1/6/05
Hi Brian,
Let me try to respond below as you ask the questions........
Greetings!
Your site is quite excellent and very enjoyable, and I hope that your
business is flourishing.
I am a lover of wood heat (and of old houses, old techniques, and old things in general), and have been fascinated by the Rumford design since I discovered it a couple of years ago while researching heating options for our home. After having read your online version of Count Rumford's essays and having looked at your design sheets for the Superior Clay prefab elements, I have a couple of design questions, bearing in mind that I am neither a professional mason nor sweep (although I have some knowledge of such things), and have never had the privilege of seeing a true Rumford fireplace at work or rest:
1. Rumford mentions in his essays that problem fireplaces should have
the throat lowered and placed closer to the fire, but you advocate
having as tall a back as possible. How is this reconciled in your design, and does it have to be taken into account when 'Rumfordising'? (Or am I misunderstanding 'tall' as meaning significantly higher than the width of the fireplace opening?)
Fireplaces in the 18th century were generally taller than modern fireplaces. Relative to a 1970 fireplace that might be four feet wide and only 27" tall, we can usually rebuild it to be four feet tall using a Rumford streamlined throat. We agree with Rumford that lowering the throat improves draft. Our Rumfords are "tuned" to have square openings. Build a 36" wide Rumford 32" tall and it will be bullet proof. Build it 24" tall and it's suck small children off the floor. Just kidding, but remember the objective is to build it so that it vents all of the smoke but not too much excess heated room air. For best efficiency build it as tall as it is wide as designed.
2. In each of Rumford's engravings, the front edge of the throat is even
with the front edge of the flue/smoke chamber, which he mentions should
proceed straight upwards thence. By design, this produces a small smoke shelf behind the back of the throat, which would seem to help calm any
downdraughts that may occur as the heated gasses are jetted into the
flue by the 'nozzle' and travel up the front portion of the flue. Your
design however has the throat centred in the smoke chamber: would not that be less efficient, and cause more turbulence and possible creosote
deposition in the smoke chamber? (Rumford seems to have been intuitively aware of the advantage, if not necessity, of a smoke shelf when he discusses in Ch. 2 the need for the throat to 'end abruptly', with a horizontal space or shelf behind it, rather than expanding 'in the form of a trumpet' to the width of the flue.)
You're a careful reader and this is a more complicated question than it might at first seem. First of all it's my belief that Rumford's "smoke shelf" was merely the inadvertent result of building a shallow reflective firebox within an existing deep firebox. See my argument in the article at http://www.rumford.com/articleOrton.html As for Rumford's comment about ending abruptly, it certainly should not be extended to imply Rumford thought there was air flowing down the back of the chimney as Orton and others do. Rumford said no such thing.
Did you pay attention in high school physics? In my class, in the introduction to fluid dynamics, we studied flow from a 6" diameter pipe into an 8" diameter pipe with a 90 degree abrupt transition. No resistance at the transition in that direction was measured. But when the flow goes the other way from the 8" pipe into the 6" pipe there is a significant resistance and reduction in flow. The sharp 90 degree reduction causes turbulence that pushes the laminar flowing streamlines down to just a couple of inches in diameter so that the flow is as it would be in a pipe with a diameter of only two or three inches. Applying this idea to the Rumford throat, the sharp flat abrupt transition at the exit of the throat works as a partial check valve letting air pass freely up through the throat but resists flow back down through the throat. I think that's what Rumford meant by his comment about and abrupt end.
So what? we might ask. Rosin didn't get the point and advocated a streamlined exit to the throat and Rosin fireplaces work well. See http://www.rumford.com/tech16.html Rosin apparently thought all turbulence was bad but I have seen no particular advantage to his "trumpet shaped" throat exit recommendation.
Perhaps Rumford's flat abrupt termination to the throat helps when there are gusty winds or perhaps a sudden change in indoor air pressure as might be induced by slamming a door, but generally the flue gasses should go up the chimney and the volume of air in the smoke chamber acts like a shock absorber to even out any vicissitudes.
One thing I have learned from experience is that the "smoke shelf" does not have to be at the back. It could be toward the front or perhaps most effectively, all around the throat opening. We have plans showing it in all these positions. See http://www.rumford.com/plans/R3636exterior.gif and http://www.rumford.com/plans/classicflyerplan.gif and http://www.rumford.com/backtoback.html (While the plan doesn't clearly show the smoke chamber, these back-to-back plans often call for the smoke chambers to be set forward so the "smoke shelf" would be in front simply because there is no room to set the smoke chambers back any farther.)
3. Rumford also mentions towards the end of Ch. 2 that one should extend the back and covings of the fireplace 5-6" above the top of the breast, which would make a straight, narrow, 4"x[n] trapezoidal channel before entry into the flue a feature of the design; almost a 'beak' to the nozzle, if you will. Your design does not appear to have this: was it found unnecessary, or is there another reason?
As I read it, that's exactly what we do. Look at Rumford's plates at http://www.rumford.com/chimneyfireplacesp.html and note that there are some variations on this point between, say Fig 6 and Fig 13. Now compare with our throat in the plan at http://www.rumford.com/plans/classicflyerplan.gif Our actual throats have a curve with an increasing radius so that the curve flattens out near the top, while I made the drawing with a constant radius arc, because it's easier. Maybe you can discern the subtlety from the picture at http://www.rumford.com/Todd.html In plan view our throat opening is trapezoidal like Rumford's. It's the back 4" or so of the firebox in plan view.
4. Would there not be a slight aerodynamic advantage to having the bottom portion of the breast above the fireplace opening a little wider before the beginning of the curve, such that there is not an abrupt corner that the room air must round before travelling up the curve of the breast? And have you ever tried or found any advantages to rounding the corners of the sides of the fireplace opening as well, to provide a smoother passage for the combustion air?
Again our throat is essentially flat (horizontal) at the top of the opening and curves to be vertical at the top of the opening. Within the limits of manufacturing clay products it is a smooth airfoil. Maybe you are focusing on the outside corners of the surround sides and top. Rounding these corners might have a slight aerodynamic advantage but it wouldn't look like most people expect their fireplaces to look and I think the advantage, if any, would be very slight. The air is gathering and flowing very slowly around these sharp edges and it's therefore not so important to be streamlined as it is farther up in the throat where the gasses are flowing faster - different Reynolds numbers and all that.
5. When doing a 'Rumfordisation' of an existing fireplace, if you use
plaster for rounding the breast, do you employ regular plaster of Paris,
and find it sufficiently durable for the task? I ask because in
Rumford's day (and into the first half of the 20th century) 'plaster'
meant lime plaster, which is much tougher but more difficult to work with.
Firstly, I would use a Superior Clay Rumford throat for a "Rumfordization" as shown in the pictures at http://www.rumford.com/rem.html We do sometimes smooth out, bring down or fill in behind arched fireplaces with plaster. You are right about historic lime plaster being different than modern plasters but we've had good luck with light weight brown coat plasters like Structolyte that we can slather on two or three inches thick. It seems to perform at least as well as Portland based mortars. Better than any plaster, of course, would be a refractory mortar like HeatStop II or Insul-Stik featured at http://www.rumford.com/store/cms.html
6. Since the firebox is comparatively shallow, how much farther must the hearth be extended for safety reasons than in a modern, run-of-the-mill fireplace?
US codes don't require any additional hearth extension to make up for the shallow box. It was discussed at code meetings but the argument that even in a deep fireplace you can build the fire out at the front won the day. The Canadian code, however, does require that the heart extension be at leas 36" forward of the fireback which would mean a 12" deep Rumford fireplace would have to have a 24" deep hearth extension rather than the minimal 16" or 20" hearth extension.
Incidentally, I see that your 'Road Show' is going to be in Herndon, Va.
later in the month, which is not too terribly far from Baltimore. What
is the event, and is the public welcome to come and see your display?
It's a Masonry Alliance for Codes and Standards (MACS) meeting and you would be bored to tears. Anyway, I just made arrangements to stay in Hawaii a few days after our meeting there so I'd have to take a red-eye to VA and am trying to get a substitute.
Do keep in touch. You've read all this stuff and I don't have many people as interested and knowledgeable as you are to talk with.
Thank you,
Brian Cardell*
Baltimore, Md.
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