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Posts Tagged ‘Heating with Wood’

Heat acts like an invisible fluid that washes over and penetrates through any barrier. The reason I thought about it this morning was the temperature in the house, the difference in temperature from outside, and the feel (comfort level) in the house.

The thermometer, placed on the counter in the center of the house, said 68 degrees, which is a reasonable indoor temperature. But should I start a fire in the stove? Ultimately, for most people, the answer to that question depends on comfort level. Ignoring the real psychological components of comfort level, comfort level depends on heat flow rate.

Heat flow rate is the reason for heat index and chill factor in weather reporting of temperature. An inanimate object feels neither extra hot when it is humid nor extra cold when the wind is blowing. The temperature at which it settles is determined solely by the average rate of vibration of its molecules (i.e. temperature). 

But when an object like a thermostated heater or warm blooded mammal or human is warmer than its surroundings and produces heat to maintain that difference, heat flow rate is crucial to comfort and even survival.

For a fuller understanding of this concept. let us posit two facts:

  1. Heat always flows from warmer to cooler.

  2. Heat Flow Rate is     Screen Shot 2018-12-08 at 6.05.10 PM,

where Q is heat, t is time, K is conductivity, dT is change in temperature, and l is length (distance or thickness) of heat flow, and the combination of dT/l is the temperature gradient.

Since in winter I heat with wood for the purpose of keeping us warm and feeling warm, I will consider the situation of heat flowing out of our bodies. Unless I am sitting by the woodstove, I am warmer than the room and heat is leaving me.

The greater the area (A) of my skin exposed to the surroundings, the faster I cool off. For this reason, you don’t expose flesh to the air on an extremely cold day because the heat flow rate is so great from any area of  your body that it can’t provide enough heat to prevent your flesh from freezing.

The greater the temperature difference (dT) between me and the environment cooling me, the faster I cool off. Our bodies are constantly radiating heat to the cooler surroundings. If you have ever worked in an unheated building with a concrete floor, you know that it is very hard to keep warm. You can feel the concrete zapping heat out of you (you radiating to it, in fact).

The shorter the distance (l) for the heat to flow to reach the cooler temperature, the faster I cool off. The thickness is the reason thicker insulation works better. More thickness of a substance that slows heat flow rate slows it more. Insulation, be it pink or down or quilt is really just a function of how much non-convecting air is trapped in the insulating layer. Air is an excellent insulator, which brings us to the next component.

And the better the material is at conducting heat (thermal conductivity (K)) off of me, the faster I cool off. Conversely, the reason air is poor conductor of heat is the distance between molecules. On the other hand, since it is a fluid, it is a decent convector (heat transfer by flow of a fluid), and it provides little resistance to radiation, since there is less matter than most materials to absorb or radiate back the heat. When you put a warm hand on a cold, wooden table it won’t feel very cold, but on a cold, metal appliance it will. This increased flow happens because the metal has a far greater thermal conductivity than an insulator like wood.

Soooooo, back to my question. Should I start a fire when the core of the house is 68 degrees? Considering this situation, I ask myself several questions. What is the temperature outside? Is there a strong wind cooling the house? Is there cloud cover to prevent cooling at night or prevent warming during the day? Is there a temperature trend up or down from the present reading of 68 degrees because of internal or external changes to the house? About this time, those of you who have thermostats that do all of this “thinking” for you should stop taking it for granted. It is a relatively old technology, but not an altogether simple or trivial one.

In terms of comfort level, it can be 68 degrees in the core of the house and feel quite chilly because the outside is removing heat rapidly. This situation will result in the peripheral (near the outside walls) temperature being several degrees cooler.  I used to have my indoor/outdoor thermometer on a window sill. It consistently read cooler than the one I have now. My new thermometer wouldn’t fit on the sill. It would probably be easier to not think about it if I had a thermometer in the center of the house and on the window sill. Then I would have an approximation of the heat flow rate out of the house, but that would make me less truly aware of my surroundings. I have to walk past the central thermometer and past the window to get to the refrigerator and the stove to make my breakfast anyway, so I feel the temperature difference from core to periphery. I do not, however, usually think much about the various components of heat flow rate, because I am only barely awake at 5:15 in the morning.

101_1604

Not time for a fire, yet (I need to reset the clock.)

P.S. If you read this far, I surmise that you are either a science geek or a particular friend. Either way, thanks for reading, and glory to God for His ordered universe and minds to make sense out of it.

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