Rumford Technical Discussion
Heating Water
And Other Heat Transfer Issues
The main issue is the basic laws of physics - the conservation of energy, the first law of thermodynamics or, as masons phrase it, "There ain't no free lunch". The Rumford fireplace is the best wood-fired high intensity radiant heater ever developed and, if you draw off some of that heat to heat water or air, it will be at the expense of the radiant heat. And, I might add. you are unlikely to make the Rumford into even a mediocre hydronic heater without a lot of research and development. There are a lot of good boilers and air heaters. But trying to heat water or air with a Rumford would be like trying to plow with a Jaguar.

I do have a friend who developed a stainless steel pipe loop to heat water in a fireplace but, after experimentation, concluded that the pipe had to be in the firebox - not buried behind the firebrick where it didn't get hot enough for effective heat transfer. In the firebox it was also accessible when it needed to be replaced.

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Tue, Feb 16, 2010

Hi,

Do you have any experience with collecting heat from a fireplace with hydronics?

I am think of placing copper tubing in the space between the firebox and backing (just behind the firebricks). This would be an outdoor fireplace and the water/glycol would be circulated through the fireplace and to tubing under a bricked patio. The heat would be used to melt snow and also extend the outdoor eating season. Some concerns I have are with the temperature in the space behind the firebox where the copper tubing would be. Do you have any idea what this temp could be?

Thanks,
Kevin Wiebe

    Kevin,

    The main issue here is the basic laws of physics - the conservation of energy, the first law of thermodynamics or, as masons phrase it, "There ain't no free lunch". The Rumford fireplace is the best wood-fired high intensity radiant heater ever developed and, if you draw off some of that heat to heat water or air, it will be at the expense of the radiant heat. And, I might add. you are unlikely to make the Rumford into even a mediocre hydronic heater for the reasons you suggest - the temperature behind the firebox, etc.

    I do have a friend who developed a stainless steel pipe loop to heat water in a fireplace but, after experimentation, concluded that the pipe had to be in the firebox - not buried behind the firebrick where it didn't get hot enough for effective heat transfer. In the firebox it was also accessible when it needed to be replaced.

    To answer your direct question about how hot it might get buried in the fire box, the surface of the firebrick may get up to about 1,800 deg. F and the outside of an 8" thick firebox, by code, is not supposed to get over 90 degrees above ambient temperature so maybe 160 deg. F. I think you would be about right to assume the temperature gradient through the masonry would be lineal so buried in the middle about 4" behind the surface of the firebox, the temperature might be around 980 DEG. F (1,800 + 160 = 1,960 2 = 980) but only after equilibrium which might take days.

    My guess is that the radiant heat from the Rumford might do a better and quicker job of melting the snow on the patio.

    Best,
    Jim

***************

2/15/07

Dear Jim,

Thank you so very much for your wonderful web site with so much information.

I have a couple of comments on what I have read. Firstly, a tepee fire is fine unless the flames are going up the chimney, then you are loosing more of your heat and if you are building a large fire it probably would be better to stack the logs horizontally. Secondly, fireplace air would be better gotten from the stale house air and the outside air used to freshen the HVAC system air. Thirdly, I would think that the make-up air being vented in the fireplace would tend to de-stabilize the air flow and reduce the flow at the top of the fireplace - thus causing smoke in the room.

We are building a 18,000 sq. ft. lodge that is 82' diameter and 4 stories high that has a great room of 1750 sq. ft. with a 20' ceiling. We have placed a 8' wide by 6' high (or 7'x6') Rumford fireplace at its focal point and would like an arched opening (similar to the 42" stone arch shown in your web-site) for esthetic reasons. Any excess heat from this large room will be drawn off by the HVAC system and stored in a 15,000 gallon solar hot water tank for future heating needs. The heavy steel chimney is 55' high and will incorporate a heat exchanger that will give high temperature water for storage in a 4,000 gal tank. There will be a remote controlled variable-closure damper at the top of the chimney rather than one in the fireplace throat. Also there will be an adjustable make-up air blower for the HVAC system to maintain a positive inside pressure in the lodge.

I am looking for some assistance on the design details that would take advantage of the lack of a damper in the throat and the arched opening by shortening the width of the throat and smoke chamber (assuming the smoke would be less likely to spill at the sides because the height of the fireplace opening on the sides is lower). Also because of the length of the chimney I would like to minimize the diameter to the absolute minimum.

Some questions: 1) What are the ideal width to height ratio of the fireplace?; 2) You apologize for the sloped back on the 7' & 8' plans, but Rumford says that when he sloped the back he got more heat than any other fireplace he constructed, why is that? 3) Can we reduce the flue size requirement because the archway would reduce the fireplace sq. ft. opening?; 4) Can we reduce the flue size requirement if we add a couple of layers of brick across the bottom of the fireplace opening to contain the ashes, thus reducing the fireplace opening?

    Dale,

    Thank you for your kind words about our website and for your thoughtful comments.

    I don't agree with you, however, about horizontal fires. See the comments at the bottom of the page at http://www.rumford.com/certification/design.html. Big or small, it's cleaner-burning and more efficient to keep the smoke and volatile gasses pouring out of the ends of the newly involved logs in the hot center part of the firebox rather than letting these gasses get mixed up with the dilution air and lost, unburned, up the chimney.

    I think I do agree with you about the ineffectiveness of outside air ducted directly into the firebox. My reasons are stated a little differently at http://www.rumford.com/tech7.html

    I think your plan is unique and will probably work but I have the following comments:

    1) Arched openings are fine. See our recommendations on how to maintain the streamlining at http://www.rumford.com/arched.html

    2) You won't be able to "draw off" excess radiant heat with the HVAC system if, as I understand your concept, you mean to store the heated air. Rumfords heat radiantly - people and surfaces not the air - like the sun.

    3) If you mean to draw off the excess heat by heating air or water with the Rumford it will be at the expense of the radiant heat. See http://www.rumford.com/Sargent.html and http://www.rumford.com/circulating.html Rumfords are the best wood fired radiant heaters ever developed but are not particularly good air/water heaters. You'd be better off with a smaller Rumford and a separate boiler to heat the water.

    4) Theoretically if you reduce the flue temperature by extracting heat from it to heat water, the draft would be less and you would have to increase the size of the throat and flue to maintain the draft. That is if our Rumford were perfectly tuned. As a practical matter, since we purposely make the throat just a little too big to make sure the fireplace draws and, because your chimney is unusually tall, you probably can draw off a little heat from the flue. But, again, a boiler would be a more efficient way to heat water - and be careful not to be seduced by the idea of getting something for nothing in violation of the principle of the conservation of energy or the first law of thermodynamics.

    5) I like your balanced ventilation system and neutral or positive indoor air pressure. That alone obviates the combustion air in the firebox idea.

    I think I've hit the questions about arched openings. I wouldn't reduce the flue size since it will need to be bigger to the extent that you are able to draw heat from it to heat water and in any case you can control the flow at the throat and the top damper. And, if you guess wrong and the flue is too small, it would be difficult to enlarge.

    Some questions:

    1) Rumfords are generally as tall and they are wide however small Rumfords tend to be taller than they are wide and large ones lower than they are wide. Ours are all "tuned" to be square in that the throat opening is designed to be one twentieth of the area of the fireplace opening. Make the firebox taller and it will be more temperamental - lower and it will draw too well and be less efficient. Make the opening lower so it will draw better because you plan to extract heat from the flue making it draw less well and you end up with a net zero efficiency gain - if your heat exchanger is as efficient as the radiant-heating Rumford is.

    2) We slope the fireback on the seven and eight foot Rumford to bridge the top of the deep firebox reluctantly because we know it to be wrong but because the slope is gentile, we can get away with it by making the throat opening a little bigger and we haven't gone to the expense of making a throat tile with a larger radius that would bridge the firebox without sloping the fireback. Rumford did in fact, in a footnote, say that when he sloped the fireback he got more heat than any other fireplace he built. But, he was talking about a very small fireplace, probably only 18" wide, in which he created a deepened firebox in order to make it deep enough (12") to support combustion. His little anomaly featured a sloped back immediately above the fire but then straightened up to perpendicular just like all his other fireplaces - nothing at all like the slope in our big Rumfords or those Orton erroneously attributes to Rumford. See http://www.rumford.com/articleOrton.html This was one little fireplace out of 600 or so Rumford rebuilt. In his second essay, written after that anomaly, Rumford insisted that the back always be plumb and the breast rounded. He never again mentioned that little fireplace, subject of the footnote in his first essay.

    In answer to your last questions about reducing the flue size because of the arch or the inner hearth being lower, I don't know but I think it would be of marginal if any benefit. Again, if you guess wrong, it would be hard to enlarge the flue and it's not necessary to reduce the size since you can control the flow at the throat and with the top damper. Probably a couple of Pascals difference in indoor air pressure would have a bigger effect.

    In spite of my cynicism I suspect you will build this monster so let me know how it turns out and what you learn.

    Warm regards,
    Jim Buckley

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