Richard Boyt, July 2003
With all the current talk about chimneys, I'm going to put in my two cents, probably too little, too late. This letter is in response to Tammy Bond's request for personal experience. I find that chimneys are useful, at least the small ones I use on my experimental stoves, and the big brick one in the center of our house that is fed by our two cast iron heating/cooking stoves.
The small chimneys evolved from balancing a stack of half a dozen tin cans to increase the draft above the combustion chambers of some of my experimental stoves that refused to stop smoking. Seeing a gust of wind topple the tin can tower, I tried ways to join them firmly together and yet still be able to take them apart again, if I choose. Ways that would make assembly and disassembly easy and that permitted me to easily vary the diameter and the height as needed.
I filed the jaws of an old pair of pliers, turning them into a crimper (see drawings). After removing both ends of the tin cans, I crimped all the way around one end of a can, and jammed it inside the un-crimped end of another. The length, depth, and number of crimps determine how tightly they will fit, and whether or not I can get them apart again. I found that if I want them permanently together, I could join them, and then pound out the crimped areas, using a section of boiler pipe as a mandrel. Lightweight, various lengths and diameters, no cost, easy to make, easy on, easy off, easy to clean, easy to attach to a permanent masonry chimney, if used indoors, or to a semi-permanent tin can chimney that goes up through the roof or out through a wall. Good for starting fires or giving one a "kick", or operate continuously during a burn.
Add a rotatable baffle to the stack to control the draft. The attachment of the stack to the stove is easily done by modifying other sizes of cans to make a "top hat" adaptor . This adaptor can be placed on the stove to hold the stack of cans that fits down loosely over it. Loose connections make the stack easy to assemble and disassemble. Drill a number of small holes in a line up one side of the lower cans so you can see if the flame rises that high. I hope the sketches explain what words find difficult. If a precarious stack of old tomato cans makes you uneasy, you can always "cheat" and use stove pipe.
This might be a good place to refer to "The Kiln Book" by Frederick L. Olsen (second edition) Chilton Book Co, Radnor, PA (1983). While Olsen is referring to the combustion chambers of kilns, I feel that his statements also apply to the combustion chambers of cookstoves. "Grate area for natural draft... wood: ten times greater than the horizontal section of the chimney." (p. 58) "Chimney diameter is approximately 1/4 to 1/5 the [combustion] chamber diameter" (p. 61). "The taper on the chimney controls the rate of draft. Tapering reduces atmospheric pressure and increases the speed of draft thereby controlling the rate of draft which ideally should be four to five feet per second." (p. 59) "A tall chimney increases velocity inside the firing chamber. Too high a chimney can cause irregular heating by pulling the heat out of the kiln, not allowing it to build up within the chamber, thereby prolonging the firing. On the other hand, too short a chimney can protract the firing by decreasing the draft rate, which allows build-up in the firebox and does not pull enough oxygen into the kiln to allow proper combustion for temperature increase." (p. 61)
Our home has a large permanent brick chimney that serves our two wood heating/ cooking stoves is quite another matter. For the past thirty years, we have lived in this 90+ year-old, 9 meter (30 ft) one-story square wood frame structure, with an unlined brick chimney rising up from its center to about a meter above the peak of a steep pyramid roof. We blocked off the old fireplace at the base of the chimney and connected above it the flue of a 1922 "Home Comfort" woodburning kitchen cook stove, and in the living room, the flue of a cast iron, space heating box stove. The 6" diameter flue pipe of each stove runs about four feet vertically, and six feet horizontally, to connect to the brick chimney.
During the winter, we run both stoves at full draft to minimize smoke and creosote build-up. No banking the fire at night means re-loading every four to five hours in very cold weather. We take down the stove pipes once a year for cleaning, removing from each about two pounds of hard, brittle, glassy-- to soft fluffy tar/ creosote. The brick chimney itself gathers hard creosote layers that are very difficult to remove, as the roof is too steep to climb, and I can reach only a little way up into it with scrapers from below.
During our thirty-year stay here, we have experienced five or six flue fires-- very scary, very dangerous. Usually, they happen in the middle of the night when we are awakened by a strange palpable roar and the crackle of over-heated stovepipe. If it has advanced far enough, we can see the landscape near our house illuminated by an eerie orange glow from the blowtorch of flame extending several feet above the chimney. Our flue fires always start from a very hot stove, work their way up and along the stove pipe, and into the chimney. With luck, we can stop the progress of the burn before it reaches the chimney by shutting off all air and by cooling the stove pipe with water. If we can't stop it, a full-fledged chimney fire takes over. If we can't put it out, at least we can slow it down. Over the years, fires have eroded the mortar between the chimney bricks. Before we re-grouted the bricks, we could see the orange glow of the raging fire inside the chimney through open cracks between the bricks. We keep a garden hose ready at all times, to keep the roof from catching fire. I also have a chemical torch that is supposed to lessen the ferocity, but have not used it yet. A full flue fire does completely clean out the chimney, so its not all bad, unless it also cleans out the house.
So why the heck don't we fix it? Tear the chimney out and build it right? It would cost less to let the house burn and buy a trailer. Abandoned for over twenty years, "this old house" has feasted colonies of termites, the foundation has hosted lush gardens of moldy dry rot, the ceilings have suffered from leaky roofs, and floors were never level or square. It is a few years older than we are, and it need last only a few years more. The important things-- the family, the forest, and the land-- will still be when we and the house are gone.
But in the mean time, we need space heat during the cold Missouri winters. In response to the recent interest in heat transfer, my next entry will be a description of a fan-driven counter airflow heat exchanger which I use on the flue of the cast iron living room box stove.