I actually did a bit of research on this topic back in High School, and as it turns out, the way it's displayed doesn't work too terribly well. In the setups we all see on youtube and such, there is no circulation, so after a rather short period of time, heat pockets start to develop, and it loses it's effect. I did a little experiment for my paper, I took a 2'x2'x2' plexiglass box, put one of those things you use to keep water from freezing in the winter in there, and recorded the temps around it. After 5 minutes, it was about 21C farthest from the heater, and close to 40C near it, and climbing. I then put a pump in the cube, with the return and supply lines leading to a 5 gallon bucket of mineral oil, and the oil stayed at 24C right next to the heater for nearly 15 minutes, if I remember right. I didn't use any fans or anything to cool the oil, which might have made it effective. Oh, and I think the cube weighed somewhere around ~70 pounds.