Heat Rises – But Why?

From primitive fireplaces to state of the art building energy management systems, every heating and cooling system operates on the principle that hot air rises. That’s why attics need insulation, different floors of the same building require different amounts of energy to maintain the same temperature, and hot air balloons can float. Why does this happen? Here’s a brief overview of the science of hot air rising from your trusted Hartford HVAC professionals, the Tradesmen of New England.

Archimedes’ Principle of Buoyancy

Ancient Greek scientist and philosopher Archimedes was the first to realize that an object will float if it displaces enough fluid to equal its own weight. For example, a boat floats because it displaces an amount of water that equals its weight. Therefore, the water will supply an upward force that’s large enough to cancel out the downward force of gravity on the boat. This principle applies to fluids that are liquid, but gases as well. A helium balloon will float in air because the gas inside the balloon is less dense than the air on the outside. That means the air that would normally fill the space the balloon occupies weighs more, and the balloon is forced upward.

Hot & Cold Air

In the same way that helium is less dense than regular air, hot air is less dense than cold air. There is more space between the molecules in the heated air inside a hot air balloon, meaning its height can be controlled by heating and cooling that air. The hot air balloon floats upward because it does not weigh as much as the denser, cooler air surrounding it. Imagine removing the balloon, and the result is the same: a pocket of hot air is forced upward as cooler, denser air falls to lower heights. And that’s why hot air rises.