Synopsis: This article details construction of a masonry heater, which uses a very high thermal mass to store and release heat slowly. The author covers construction of the firebox and internal passages that make the heater so efficient.

A century ago, Mark Twain declared that masonry heaters could heat a home comfortably all day after one firing, consuming “no more fuel than a baby could fetch in its arms.” In comparison, he called the American woodstove “a terror” that wastes so much wood it makes you “think you have been supporting a volcano.” No wonder he was perplexed that the U. S. had yet to adopt the masonry heater as its own.

Nowadays, site-built and factory-built masonry heaters are appearing in the U. S. with a flourish that would have dazzled Twain. Nevertheless, they are still virtually unknown in my hometown. That’s why I set out to build a showcase model. As a masonry contractor, I wanted to demonstrate how effectively such a fireplace (as opposed to a standard masonry fireplace) produces and stores heat, releasing it uniformly over a long period of time. I also wanted to show how adaptable it can be to a home’s style and decor.

My clients, John Hosemann and Joan Carney, were ideal.They were in the early stages of designing a new home and were searching for a fireplace that would actually heat it. John had lived with conventional masonry fireplaces in two previous homes and didn’t want another “hole-in-the wall.” Joan insisted only that the fireplace be beautiful. They both wanted to be able to view the fire.

They also gave me an excellent location to work with—right near the center of the house.The front of the fireplace would face the living room, dining room, and kitchen; the back would face the master bedroom. Not only was this spot ideal for heating, it would make the fireplace the focal point of the house.

When I first looked at the house plans, it was clear that a simple boxlike fireplace would not do. The design would have to feature intersecting planes and angles galore (like the house), while incorporating a suggestion of raised levels.

Working out such a design is not merely a matter of erecting an elaborate facade around a square firebox. For heat to be effectively transferred in a masonry fireplace, internal heat-exchange chambers must fall into alignment with the fireplace’s outside walls. Fortunately, I had many prototypes to choose from.

Following the Finns

Efficient masonry heaters are not new. Hundreds of models have evolved of necessity around the globe, whenever and wherever wood-fuel supplies have been threatened by vanishing forests. These heaters have one common denominator—a huge mass of masonry that holds heat. This heated mass supports combustion temperatures that exceed 1500° F, hot enough to burn wood completely (including the volatile gases that comprise more than two thirds of the wood’s fuel value). In fact, the efficiency ratings of masonry heaters begin where those of the best metal stoves quit, and range from 80% to more than 90%.

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