and Canada, we like to put marshmallows on a stick and roast them over a campfire, or at home in the fireplace. Such marshmallows are delicious, by the way, and can also be sandwiched between two graham crackers, although I prefer mine a bit charred, which doesn’t work so well for the sandwich thing. The sandwich is called a “s’more” (probably a contraction of “some more, please!”) Anyhow, the point is that we roast marshmallows over small, controlled fires, and only a very special person would do so over a large fire. I leave it to you to guess what kind of person that would be. Note also that “shove” has somewhat reckless or violent connotations.

You'll need to give us some context. All I can say is that a furnace is extremely hot.