This subreddit of course loves hot water because it is used to make that magical bean juice that we love. A common theme I constantly see on here however is people fretting over the material that their favorite brewer or kettle is made out of. While this does have an impact on heat loss, it’s a bit of a misconception that this will ultimately make a HUGE impact on your final brew temperature. And frankly it’s driving me nuts that I keep seeing this here.
Heat loss does occur by a hot object transferring heat to the air, but what often gets overlooked is how fast this actually occurs. Conductive/Convective and Radiative cooling are how for example a hot kettle will give off heat to its surroundings. This is when the kettle gives off heat by emitting infrared radiation (raditation cooling) or by interacting with the air/other surfaces to transfer heat (Conductive/Convective cooling). While these are common ways we might think of an object cooling off, these modes are actually quite slow. Radiative cooling especially is very slow, that’s why vacuum insulated bottles are actually quite effective at keeping your drinks hot or cold even though they are made out of metal. So for the purposes of coffee making, it’s not really relevant. Thus let’s focus instead on how water loses heat to the air.
In addition to Conductive/Convective, Evaporative cooling is another way heat is lost. This is when water gives off water vapor to the surrounding air. In the process heat both is requires to generate the vapor and heat is also carried away by the vapor. This is by far the fastest way a body of water will cool down and it’s not even close. To really show this off here’s a quick experiment you can do yourself. Take your favorite kettle and heat it up. Measure the temperature at the start and then let it sit for 10 minutes and measure it again. Try to avoid keeping the lid open for long during this first one if you can. For the second run heat up the water, measure the temperature, and then leave the lid open for 10 minutes. Measure the temperature after 10 minutes. You’ll find there’s a massive difference. Here’s some results I took using a thermometer gun.
Lid closed for 10 min Initial:
Lid closed for 10 min End:
Lid open for 10 min Initial:
Lid open for 10 min Final:
And actually this kettle is not perfectly sealed, even with the lid closed there’s still some vapor that escapes through the pouring spout. This is far from a top of the line kettle as well. You could probably find one with better insulation. But even so, the heat loss is still pretty low so long as the kettle remains shut. And before someone chimes in and complains about the experimental set up. Do it with whatever high precision method you want! You’ll get similar results. This is well studied physics that has been known for centuries now.
It can take 10 minutes or more for an economy metal kettle to lose 2-7 degrees C in temperature. With an open kettle on the other hand it lost almost 20 degrees C in the same time! This will depend of course on the kettle as different kettles have different heat loss rates depending on the material and surface area. But when it comes to coffee making this largely is irrelevant for two reasons. First most coffee brewing methods will not require you to leave the kettle sitting for this long before you use it. French Press? You’ll pour and most of the time will be consumed by waiting for the brew. Pour over? Not going to take more than a few minutes between adding water even with the bloom. Moka pot? You’re heating it anyways. You get the point. Second if you use an electric kettle you can simply reheat the water right before you pour. This is not what is going to be ruining your brew temperature if you handle it properly!
The biggest heat loss is when you actually pour the water and expose it to the air! Not just because the air is stealing heat, but because your water is losing heat via evaporation! This I might add is why your body uses water to cool itself. Your body uses evaporative cooling by way of sweating because it’s way more efficient way of giving off heat than other methods. Inside of a kettle the water vapor is largely trapped inside. This both decreases evaporation rate because the air inside the kettle is saturated and also keeps heat from being transported away by water vapor. If you’ve ever been to say Flordia or other humid areas, it’s harder to sweat in humid air than dry air. Same physics apply here.
Evaporation will likely be influenced by things such as pour height, surface exposed to the air, etc. I would have to test pour height, but the reasoning there is the act of moving through the air increases evaporation and air contact time. It’s the same principle to how a fan cools you off, moving air (or moving water through the air) will increase heat loss. Also higher pour heights may cause the water to break up into smaller droplets which increases surface area and thus more heat is lost to evaporation and the air.
And even after you pour, very little heat is actually lost to say the walls of a V60. This assumes if you preheat it of course as pouring into a cold V60 will cause the water to heat up the V60. But once the V60 is up to the same temperature as the water, it’s a much slow process to go from water > V60 > air. The primary heat loss will always be through the top of the brewer that’s open to the air not the walls. So if you want to focus your energy on keeping your brew temperature as high as possible, figure out ways to reduce heat loss via evaporation!
Focusing on the kettle apart from just making sure to use the water right after you heat is a largely a waste of time from a physics perspective. What happens after the water leaves the kettle is more important for keeping heat up.