Build a High Efficiency Fireplace (90% Efficient!)

It's possible to build a high efficiency fireplace using cast clay for the firebox and sheet metal for the stovepipe.


| November/December 1980


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

Nevertheless, using information they'd gleaned from the university's seminars — with the aim of remedying the disadvantages of the design — Christensen and Sanchez designed and built a five-piece, cast-clay firebox that weighed only 280 pounds, cost them $244 in new materials, and took up an easy-to-live-with 18" X 18" X 27" space on the living room floor. Best of all, when the two tinkerers conducted performance tests on their compact creation, they found its efficiency to be virtually equal to that of the "Bolshevik behemoths." (In all fairness, we should point out that, although the efficiency of the large and small models is about the same, the volume of heat rendered by either room oven will vary according to the amount of wood burned ... and the larger hearth is, of course, capable of absorbing more thermal energy before releasing it to the living area.)

How They Did It

The Southwesterners' secret is not an intricate interior labyrinth (their molded refractory firebox is almost Spartan in this respect, though its metal flue pipe is somewhat convoluted), but the fact that cast masonry stores and liberates heat better than does metal. The team opted to fabricate the stove from Kast-Set, a refractory clay, because [1] it's easy to work with, [2] it's strong, [3] it can withstand 2500°F temperatures, and [4] it doesn't require kiln drying.

William Pople
1/21/2012 1:58:23 AM

I would like to get some more information on this plans, instructions, and/or pictures. Is there any more information you could provide for searching, plans or any thing like that?


PAT HITE
2/2/2010 12:24:58 PM

I ATTEMPTED TO GET IN TOUCH WITH SANCHEZ AND CHRISTENSEN TO PURCHASE A COPY OF THE PLANS, BUT SO FAR HASN'T PANNED OUT. COULD YOU HELP? BUILDING A SUPERINSULATED "GREEN" HOUSE IN WASHINGTON STATE, AND THIS LOOKS LIKE A GREAT WAY TO HEAT IT.


deborah_37
8/6/2007 12:38:46 AM

best way and cheapest way to replace the firewall next to the bricks is -buy a bucket of cement for stone flooring you can get it at lowes or home depot for about 20 bucks and place rocks or stone flooring.any kind of rocks are fine. they are easy to clean and they dont burn and they arent a fire hazard.






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