Build a High Efficiency Fireplace (90% Efficient!)

By David Gustadson
Published on November 1, 1980
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The completed high efficiency fireplace will look something like this. Its convoluted stovepipe furnishes a quick blast of heat after the initial lighting.
The completed high efficiency fireplace will look something like this. Its convoluted stovepipe furnishes a quick blast of heat after the initial lighting.
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[5] Bob tacks more of the galvanized veneer to the upper half of the exterior form. [6] The completed casting frame prior to spooning in the Kast-Set. All the metal surfaces have been coated with oil or grease before the
[5] Bob tacks more of the galvanized veneer to the upper half of the exterior form. [6] The completed casting frame prior to spooning in the Kast-Set. All the metal surfaces have been coated with oil or grease before the "mud" is laid in.
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[1] After making a cardboard template, Bob Christensen outlines the borders of the modified Russian fireplace. [2] Alex Sanchez prepares to halve the wooden mold at its midriff.
[1] After making a cardboard template, Bob Christensen outlines the borders of the modified Russian fireplace. [2] Alex Sanchez prepares to halve the wooden mold at its midriff.
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[7] One week later, after the refractory clay has set, the forms can be removed. 
[7] One week later, after the refractory clay has set, the forms can be removed. 
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[3] The inner and outer braced plywood forms ready for covering. [4] Alex finishes fastening a sheet-metal
[3] The inner and outer braced plywood forms ready for covering. [4] Alex finishes fastening a sheet-metal "skin" to the inner section of the mold.
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[8] One part of the room oven's finished body, which must be damp-cured, then sun-dried for two or three weeks.
[8] One part of the room oven's finished body, which must be damp-cured, then sun-dried for two or three weeks.

The “rebirth” of wood heat has been a real education for a good many people who grew up thinking that winter warmth is created by a turn of the thermostat. Not only has the growing popularity of fireplaces and wood stoves forced a number of folks to put a time-and-sweat — rather than a dollars-and-cents — price tag on their fuel, but it’s also stirred up the curiosity of a few individuals regarding the actual levels of performance that can be expected from the “new” appliances.

Increased Efficiency

Surprisingly enough, the folks who have looked into such matters have found that the efficiency of the woodburners (calculated by measuring the amount of heat vented into the atmosphere, subtracting that figure from the total number of BTU available in the given weight and kind of wood used, and converting the result to a percentage) can range anywhere from near zero (with a poorly designed fireplace) to a high of perhaps 72% (in a good airtight stove).

Not long ago, in an effort to learn why some heaters work better than others — and with the eventual goal of building an effective, affordable high efficiency fireplace — New Mexicans Bob Christensen and Alex Sanchez began to research various stove designs. The pair initially concentrated on modifying the basic shape and configuration of the “standard” firebox … but when they discovered that plate steel couldn’t be easily conformed to the pattern they wanted, the inventive Southwesterners decided to try a different tack.

Coincidentally, the New Mexico Energy Institute at the University of New Mexico (an Albuquerque-based research organization dedicated to the promotion of alternative energy) was conducting a workshop project on so-called “Russian fireplaces” … devices which were testing out at 90% efficiency! The excellent performance of these unusual heaters stems from two characteristics: They contain enough masonry to provide a massive heat sink, and the interior flue path of each unit is arranged in a serpentine pattern so that hot waste gas has plenty of time to transfer its thermal energy into the walls of the stove. Unfortunately, the typical Russian fireplace weighs about 11 tons, stands seven feet tall, and can cost as much as $2,000 if built by a professional mason.

A Tempest in a Teacup

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