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If air is a bad heat conductor, how does fire heat up a room?
Could someone help me, as I really don't get this?

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    $\begingroup$ compared to good conductors it’s a bad conductor, but compared to BAD conductors it’s not that bad 😎 $\endgroup$
    – szulat
    Jun 28, 2021 at 13:03
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    $\begingroup$ Being a bad heat conductor doesn't mean having no thermal conductivity at all. $\endgroup$
    – DKNguyen
    Jun 28, 2021 at 22:44
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    $\begingroup$ Radiation, Convection. Air loves to blow around, spreading the love. And air is virtually perfectly transparent, allowing the radiant heat to pass right through it.... Conduction is almost completely irrelevant. $\endgroup$
    – PcMan
    Jun 29, 2021 at 9:33
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    $\begingroup$ if air were a good conductor, you would be instantly burned by the fire meant to heat up a room. that's the matter of perspective! :) $\endgroup$
    – szulat
    Jun 29, 2021 at 20:22
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    $\begingroup$ Same way the Sun heats up the Earth even though vacuum doesn't conduct heat directly. $\endgroup$
    – Paddy
    Jun 29, 2021 at 22:54

8 Answers 8

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There are three mechanisms at play: conduction, convection and radiation. Radiation is the most immediate. Your environment irradiates you with black body radiation at room temperature (assuming that you are in a room at 20 C/ 293 K). As soon as your fire burns it emits black body radiation at a temperature of a 600 C (900 K), that is mostly in the infrared. The power emitted by a black body is proportional to $T^4$ (Kelvin), according to the Stefan-Boltzmann law, and this radiation is quite intense. Also the air will be heated by your fire and hot air will reach you by convection. The least important effect is air conduction.

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    $\begingroup$ I think it would be better to use Kelvin in this answer. Otherwise the scaling with $T^4$ might be misinterpreted. The ratio of power emitted by fire compared to air is approximately $\left(\frac{800 K}{300 K}\right)^4 \approx 50$, not $\left(\frac{600 C}{20 C}\right)^4 \approx 800000$. $\endgroup$
    – Poseidaan
    Jun 28, 2021 at 9:08
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    $\begingroup$ Do you have any evidence that a bonfire emits a black-body spectrum? This is not to be expected since this radiation comes from a irreversible chemical reaction, i.e. the bonfire is far from thermal equilibrium. Mechanically, the radiation is not allowed to scatter or interact sufficiently with the environment before hitting your eye. $\endgroup$ Jun 28, 2021 at 14:10
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    $\begingroup$ @ThibautDemaerel Perhaps you assume that the chemical reactions are radiative, but they are not. They produce hot gasses, which emit black body radiation. $\endgroup$
    – my2cts
    Jun 28, 2021 at 14:38
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    $\begingroup$ Could just replace "blackbody radiation" with "thermal radiation" to avoid having to worry about whether a fire (or the room for that matter) actually produces a black-body spectrum. $\endgroup$
    – gardenhead
    Jun 29, 2021 at 0:03
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    $\begingroup$ I find it very confusing why there is so much talk about black body radiation here. The term is "(thermal) electromagnetic radiation". Black body is a tiny part of that, and a theoretical construct at that. Since OP is obviously quite new to thinking in these terms, I would find it much more useful to introduce them to the concept of electromagnetic radiation in general (i.e., that the same thing that we perceive as light, or which allows us to transmit radio signals, also is a direct energy transport mechanism if intensity becomes higher). $\endgroup$
    – AnoE
    Jun 29, 2021 at 9:59
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Air may be a bad conductor, but it's not that bad a transporter. What I mean, is that the room is heated by a process called convection, and not conduction.

So basically, fire heats up its neighboring air, which gets lighter and rises up and is eventually displaced by cold air which again gets heated and rises, and the process continues!

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    $\begingroup$ Yes, conduction is just one of three heat transfer processes. This means that with a limited conduction, the heat transfer could still be high. It's the radiation that transmits more heat, though. $\endgroup$
    – apaderno
    Jun 27, 2021 at 22:33
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    $\begingroup$ @apaderno radiation is certainly most important for warming the person gazing into the fire, but a thermometer in the far corner of the room with no line of site (even for reflected IR) will still register a rise in temperature. For this, convection is most important $\endgroup$
    – Chris H
    Jun 28, 2021 at 7:48
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    $\begingroup$ @my2cts it certainly might, but you could easily set up a series of baffles to block that and the temperature would still go up - or make your thermometer out of something with very high IR reflectivity $\endgroup$
    – Chris H
    Jun 28, 2021 at 9:05
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    $\begingroup$ Convection current plays a major role imo $\endgroup$ Jun 28, 2021 at 9:19
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    $\begingroup$ The problem with convection as an answer here is that a old-style open fireplace (when working properly) draws air from the room and up the chimney. $\endgroup$
    – JimmyJames
    Jun 28, 2021 at 20:32
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Preamble
In physics, when one says that something is big/small or good/bad, these terms are never meant in absolute sense, but relative to something else. E.g., we cannot say whether 1 meter is a big or small distance: it is small when we talk about stars and huge when we discuss atoms.

Is air a bad heat conductor
I assume that conductor in the OP means heat conductor. Air is a bad heat conductor, if you compare its heat conduction with that of a piece of a metal or a single layer of glass. Thus, windows in northern countries are often made of a double glass layers, separated by air, so as to preserve the heat in the room from leaking outside. Still, the windows remain the main route for the heat loss, unlike the walls filled by better isolating materials, i.e. the materials that conduct heat even worse than air. Why do we not fill windows with the materials that are better isolators than air? - because we want them to be transparent.

Similarly, although air may be a relatively bad heat conductor, we do not have much choice when heating our rooms than to allow it to be filled with air.

Convection and radiation
It has been pointed out in the other answers that the heat exchange between the fire and surroundings is mediated not only by air, but also through the radiative heat transfer. Let me characterize both in terms of easily observed physical phenomena.

Convection
Firstly, convection is not the same as heat conduction: conduction occurs through an essentially static material, whereas convection is displacement of volumes of already hot air to another place. The flow of air is always present in ventilated human habitats - which is what we encounter in everyday life. Even if no particular effort is made to ventilate a room, the warmer air typically accumulates at the top of the room and eventually leaves it, whereas a cold air enters along the floor. This could be tested in an easy experiment, by placing a candle in the door opening at different heights from the floor.

When a room is heated by the stove, the air around the stove becomes hot, raises up, and spreads along the ceiling or penetrates at the higher floors. Since air is a relatively bad heat conductor, it may take quite a bit of time until the heat propagates down, and the floor in rooms heated by a fireplace often remains cold. Note also, that maintaining a fire in a stove requires quite a bit of convection to supply oxygen for combustion, and stoves are explicitly designed for this purpose.

Radiation
Radiation plays an important role in heat transfer between fire and its surroundings. If we sit near the fire we feel heat even though we are not in the stream of the hot air rising upwards. Wet clothes placed in front of a fire dry rather quickly. These effects take place, even if we reduce the contribution of convection to a minimum - e.g., when sitting in front of the camp fire, where hot air escapes completely.

Note also that bad heat conductors are also characterized by higher heat capacity. Thus, a metallic stove may heat very quickly, but would not keep any heat after the fire is extinguished. On the other hand, a brick fireplace may take a few hours to heat up (and a few hours to heat the room, since air heat conduction is slow, and there is no much radiation coming out of a closed fireplace). However, once hot, it may keep the room warm throughout the night (it is necessary to note that it similarly heats the room via convection and radiative transfer in the infrared range - it is this infrared light that is perceived as heat when we are next to a fireplace, but do not directly see the fire).

Remark: It was pointed in the comments, the windows are nowadays filled not necessarily with air, but with other transparent materials having poor heat conductance (including vacuum). While this is an interesting piece of information, the point I was making is that the windows are filled with air not just because it is a good insulator - the need for transparency restricts the range of materials that we can use, and air is not the worst choice.

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    $\begingroup$ We do actually fill windows with things other than (normal) air. When my parents were getting new windows, they were shown windows that were filled with, IIRC, argon because it was an even better insulator than the nitrogen, oxygen, carbon dioxide, etc. that make up normal air. Air has a thermal conductivity of 26 mW/m·K at STP, while argon has a thermal conductivity of only 16 mW/m·K. $\endgroup$
    – Andrew Ray
    Jun 28, 2021 at 17:24
  • $\begingroup$ @AndrewRay I didn't know about this, but it makes sense $\endgroup$ Jun 28, 2021 at 18:27
  • $\begingroup$ @AndrewRay I remember learning in school that if not done well, air can make insulation worse, due to convection happening in the pane. I believe the state-of-the-art today is triple-pane vacuum insulated windows, which are apparently not losing significantly more heat than a well-insulated wall. $\endgroup$
    – gerrit
    Jun 30, 2021 at 11:54
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Probably the most important heat transfer mechanism in the development and spread of a fire in room is radiation which can raise the temperature of the materials in the room to their ignition point. The main role of the air in the room is to provide oxygen to enable ignition and self-sustained combustion.

Hope this helps.

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  • $\begingroup$ I don't think the question is about the spread of a fire, but about heating. Fire spread, as in a house burning down, involves many other mechanisms. $\endgroup$
    – jamesqf
    Jun 28, 2021 at 15:36
  • $\begingroup$ @jamesqf You're free to your own interpreting of the question and I to my own. $\endgroup$
    – Bob D
    Jun 28, 2021 at 18:32
  • $\begingroup$ @jamesqf his answer is a good example of how air isn't required to make things really hot. $\endgroup$
    – RonJohn
    Jun 29, 2021 at 3:33
  • $\begingroup$ "You're free to your own interpreting of the question and I to my own." - but jasmesqf is not questioning your right to interpret the question, but is rather trying to be helpful by pointing out that you may have misinterpreted it. You are, of course, free to disagree, or, indeed, ignore the remark. I'm just pointing out that it's not an "attack". $\endgroup$ Jun 29, 2021 at 9:11
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Let's flip it around: why doesn't it the heat the room up more?

For a good visual, imagine you're out in just-above-freezing temperatures. Which would you rather do:

  • not have a glove on your hand, exposed to the air
  • put your hand into nearly freezing water
  • put your hand on a flagpole and hold it there

... the first! Having some skin exposed to cold air isn't nearly as bad as cold water, or cold steel. Because the air doesn't conduct nearly as much heat.

Now think about that fire. It's something several hundred degrees warmer than your body is equipped to handle... yet you're able to stand a meter away from it! When you think about it, the air's doing an awfully good job of insulating you away from that heat (and most of the heat you feeling isn't even coming from conduction, but from radiation).

Now imagine that the large fire was build atop a thick metal plate. Do you think you'd be able to stand on the metal plate that close without burning your feet?

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Most of the heat is transported by air flow. Often this is just convection*, but if you use a wood stove for heat (as I do), you'll find that it heats much more effectively if you use a blower fan to help move heated air away from the stove and into more distant areas.

A second significant transport is radiation. That's one reason (the other being aesthetics) why many wood stoves have large glass windows.

*Note that most insulation - glass wool, foam, even the fluffy fabric of a sweater - is designed to trap air, and thus minimize convective heat transport.

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  • $\begingroup$ I always thought that the blower fan was more about replacing the used up oxygen near the burning surface, rather than scaling up the convection. Otherwise, this is spot on, particularly the explanatory bit about how air is used as an insulator. $\endgroup$ Jun 28, 2021 at 13:56
  • $\begingroup$ @João Mendes: No, at least not in any wood stove I've seen. Combustion air follows a completely different path. You really wouldn't want it to, since forced air, as from a bellows, creates a much more intense fire and uses up fuel faster. So you would use forced air for e.g. a blacksmith's forge. $\endgroup$
    – jamesqf
    Jun 28, 2021 at 15:39
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    $\begingroup$ @JoãoMendes there are also ducts behind the fireplace; for example, in Franklin stoves. $\endgroup$
    – RonJohn
    Jun 29, 2021 at 3:36
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Consider also that fire, and any heated surface, emits infrared light, which you feel instantaneously. Thus, if you turn on a heating plate, you may instantly feel a change in temperature.

Air is a terrible heat conductor, but it is also dynamic. Warm air simply gets out of the way to make space for cold air, which increases the rate of heating. Because the hotter something is, the slower it heats further, as the heat gradient becomes smaller. Thus, if the hot air moves out of the way, making space for cold air, that heats faster, it increases the perceived heating rate.

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  • $\begingroup$ +1 This answer explains correctly how fire can heat up a room, and does it in simple terms. It should help the OP much more than some of the rather in-depth and more theoretical answers. $\endgroup$
    – Timm
    Jul 1, 2021 at 9:15
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Turbulent vs molecular transport

Molecular diffusion of scalar quantities by fluids is often very slow. If you open a jar with a stinking substance, you won't e able to small from some distance due to molecular diffusion, but due to turbulent transfer. The fluid in a typical room is moving chaotically and carrying scalar quantities with it.

For heat (internal energy) or temperature, we call the molecular process conduction (and we use a different coefficient and the Prandtl number instead of the Schmidt number, and ...), but it really works the same way. The turbulent Prandtl and Schmidt numbers (the ratios of turbulent viscosity to turbulent diffusivities of heat and passive scalars) will actually be almost the same so the efficiency of diffusing heat and passive scalar quantities by turbulent motions is the same.

Convection

Convection is an ambiguous or overloaded word. Often, depending on the subject of sturdy, it is used for what other would call advection but in many engineering disciplines using "convection" with multiple meanings seems to be preferred. The term "advection" has the advantage of being non-ambiguous. The turbulent motions of air in the room with a fire or with an open jar with a stinking substance advect the heat or scalar concentrations with it.

But convection has also a different meaning. convection is the turbulent motion generated by thermal stratification that is unstable, typically by heating lower parts of the room, but can also be started by cooling the upper parts of the room. Convection causes large organized turbulent motions that are very effective in mixing scalar quantities in the room.

Convection will start when the Rayleigh number is large enough. For a fixed room with air it means when the temperature gradient becomes large enough. The temperature difference necessary to start convection is quite small and any room heating will start it. That's why radiators are able to heat a room. They would not be able to do it just by molecular heat conduction.

The term "convection" also appears in the terms "forced convection" vs. "free convection" often discussed in engineering, that are related to the present problem, but I will not touch thme.

Radiation

Any solid body (or even the fire) with nonzero emissivity will emit radiation based on the Stefan-Boltzmann law. The intensity is relative to the fourth power of temperature (~T^4).

Therefore, objects in direct view of the fire will be heated by radiation. That radiation happens mostly in the visible and the infra red-part of the spectrum (depending on the temperature of the object).

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