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Rumford Fireplaces: Safety vs. Efficiency


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The Rumford Fireplace: Safety vs. Efficiency

Count Rumford, actually an American by the name of Benjamin Thompson, gained fame by rebuilding more than 500 fireplaces in England. It became a matter of pride to have a fireplace constructed by his masons.

In the 80's, as a journeyman mason, I began to study and build his fireplaces which became a real problem with the Montgomery County, Maryland building department. The Rumford breaks all the rules. In the end, I convinced them to permit the shallow firebox in trade for a deeper and wider hearth.

Another challenge to constructing a Rumford was the cast iron dampers were too big. I was able to overcome this by bringing the top of the firebox back forward of the damper flange, but if I had it to do again I would use a chimney top damper.

Anyone that I ever built a Rumford for will happily tell you that the heat they produce will drive you from the room.

Before we consider the design of the Rumford it would be helpful to understand what are the most common problems with a masonry fireplace.

What causes a masonry fireplace to smoke?

1. An insufficient throat area. If there is not a sufficient space just above and behind the fireplace opening and below the damper for smoke to collect and roll prior to passing through the throat it will come back into the room. The height of the opening is not critical, but this area behind the top edge of the opeining is.

2. A flue that is too large. That sounds crazy, but bear with me. Smoke is actually heavier than air. As smoke cools, it becomes heavy and the moisture (sap) in it begins to condensate on the sides of the flue in the form of creosote and glazing.

In the 80's I constructed a radical masonry wood stove chimney flue under the direction of Professor Jess Brown from the pyrotechnics department of Auburn University. The wood stove exhaust was an 8" diameter. We used round terra cotta flues all the way up and surrounded them with perlite insulation (good to 2000 degrees) Three years of monitoring revealed that the flue never needed cleaning. All heat and creosote remained in gas form until released from the top of the flue.

3. Taller objects near the flue. The top of a tree or roofline that is near and taller that the chimney can cause wind to curl over and down into the flue.

What are the unique and successful features of the Rumford?

1. The fireplace opening was huge by todays standards. Most masons would look at a Rumford and conclude that it's going to smoke, but it doesn't.

2. All firebox surfaces were angled to radiate the heat directly into the room.

3. The throat was a mere 4", just large enough to prevent smoking while minimizing heat loss.

4. The smoke shelf was just large enough to permit the passage of exhaust and return air.

5. A plumb bob held in the center of the flue also hits the center of the firebox floor. (Heat rose directly up without any restriction.)

Unfortunately, to meet todays safety concerns and standards the masonry fireplace has returned to the inefficient dark ages.

Next: Why brick veneer was falling out of high rise buildings onto parked cars in Texas.

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I have a question about smoke shelves, which seem mysterious even to experts who teach educational seminars. You mention that smoke shelves permit the passage of exhaust and return air, which makes sense, but which also seems slightly oxymoronic. What exactly is the purpose of a smoke shelf, and what are the underlying principles that allow it to properly perform?

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Well John, it's funny you should ask with your ninja suit and sword. I practice Aikido and Thai Chi. Some of fireplace function and theory is as mysterious as martial arts, and I wish someone would confirm some of it through aerodynamics. But, allegedly coming down through that spiral of up rising exhaust is return air. Allegedly. if you had closed glass doors, return air coming down the flue will continue to fuel the fire. I readily admit, it's beyond my understanding. I just built them and am not a master of aerodynamics or pyrotechnics. Much of masonry fireplace construction is tradition and what has proved to be reliable. But, more than any other mason, Benjamin Thompson actually seemed to understand what he was doing and why. Every dimension of his fireplace construction was ratios based upon the original fireplace opening.

In fact, I love listening to and reading books about the founding fathers. John, it appears that those guys back then were so much more educated than we are today. Their understanding of mathmatics, language, science, and botany was sobering. I fear that many of those men would consider us in comparison dunces today! Sad, but true. I've spoken to men from Europe that have made the same claim. I've been told that our college was European High School! Kinda scary.

At any rate, to answer your question as best I can, cold return air bounces off the smoke shelf and joins with exhaust to rise again up the chimney flue. That's the best I can do with this mystery. For more info refer to "The Forgotten ARt of Building a Good Fireplace" by Vrest Orton

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I'd like to add to the list of:

"What are the unique and successful features of the Rumford?"

6. The "rounding off the breast" in the throat to create a venturi nozzle effect to draw smoke up into the chimney.

2006220194439_rumford.GIF

Unfortunately I see many 18th C. Rumfords that no longer function due to 20th C. alterations.

1. Cast-in-place liners that are formed around the inflated tube always change the critical flue ratio needed for it to work.

2. Adding damper assemblies that create turbulence in the throat.

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We have quite a few Rumfords around here, somebody in the 70's was an accomplished Rumford builder. I have a friend who put 2 on his patio. They really are quite amazing. The wood usually needs to be burned upright due to the shallow box. If correctly designed, the back of the fireplace is actually pretty much a parabola which tends to explain the heat output.

Rumford was a pretty amazing guy. I remember a good artical about him in Natural History about 10 years ago.

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Fritz,

We take so much for granted these days. People like Benjamin Thompson, Thomas Jefferson, Benjamin Franklin and John Adams accomplished so much without the aid of laptops, slide rules or calculators. Many of these guys spoke two, three and four languages fluently and we think we're so smart. They would probably consider us lazy and mentally challenged. :-)

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Everyone knows I love old books, so I remembered a diagram in an old Home Mechanics Book from VanNostrand that shows how a "fireplace" works. When I first saw it, I knew it was a Runsford stove, but it was published 70yrs ago - - - -.

Photos of book a little blurry, but gives you the idea.

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  • 2 years later...
  • 3 weeks later...

We are starting the construction of a Rumford fireplace, and trying to follow the specs as much as possible. One point that we do not feel comfortable with is the flue size. The only info we have found for sizing the flue says that the flue should be 10% of the fireplace opening. Our fireplace opening is large, @ 60" square. This 3600 sq. in. X 10% = 360 sq. in. Using a square flue, this would be 19", which seems like overkill.

Also, in this discussion board, there is mention that by keeping the flue size down, the creosote, etc., stay in gas form, rather than cooling and condensing in the flue.

Any advise on this would be greatly appreciated.

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Go here:

http://www.rumford.com/mfg.html

The proprietor, Jim Buckley, went into the business of Rumfordizing conventional fireplaces way back in 1982 and over the past 26 years has probably learned more about these babies than anyone else in the country. He can probably tell you what's what without even lifting a gray matter pinkie.

ONE TEAM - ONE FIGHT!!!

Mike

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