Why Does Convection Work the Way it Does?

The principle of convection says that warm air always rises, and cool air always falls. Actually the principle extends to any fluid, but that’s a subject for another day. For now, let’s talk about air.

When air warms up, it expands. As it expands, it becomes less dense in relation to the cooler air around it. And because it’s less dense, it tends to “float,” that is, to rise. The most dramatic example of this phenomenon is a hot air balloon, which works because the burner heats the air up inside the balloon so that it becomes so light it floats in relation to the other air of the atmosphere. It is so light, in fact, that it floats even though it’s carrying a basket and one or more passengers with it.

When air eventually cools in relation to the air around it, the some principle says that it will fall, and the cycle will repeat itself.

Designing buildings that work is all about using the principles of convection so we’re not fighting against them but rather are using them to our advantage.

Our favorite example of this is our trusted wood stove, which provides all the heat for our tiny apartment in the pole barn where we live. We tell people wood stoves warm your cockles, and the reason they do is convection.

Our stove, like most of the modern stoves these days, is a metal box sitting away from the wall so that air surrounds it on all sides. As we heat wood in the stove, convection causes the air around it to rise. Although we can’t see it doing this, it immediately begins fanning out across the ceiling of the room. As it does, it displaces cooler air, forcing it downward. As the stove continues to heat, it continues to cause this convection pattern, and eventually, all the air in the room is warmed. We never are aware of a fan or any air moving device. All we need is convection.

That’s why we say the wood stove warms your cockles, because you slowly become aware that you are warm and comfortable and you never feel a draft of any kind. And lest you think this takes all day, we typically feel comfortable within 30 minutes after starting a fire in the stove.