Recently, I wrote a little article about the stack effect to explain that the flow of air and heat is upward in winter but downward in summer. Turns out, the stack effect is a hot topic. That article has gotten 25 comments so far. When I posted it to the RESNET BPI group on LinkedIn, it got another 22 comments. And John Brooks started a stack effect discussion here at Green Building Advisor that has 61 comments as I write. Based on all this discussion, I’d say that heat does rise!
My response is: Relax and repeat after me: Heat rises. Let’s say it again, just to help you get over your phobia: Heat rises. What is it about that statement that’s wrong? Nothing! It doesn’t ascribe a cause. It doesn’t say the only thing heat can do is rise. It’s just stating an observation. Heat can move upward. It also can move downward, sideways, diagonally, or any direction at all.
In the winter stack effect, heat rises because it’s moving with the warm air that’s less dense than the colder surrounding air. In summer, it moves downward as it follows the cool, dense air. I described all this in the previous article. I also stated clearly at the top of the article that it’s the Second Law of Thermodynamics that drives heat flow, and that law says that the natural flow of heat is from hot to cold.
Yes, it’s true that a lot of people are confused about the nature of heat flow. It’s also true that many in the home performance community are quick to point out that, No, heat doesn’t rise – warm air does. Well, if the warm air is rising, isn’t the dang heat rising, too?
Warm air also rises
The other argument against my explanation was that not only does heat not rise but warm air doesn’t rise either. Nope. It gets pushed up by the cold air below, some say. Bud Poll and I exchanged several comments, and he commented in the Green Building Advisor thread as well. His main beef is with the use of certain words to describe the process:
Avoid the words “pull” and “replacement” in conjunction with warm air moving up.
Here’s old and new for stack effect (up north).
Old: Warm air rises inside our home and moves up and out those leaks in the upper portions of the building while pulling in its replacement air through leaks in the lower areas.
New: Cold air pushes into the lower portions of our homes forcing the lighter warm air up and out through leaks at the top.
As it turns out, a couple of other physics principles shed some light on what’s going on here. First, there’s Newton’s Third Law of Motion, which states: For every action, there’s an equal and opposite reaction. Then, another aspect of the Second Law of Thermodynamics says that air moves from high pressure to low pressure. The cold air and the warm air, in other words, work in concert with each other. Each does its thing because of the assistance of the other.
In the end, I agree with Martin Holladay, who said of Poll’s mission to change the way we talk about stack effect, “I’m skeptical that your new method of explanation gets us any closer to clarity and understanding.”
Rather than arguing about how many angels can dance on the head of a pin, let’s focus on what’s important and try to help folks like the Hartfords in Maine, who had to resort to asking the fuel oil company to take their car in exchange for another tank of fuel oil.
If you agree, repeat after me, Heat rises. (Just don’t forget that it can fall and go sideways, too.) If you don’t agree, I’ll see you at Building Science Fight Club.
Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.