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Archive for December 17th, 2017

As I said in a recent post, I have taught whole science lessons by using the illustration of the woodstove. See if you can name the concept being taught by the following story before I name it at the end.

Heating with wood can be interesting and exciting (chimney fire!) but also mundane. When something interesting happens it gets your attention. One bright morning in early Spring in the Horseshoe I got up and ate breakfast. I had not started a fire the night before because it wasn’t that cool and we like to sleep cool. My wife said, “Don’t you think it’s cool enough for a fire?” Of course, there is only one right answer, so I gathered wood and kindling and sat down to build a small fire to “knock the chill off.” I had only just lit the paper and cardboard when the smoke started billowing out of the top of the open door into the living room. My immediate response was to slam the stove door followed by jumping up to check to see if the stove pipe damper was open. The smoke started oozing out around the door dampers, so I hastened to screw those shut, only to continue more slowly coming out around several small gaps around the doors. All there was left to do was to open the front door and start fanning. After a few minutes the smoke was cleared and the living room really needed a fire in the stove. I got my flashlight, cautiously opened the stove door and peered into the exiting stove pipe at the back of the stove. There seemed to be no obstruction. I even used a mirror but couldn’t see around the curve for the smoke. There must surely be an obstruction in the chimney, be it birds or squirrels or bats or nest thereof. I was determined to get to the bottom of the problem. I put on a coat and hat and went outside to get the ladder and lean it up against the house. As soon as I got out from under the back door porch, I realized that the morning was warming up quickly in the bright sunshine. It was warmer outside than inside. Maybe I should just open the doors and wait. But instead I continued on the mission of solving the mystery stoppage in my chimney. I climbed up onto the red metal porch roof with the flashlight and then scampered up the steeper main roof and grabbed ahold of the chimney. Straddled across the peak of the roof, I removed the chimney cap and pulled the flashlight out of my back pocket. When I shown it down into the opening, I had to wait for my eyes to adjust and move my face closer. I could see clearly down most of the length of the chimney. There was very little creosote build-up. But toward what I judged to be the bottom where I should have seen the pipe coming into the chimney from the stove, the view went fuzzy. I could not tell what I was looking at. I tried to adjust the flashlight and my head to see better, but to no avail. Frustrated, I thought, “I’ll fix that,” shut off the light, slid down the roof to the porch, bounded down the ladder, and stepped over to the old smokehouse, now a storage shed. I strung up my 100′ extension cord with a plug-in socket and light bulb on the end and retraced my course back up to chimney side. Next, I lowered the lit bulb down into the chimney, which was very well lit now. When I lowered the bulb down to the vague area, I could see that there was smoke hovering low in the chimney. I lowered the light into the smoke and it totally disappeared after several inches more of lowering. It would appear and disappear as I raised and lowered it out of this dense smoke layer. I turned my head aside for a moment to consider, since fresh creosote is not the most pleasant smell. Just as I turned back to look again I got a face full of smoke that just kept coming. I choked on the lung full I received and must have called out in alarm because my wife yelled up to ask if I was OK. I hastily pulled the light out and made my way down to the porch with the smoke settling down the roof behind me. What in the world was happening? The smoke cleared quickly, because there really wasn’t all that much. I climbed back up on the peak of the roof and again lowered the light. The chimney was clear.

It took me awhile to figure out what had happened. Have you figured it out, yet? There are several hints in the story having to do with temperature. Before you read below you may want to go back and reread with the hint.

The temperature was warmer outside than inside. Cold air is denser than warm air. Since there was not a fire the night before, the air in the chimney was cooler than the quickly warming exterior air. When I started the fire, smoke was not able to move through the pipe because the air there was heavier than that higher in the chimney where the sun beamed down. There was a thermal inversion in the chimney restricting the air from rising. When I left the light bulb in the smoke layer for a few moments of slow reflection, it warmed the smokey layer so that it was lighter than the air above it and began to rise. I got a face full of lesson about thermal inversion. This situation happened again years later in the home we presently live in. I didn’t even hesitate. I climbed up on the roof and lowered a light bulb down into the chimney. The smoke rose. I climbed down and started a fire and had a warm feeling inside knowing that knowledge and experience go a long way toward clearing away smoke that obscures your path.

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