1. Fireplaces are net heat losers.
If this were so it would mean that people whose homes' only source of warmth is fireplaces use them for air conditioning instead of heating, including the pioneer sodbusters who heated, not cooled, their homes with fireplaces fueled with buffalo chips. What this false claim means is that if a fireplace is not in use and if the damper is left open and if another heating device is in use, then some hot air is lost up the chimney. As a practical matter, some fresh air is needed (which displaces air already inside the home) or the air would soon smell like a rather fresh buffalo chip.
Photograph by Hilary Turner Photography
2. Open fireplaces are a waste of energy.
Sure they are, and so is everything else. In the real world, there are no isentropic reactions. Everything we do is inefficient. Fireplaces convert only about 10% of the wood fuel heating value into net room heat. Furnaces are about 30% efficient, automobiles about 20%, light bulbs about 10%. The fact remains that fireplaces do provide heat and a pleasant appearance.
3. Glass doors increase fireplace efficiency.
Actually, the glass decreases the efficiency by over half, because it blocks radiant heat from reaching the room. A closed fireplace door only becomes useful when the fire has died down enough to suck up more heated air than it yields. Metal doors are cheaper and stronger than glass.
4. Fireplaces should be deep and wider than they are high.
Count Rumford proved just the opposite but, being a Tory at the time of the American Revolution, nobody believed him. The Canadian Centre for Research and Development in Masonry has just again proved the efficiency of shallow, high fireplaces, but I guess we can't believe them because they spell "center" a funny way.
5. Fireplaces must have smoke shelves.
The Canadian research has shown that smoke shelves are not needed, but fireplaces should have a throat and a flue close to the front of the fireplace and a chimney top damper instead of or in addition to a throat damper.
6. Hardwood is a better fuel buy than softwood.
All dry wood has about the same heating value per pound. Hardwood does become a better buy when it is offered at the same price per cord as softwood, because it is denser. Wet or green wood should be thoroughly dried before use or most of it's heating value will be wasted in boiling off its moisture. The same goes for wet buffalo chips.
The Rumford Fireplace
Sir Benjamin Thompson, Count of Rumford (now Concord, New Hampshire), did the original research on the design of energy efficient fireplaces 200 years ago. Unfortunately his work was ignored in America because Count Rumford was a Tory and because his fellow colonist, Benjamin Franklin, was inventing a stove which was even more efficient that the Rumford fireplace, although less cheery. More recently the Centre for Research and Development in Masonry, in Calgary, Alberta, has done sophisticated testing of the Rumford Fireplace and has proved its high efficiency.
When I was in grammar school, our science teacher asked the class how many ways a book could be moved across the room. After much discussion, we said that a book could be carried across the room. Fine, the teacher said; that would be tantamount to convection. Then we said a book could be passed from student to student; that would be conduction. Finally we said a book could be thrown across the room; that would be radiation.
Most useable heat from a fireplace is in the form of radiation as opposed to convection (which goes up the chimney) or conduction (which soaks into the masonry and is then radiated). The Rumford firebox approaches the shape of a flashlight's shallow, parabolic mirror, and maximizes the radiation of heat and light into the room. Conventional fireboxes are so deep and low that a lot of the radiation bounces back and forth and is lost.
Notice the different dimensions between similar conventional and Rumford fireplaces:
The Canadian research has resulted in some other fireplace improvements: 1. A fireplace does not need a smoke shelf. 2. The flue should be near the front of the fireplace rather than the rear and there should not be a mantel. 3. The fireplace should be inside the house, not against an exterior wall. 4. The damper should be at the top of the chimney instead of (or in addition to) the throat. 5. Glass doors should not be used; metal doors should not be closed until the fire has died down to embers. 6. the fireplace should be at the floor level rather than raised.
A fireplace does not need a smoke shelf to keep it from filling the room with billowing smoke. That will be prevented by building a proper throat (3-1/2" deep and the full width of the fireplace), by constructing all the heat passages with smooth surfaces to minimize turbulence, and by providing a large enough flue (at least 1/8 of the fireplace opening area).
The flue should be near the front of the fireplace to recover some of the other heat - the convection and conduction. Convection carries hot gas up the flue and conduction transmits some of its heat to the surrounding masonry. The masonry face of the fireplace gets hot and radiates heat into the room.Because the face is intended to get hot, there should be no mantel or any combustibles adorning it.
The entire fireplace (except its foundation and chimney top) should be inside the house. Even though the flue is toward the front of the fireplace, the backsurface of the fireplace does get warm. This warmth should be used rather than wasted, which would happen if the fireplace were built in an exterior wall. An interior fireplace has another benefit, even when there is no fire in it. Its mass acts as a heat sink, helping to maintain an even temperature within the house. That is why wine cellars and dynamite magazines are built with masonry. If the fireplace is positioned close to a south facing window, then the sun can sink heat into it too.
The damper should be at the top of the chimney to keep out cold air which would otherwise make the chimney a "cold" sink. It is so cold in Canada that they recommend two dampers, at the top and at the throat. The damper should be so balanced that, if the control wire breaks, the damper will fall open instead of closed. The Canadians, again, recommend a second control to open the damper in case of ice jamming.
Because of the shallow depth of a Rumford fireplace, the fire itself is close to the front. Its intense heat would destroy glass doors. The New Mexico Energy Research and Development Institute has determined that glass doors are impractical even with conventional fireplaces. They cause more radiation heat to be lost than convection heat is saved. When the fire dies down to embers so that it is not radiating much anymore, then the opening can be blocked with a metal sheet to prevent warm room air from going up the flue. Of course when the fire is completely out, the damper should be closed for that purpose. The forward location of the fire explains why a large hearth is needed to keep sparks from jumping onto combustibles.
The firebox floor should be at the room floor level, because a raised hearth would make a radiant heat shadow. Also a raised Rumford hearth would jut considerably into the room.
John McNear, 4/11/11
Buckley Rumford Fireplaces
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