Chimney sweeps certainly have seen flames before, but not like this. A CSIA instructor wanted to demonstrate the principles involved with hot air, so he brought in one of those sky lanterns you see lit and allowed to float outdoors.

(A sky lantern, or Chinese lantern, is a hot air balloon made of paper, with an opening at the bottom where a small fire is suspended.)

CSIA’s laboratory is not 100 percent climate controlled, so it can get a little humid and warm on a hot summer day. It was a good place to show that even in a room full of hot air, even hotter air will show a tendency to rise.

“Hot air rises because when you heat air (or any other gas for that matter), it expands. When the air expands, it becomes less dense than the air around it. The less dense hot air then floats in the more dense cold air much like wood floats on water because wood is less dense than water. This floating effect in a less dense medium is called a buoyant force or a displacement force.” [source: UC Santa Barbara]

Hot air comes into play for chimney sweeps in regards to the homeowner and the function of their fireplace.

If your chimney is functioning correctly, the heated air from the fire is exiting up and out of the firebox. The chimney also exhausts the products of combustion — again, when it’s working properly.

The lantern in our demonstration (used as part of our two-day Chimney Physics class at CSIA) did not automatically float — the flame inside had to heat the air in the interior sufficiently to cause the full balloon effect, which took place after a few minutes. The balloon also had to overcome the weight of the safety string tied to the bottom to keep it in the instructor’s grasp.

WATCH THE VIDEO: Chimney Physics demonstration involving a sky lantern

CSIA Instructor Michael Van Buren of Essex Junction, Vermont, explained: “The whole purpose was to get across the fact that hot air rises, and to get them (students) to think of the house as a system — and to think about the house as a hot air balloon. To show that air rises up in a house.”

“As it burned, it got lighter,” Van Buren, who was teaching CSIA’s two-day course, explained. “Just a visual that they will remember. A kind of a wow visual … it gets the point across.” The video will be added to our online chimney physics course.

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Ashley Eldridge, CSIA Director of Education, added that the consumer can learn a lot from this lesson: “Most people are familiar with the concept that hot air rises. If you believe that, you have to accept that cold air falls. So the warm air is being displaced by the cooler/unheated air. ”

“That may be the reason your fireplace doesn’t work when it’s not especially cold – the temperature differential is not that great,” Eldridge explained.

Contact your local CSIA Certified Chimney Sweep if you have questions about any aspect of your chimney. Use our zip code finder if you don’t already have one in your community.