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One Hot Fireplace

Rumford expert Jim Buckley explains how to build a better fireplace using 18th century engineering.

<p>This Old House TV: San Francisco house project</p>

This Old House TV: San Francisco house project

Early one morning, supplier Jeff McNear and I left a brick yard in San Rafael and headed for San Francisco to watch the This Old House crew shoot the construction of a Rumford fireplace in a 1906 Arts and Crafts-style church.

The Rumford fireplace, built in the sanctuary where the altar used to be, was to be the focal point of the enormous living room. The fireplace—a log-swallower at five feet wide and four-and-a-half feet tall—would take advantage of ingenious engineering from the 18th century that minimizes smoke and maximizes energy efficiency.

The secret is in the Rumford's design. In the mid-1700s, Count Rumford (born Benjamin Thompson in Woburn, Massachusetts), the fireplace's creator and namesake, realized that the only useful heat generated by a fireplace is radiant heat, and that in traditional fireplaces, much of this heat mixes with smoke and goes right up the chimney.

In a traditional fireplace, the fireback slopes forward to direct smoke up the chimney. Incoming air spills over the sharp edge of a steel lintel eight or nine inches below the damper, mixes turbulently with the smoke and "rolls" upward. While this construction prevents the fireplace from smoking, it also loses some of the fire's heat up the chimney.

Count Rumford designed a fireplace with a high, wide opening, a shallow firebox and widely splayed jambs to reflect as much radiant heat out into the room as possible. But Rumford's real genius was straightening the fireback and rounding the front wall of the throat, essentially creating a nozzle—like an inverted carburetor—that shoots smoke up through the damper and out the chimney, wasting less heat in the process.

Rumford fireplaces were common from 1796—when Count Rumford first wrote about them—until the mid-1800's.

Thomas Jefferson had them built at Monticello, and Henry David Thoreau listed them among the modern conveniences that everyone took for granted, along with plaster walls and Venetian blinds. By mid-century the word "Rumford" was synonymous with "fireplace." Trouble was not all Rumfords were equal. There were misinterpretations of what constituted a Rumford fireplace right from the beginning. Jefferson himself made some drawings in an effort to improve the Rumford design, but he neglected to round the breast. This and other misinterpretations of the Count's design caused Rumfords to fall out of favor.

Today, with the extensive restoration of old and historic houses and the renewed popularity of early American and classical architecture in new construction, true Rumford fireplaces have made a comeback and are more popular now than at any time since 1850. The traditionally tall Rumford looks appropriate in today's classically designed homes with high ceilings. The San Francisco church was no exception. Crafted in unusual red firebrick with clean lines and a uniform color, the new fireplace transformed the cavernous church sanctuary into a homey new living room.

Jim Buckley provided technical help to architect Barbara Chambers on the design of the San Francisco house's Rumford fireplace.