This week we’re going to talk about the type of heat transfer that I personally find most confusing: convection. You might find it easiest to start with the previous article on conduction if you haven’t read it already. (The third type of heat transfer, radiation, we’ll discuss in the final article in the series.)
To recap, heat energy is essentially a measure of how, well, energetic the microscopic particles of a substance are. The more those molecules are bouncing around, the more heat energy you’ve got. And conduction is simply the term for heat energy that travels through a substance as those particles bump into each other, sending that energy down the line in a sort of chain reaction.
Although it happens at different rates depending on material properties, conduction happens within solids, liquids, and gases. This is not true of convection. Convection only happens within fluids, which is a term that (in this context) includes both liquids and gases. You can’t have convection within a solid, such as a hot poker.
Convective Heat Transfer Explained
The key to understanding “convection” is the root word hiding in there: “convey.” Convective heat transfer happens when heat energy is conveyed from one location to another by a fluid like air or water. Rather than waiting for that heat energy to spread through a series of microscopic collisions (as in conduction), convective heat energy gets from point A to point B by hitching a ride on a sort of fluid conveyor belt.
This convective heat transfer can happen really quickly, like it does when your furnace kicks on. After your furnace has heated some air, its blower pushes that heated air through your duct system until it reaches your rooms. Again, you’re not waiting on a series of tiny molecular bumper car collisions to spread the heat throughout your house—that heat energy is being pushed in your direction by convection.
In the furnace example, convection is being driven mechanically by your blower motor. That’s called forced convection. However, convection is also naturally occurring within any given room in your house. That’s called natural (or free) convection, but you’re already familiar with the concept through a familiar phrase: “heat rises.”
Technically, it’s not “heat” itself that rises—remember, heat is just a name for microscopic energetic jiggling. For example, heat doesn’t “rise” within a solid; rather, it slowly dissipates or spreads throughout the solid in all directions. It’s more accurate to say that hotter fluids rise relative to colder fluids, assuming there is gravity. (OK, I admit that phrase is much less catchy than “heat rises.”)
Because molecules of hot air or water are more energetic, they are more spread out—less dense—than the lazier molecules of cold air or water, which lounge around in closer proximity to one another. In the presence of gravity, the denser cold air or water sinks to the bottom, and the less dense warm air or water rises to the top. That’s called buoyancy, and the end result is natural convection, with the “heat rising.” Most importantly for our discussion: the heat energy is being moved around by a fluid, not through a fluid.
Convection and Conduction Combined
As you may have guessed, convection and conduction are often, if not always, happening at the same time. The fact that air or water is acting as a conveyor belt for heat energy doesn’t stop those microscopic collisions from happening and spreading heat energy around through conduction. However, the types of heat transfer happen at different rates under different circumstances, usually with one effect dominating. Engineers have fancy math to figure that sort of thing out. The important thing is to realize that although they may be happening simultaneously within the rooms of your home, conduction and convection are best thought of as separate things.
Convection in the Kitchen
As discussed in the previous article, cooking is all about controlling heat transfer, and the kitchen is a great place to find illustrations of the different types. While at the local level of an individual piece of food, cooking is a largely conductive process—the outside of the food gets heated, which conducts to the interior, until it’s fully cooked—convection does play a few major roles in the kitchen.
The most obvious might be in convection ovens—it’s right there in the name!—but let’s start with a conventional oven. In addition to conductive heat transfer through the oven’s air (discussed in the previous article), natural convection occurs within every oven as hot air rises from the heating element. As the hot air rises, cooler air sinks to the bottom and is heated by the element, and the cycle continues. The air movement is important because the rising hot air continuously replenishes the heat energy absorbed by the food being cooked. This conveyor belt of new warm air cooks food more quickly than if we were cooking in a zero-gravity, no-convection environment.*
Convection ovens amplify this effect by using a fan to create forced convection. More hot air blows across the surface of your food than through natural convection during a given period of time, with the result that cook times are typically reduced by about 25%.
You can also see convection at work on the stovetop, most clearly through boiling water. The magic thing about boiling water isn’t the amount of heat per se, it’s the amount of fluid motion created by all the bubbling as the water evaporates. This creates another conveyor belt effect, with more hot water coming in contact with the surface of your food during a given period of time than if you were relying on conduction alone.**
Convection and Material Properties
As with conduction, some materials are better at convection than others. In general, for example, liquids are much better at convection than gases are. I believe—and again, I struggle with convection so take this with a grain of salt—this is due to the greater density of liquids, which results in more efficient heat transfer into the fluid before the heat is conveyed away.
In the article on conduction, I mentioned that 50º water will cool you faster than 50º air. I think it’s partially accurate to think of that as an effect of conduction at the point of contact with your skin, but now you understand that the cooling effect is enhanced by the fact that both water and air act as conveyor belts that whisk heat energy away from your body and then bring new water or air in contact with your skin to repeat the process. Water is simply capable of taking up more of that heat energy at each moment of contact with your skin than air is.
Water and air are probably the only fluids that you have direct experience with, convection-wise, but other fluids are indirectly relevant to your comfort. In particular, the fluids used in air conditioning and refrigeration are chosen for—among other properties—their ability to quickly convey heat energy through the system. (Things aren’t really ever “cooled,” but instead their heat is moved elsewhere.) It actually took many, many years for engineers to find safe, effective, reliable refrigerants. If you’re curious about the topic, the book Refrigeration Nation (affiliate link) has a great summary, and details the long and crazy road we took to arrive at having cold food in your kitchen all year. Many of the same concepts apply to the air conditioning in your house and car.
Convection around the House
Let’s return to the subject of heat transfer and comfort in your home. In the previous article, we talked about how slowing down the process of unwanted conductive heat transfer in and out of your house is called insulating. That’s because insulation is any substance—including air—that is relatively bad at conducting heat.
So how do you minimize unwanted convective heat transfer in and out of your home? The answer is a Green Old Home favorite: air sealing. If you’ve read the original article on air sealing vs. insulating, or even if you haven’t, take a look now. I think you’ll realize the whole discussion is really about the distinction between conductive and convective heat transfer—I was just trying to avoid those technical terms. If you take steps to minimize the gaps in your thermal envelope, you’ll be interrupting those conveyor belts of air that pull heat energy in and out of your home. (Insulation does not usually stop those pathways; remember, insulation slows conduction.)
You might have also realized while reading this article that (natural) convection is directly related to another concept we talk about a lot at Green Old Home: stack effect. Again, take a look at that earlier article and you’ll notice that the whole phenomenon can be thought of in terms of convective heat transfer. As discussed in that article, the effect is exacerbated by a leaky house that allows warm air to escape through the top and replacement air to come in near the bottom.
However, even if your house is well sealed, its top floor will always be warmer than the bottom floor due to natural convection, as will the top half of each individual room. Well-designed HVAC systems attempt to counter these natural effects by strategic placement of supply and return ducts, but if you live in an old home you may not be so lucky. One thing that can help in the winter is to run ceiling fans in reverse, which draws the cool air up from the floor and displaces the warm air down towards you—effectively applying the convection oven principle to your room!
But using fans in warm weather is probably the best example of how convection can work with you instead of against you. If you stand in front of a fan blowing room-temperature air, why do you feel cooler? There are two main answers, both of which have everything to do with convection.
The first reason fans make you feel cooler is similar to why a convection oven fan blowing hot air cooks food faster: whatever capacity the air in the room has to remove heat energy from your body through conduction, a fan speeds up that transfer by blowing that warmed air away from your body and providing a conveyor belt of new air to absorb more heat. For this process to be effective, the air really just has to be cooler than body temperature.
The second reason fans make you feel cooler has to do with an effect we haven’t really talked about yet, which is sweating. We’ll talk more about why sweating cools you off when we talk about the effect of humidity on comfort, but the key concept is that sweating relies on evaporation into the surrounding air. And similar to how fans accelerate the rate at which heat energy can transfer to the surrounding air, fans can accelerate the rate at which your sweat can evaporate into the surrounding air—again, by carrying the “sweaty” air away and providing a conveyor belt of new drier air to absorb more of your sweat. In theory, this process will help to make you feel cooler even if the air is warmer than body temperature as long as the air has the capacity to absorb water (or rather, sweat) vapor.
Maybe you can see now why it was fans that got me thinking about the different types of heat transfer at the start of this series. To really understand what tools to use and when, it’s useful to understand precisely what you’re trying to achieve. For example, you’re not going to cool an enclosed room by using a fan. Think about it: there’s nowhere for the heat energy to go! However, you can use a fan to speed up the transfer of heat energy away from your body.
We’ll talk more about fans in future articles—there are a lot of different types of fans, and a lot of different ways that they can be used to help keep you comfortable year-round in addition to those mentioned above. (And they cost a lot less energy (and money) to run than air conditioning, which is why we love them here at Green Old Home.)
But first, we’ve got to talk about the last type of heat transfer: radiation.
*I’ll admit it, I can’t prove this. But it feels true! If you know more than I do, please feel free to comment or leave a message. One other thing that’s on my mind as I write: it seems to me that at the exact point where oven air meets food, the method of heat transfer has to be conduction. That is, the food is getting hot because the vibrations in the air molecules are translating to the food molecules. (Or more generally, it seems that the mechanism of heat transfer between two adjacent substances is always technically conduction at the point of contact.) Yet the supply of heat energy is being replenished by convection, so the two are sort of working together. If this isn’t true, I hope someone can explain to me why!
**I think that steaming vegetables is probably another example, with the rising steam representing natural convection, though I don’t know enough about the relationship between the water vapor and the air to be totally sure if that counts.