It's not a question of surface area, but that the air you're paying to warm up will all go right to the ceiling. You can easily get temperature gradients of at

It's not a question of surface area, but that the air you're paying to warm up will all go right to the ceiling. You can easily get temperature gradients of at least 5 C on a cold day with a two-story high open set up. That is where ceiling fans come in, though.