ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Sunday, April 28, 1996 TAG: 9604300062
SECTION: HOMES PAGE: D-3 EDITION: METRO
COLUMN: Hints for Homeowners
SOURCE: STEVE ELDER
Everyone familiar with energy efficiency knows that fireplaces are notoriously inefficient sources of heat, particularly when compared to modern, air-tight, wood or pellet stoves.
Most of the heat from a fireplace goes up the chimney along with the smoke. Nevertheless, as everyone also knows, it is possible to be warmed by a fireplace. The key lies in knowing how this happens.
Fireplaces actually warm you by producing radiant heat. The firebrick absorbs radiant heat from the burning logs and re-radiates it into the room. Objects or people in line with the radiant heat rays will be warmed. Objects off to the side will not be warmed because the fireplace is not heating the room air; it is heating objects in the room. These in turn will tend to warm the air around them.
The principle is the same as standing under a porch roof on a winter day: if you step out into the sunshine, you will be warmer, even though the air temperature may be 20 degrees in both places.
The concept of radiant heat was discovered 200 years ago by Benjamin Thompson, born in 1753 at Woburn, Mass. A true 18- century man with many interests and talents, Thompson invented among other things the double boiler, the drip coffee pot, a revolutionary cookstove, a calorimeter and a better fireplace.
His many inventions and innovations in military equipment, strategy and catering earned him many accolades, including knighthood from the British Crown and the title of Count Rumford from the Duke of Bavaria. Despite his fame at the time, he is hardly known to us today, possibly because, as a British loyalist, he may have been written out of our history books.
I first encountered a Rumford fireplace about 10 years ago while inspecting a town house built around 1800 in Alexandria. My initial reaction was, ``This thing will never work! It's too high and too shallow.'' (The owner promptly built a lively, warm fire in it, causing me to revise my expert opinion to, ``Oh!'')
A modern fireplace is deep and has a greater width than height. The lower height is designed to prevent the turbulent mix of air and gases above the flame from spilling out into the room.
Rumford, with his keen intuitive understanding of physics, figured that the best way to ensure a good draft was to remove the turbulence. Instead of building a deep fireplace with the back wall slanting forward, he designed a shallow fireplace with a straight back wall, so that the smoke goes straight up the flue. He also raised the height and rounded the bottom of the lintel (the top front of the firebox).
The rounded lintel is the key to the Rumford fireplace. It creates a laminar flow of air from the room over the top of the flames, effectively pinning the smoke against the back wall. The room air does not mix with the gases to create an area of turbulence; instead, both smoke and air flow smoothly up the flue in layers. The design goes against everything modern brickmasons are taught, but it is based on sound physics and it works.
Rumford improved his fireplace further by angling the side walls up to 45 degrees, causing a much greater amount of radiant heat to be cast out into the room. Conventional fireplace side walls are almost parallel, so that the radiant heat is reflected back into the fire itself. The combination of the higher walls and their angle of reflection makes the Rumford fireplace a much more satisfying and efficient source of heat than the conventional design.
A final note: Glass doors are not recommended, because they block 80 percent of the radiant heat that is the great forte of the Rumfords. You might want to install them anyway, as a good safety feature for unattended fires; but you should keep them open when the fireplace is in use and attended.
For those homeowners or professionals who would like to to build a Rumford fireplace, there are instructions in an excellent article by Jim Buckley in the March 1994 edition of The Journal of Light Construction. Buckley also helped develop the special Rumford curved throat and smoke chamber that are commercially available from the Superior Clay Corp., P.O. Box 352 , Uhrichsville, Ohio 44683. Their products are distributed in Roanoke by Blue Stone Block on Orange Avenue NE (982-3588).
Steve Elder is a Roanoke home inspector. Questions and comments may be sent to him in care of The Roanoke Times, P.O. Box 2491, Roanoke, Va. 24010-2491.
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