Reader Comments
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Comments on Balanced Ventilation 12/5/13 from a MHA chat, with permission from Gary Sharp - That's not surprising (super tight houses and strong exhaust fans). Some of these range hoods now are blowing up to 1200 cfm out of the house. (A clothes drier is 150 cfm for comparison). This can significantly depressurize a house, so air is trying to get in anywhere it can to equalize the pressure. If a passive vent (i.e. a hole in the wall) is used to allow enough make-up air to compensate for the big range hood it needs be about the size of a basement window. Obviously, this is crazy to cut a hole this size in your air-tight house, which is why no one puts them in. So the make-up air sees the chimney flue and the outdoor air supply as a way into the house. I usually try to explain to home owners that if they want to use a big range hood like that, they need to also install another fan of the same size blowing into the house (that runs at the same time) to keep the pressure balanced. Homeowners don't like that idea much. I have heard of one builder who installs an inter-lock on the range hood to a kitchen window. The range hood won't start unless the window is open. You guessed it - home owners don't like that much either, but it does drive home the point that the air for the range hood has to come from somewhere, and it's better to come from a window than down the chimney. It's not a fireplace problem - it's a mechanical equipment problem, but you guys get blamed for it. The companies making the equipment that takes the air out of the house need to also be supplying the solution to get the air back into the house. (balanced flow) Every range hood should be supplied with a make-up air fan and instructions for how and where to install it.
Gary Sharp
I understand by adding an outside air intake into the return air side of the HVAC system below the neutral level of the home would resolve down drafting or negative pressure but I do not understand how an air to air heat exchanger would help. My definition of a air to air heat exchanger is a heat recovery ventilator HRV or a energy recovery ventilator ERV (depending what part of the country you live in) These devices are to be equally balanced meaning the outside intake air should be the same as the exhausted air from the space. Therefore, equally replacing or changing the air in the space. Maybe I'm not seeing the whole picture. Please explain.
I am an HVAC contractor dealing with negative pressure in a large custom home with 2 Rumford fireplaces and am trying to solve the problem. I have concluded that the house is very tight and there is not enough make up air. Unfortunately the lower level is finished and can not introduce outside air without major construction and opening a window when using the fireplace is not an option. Any suggestions?? Do you have a "rule of thumb" for calculating make-up-air requirements?
Many Thanks
Thanks for spelling this out so well. I've been sending clients with odor and draft problems to your site because sometimes it seems they understand better reading it from someone else other than me just telling them.
I noticed you have a new date of 1/5/06. Did you change what was previously written?
I didn't save a copy of the previous writing.
My only suggestion is to mention attic doors being sealed well as this seems to be a problem area in our area.
I just heard about a product called the attic tent which appears to be a good product but I have not had any experience with yet. I would be interested in reading your thoughts on it.
Thanks again!
Dear Jim:
I believe that the Canadian mason was suggesting that "tight" Canadian homes making suitable draft was problematic without an outside air source. I suspect that an outside air source located close to the firebox would be an important addition in any "tight" well-built home, regardless of geographical location. My home in Florida was constructed with care, and my wood burning stove does not draw properly when all the windows are closed. However, when I open the outside air vent located under the stove the stove draws beautifully with the house closed up tight.
Sincerely,
Mike,
There have been some disastrous consequences as a result of building tight homes in Canada without adequate ventilation - mildew and indoor air pollutants caused by back drafting appliances just to name two. The R 2000 homes in Canada as well as the energy code in Minnesota modeled on the R 2000 program are all but unbuildable and defunct.
We belive in balance. Bring in as much fresh outside air as the stale air you pump out. That way your appliances are not all in competition with each other and they all work better. And your home is more comfortable and more energy efficient because, with neutral indoor air pressure, you are not taxing your envelope so severely, sucking in outside air through every crack or leak. If you want more efficiency and fewer air changes, limit the exhaust - use smaller fireplaces and kitchen fans, use whole house systems for bathroom venting, etc.
Fireplace do take some air. See http://www.rumford.com/smoky.html#air for the calculations. But you can use the fireplace to vent stale air instead of an air change fan if you think of your ventilation system as a whole. For more on balance see http://www.rumford.com/ventilation.html and for why not to put air directly into the firebox - or even next to it - see http://www.rumford.com/tech7.html
Best,
Jim: I couldn't agree more - windows are wonderful things. Mike
Back to Balancing the Ventilation System
Comments on Chimney Clearances
On Mon, Nov 5, 2012 Dann Batting
Jim;
Thank you for your prompt response. The Code as you have stated is less than clear on the subject of clearances to combustibles.
I am attaching, below, a copy of what I sent to the ICC that I had intended on sending to you originally.
As I further study this issue I am perplexed even further. If I am reading the codes correctly, and this refers both to the IRC as well as the 2010 NFPA 211, the language seems to indicate that an interior chimney within in an enclosure need only have a two inch air space from combustibles. I assume that includes a wood stud chase wall, which is how every chimney I have ever seen has been constructed. The 2010 NFPA 211Table 7-2 also indicates that, for a residential chimney, the chimney wall thickness can be 4 inches of brick with a 5/8" fireclay flue liner and a 2 inch air space between combustibles.
The other interesting observation relates to the figure 2113.19 in the 2009 IRC where the exterior chimney is shown, as if constructed with an obvious 4 inch masonry wall unit and a clay liner, can be within an inch of the wall construction while the ubiquitous combustible material abutting the masonry on the exterior needs to be 12 inches away from the flue. I would think that if the 12 inch rule truly applied applied the air space would need to be about 7-1/2 ". All in all I think the codes present a lot of confusing information.
In my 40 plus years in this profession I have always tried to play by the rules but it is very frustrating when the rules are unclear.
Thanks again for you interest,
dann
I am an Architect in New Hampshire and have a question regarding clearances to combustibles as defined in section 2113.19 of the IBC and section R1001.15 0f the IRC and specifically to the requirement for a 12 inch clearance from the inside of the nearest flue lining as depicted in illustration figure R1001.15. The illustration clearly shows that a 1 inch air space is required from the chimney and the exterior wall sheathing while a 12 inch clearance is required to a combustible piece of trim. This seems confusing and the illustration also seems to depict a chimney on the exterior of a building.
I have a client who is reconstructing a chimney on the interior of their home with two non modular 8" x 8" clay flue liners enclosed by a single wythe of brick with a single wythe of brick separating the two liners. This new chimney will service an existing oil fired furnace for one flue and a future wood fired furnace for the other flue.
On two walls the new chimney is separated by the minimum ( 2" ) two inch airspace required to combustibles which has always been a standard. The client wishes to leave exposed the two remaining sides of the chimney and has been told by the local Building Official that any trim including the wood flooring will need to be no closer than 12 inches to the inside of the clay liner. If done this way there would be a 7 inch gap between the chimney and the finished flooring while the rough framing and subfloor boards will be only 2 inches away and properly fire-stopped.
This issue has been reviewed with the local fire chief , the mason who is building the new chimney and other code officials who all have never heard of this requirement before.
Needless to say that we are all frustrated and are looking for clarification. There has been some indication that if stainless steel liners conforming to UL 1777 were added that the clearance issues would go away.
Can you advise if possible as to the specifics regarding this issue.
Thank you,
Dann,
Thank you. I will keep your comments for the next time we submit a code change proposal, probably for the IRC next year.
Responding to the exterior/interior chimney you raised, the 12" from an interior lining surface applies to both interior and exterior chimneys and means that the chimney wall must be at least 12" thick in order to be in contact with combustibles. That would be true for an interior chimney as well. If you corbelled your chimney out so that it had 12" thick walls around your 8"x8" flues then combustibles could be in contact with it. We agree with the concept of making the chimney walls thicker if they are to be in contact with combustibles. We just think 12" is excessive and we have historical data and are working on test data to show that 8" thick chimney walls in contact with combustibles are more than adequate and at least as safe as 4" chimney walls plus 2" of air space currently permitted.
In response to your comments addressed to Chris ....
1) It's not a 7.5" gap that is required. Only a 2" gap is required. But if flooring or other combustibles are to be in contact with the chimney, the chimney walls must be 12" thick so that the flue liner is 12" away from any combustibles in contact with the chimney. An alternative would be to trim with some non-combustible material.
2) You are correct that you could avoid the problem by using a liner listed to UL1777 but we would like to avoid this solution due to complexity, cost, short life of these systems and the fact that these lining systems are generally intended to line the entire chimney. There are some masonry lining systems that meet the UL1777 standard and would have a longer service life. Maybe you could use those systems to line the small portion of the chimney as it passes through the floor or roof where the problem exists and not have to line the whole chimney. It's interesting to note that most of these UL1777 liners are able to pass the test by adding only 1/4" or 1/2" of insulation around the liner.
For Chis's benefit I have, again, attached the paper on the issue that I presented at the recent IBC code change hearings.
Best,
Best to supply air to the room or house not the fire box. When you go up to supply air to the fire box then it can act as a chimney too but if you have to follow code you can do that then plug it after inspection. Its called fake!!! air supply. Sorry but some codes just sucks. The worst place for air supply is inside the firebox,the air should always come from the fireplace opening so it draws like it is supposed to. If you get big wind from the right direction it can force smoke back in the room.When I have to put them in I tell the client to plug it with the rock wool I give them .The ones that dont always call me about smoke odors. - Wilcox Masonry, 2018
To: Jim Buckley
We were using our Rumford for the first time this fall and a friend noticed that the back vent got so hot that it started to set the siding on fire... So we won't use it again until we figure this out. Our contractor had a mason do the work, his first fireplace but he had the materials you provide with the kid available, so what did we do wrong?
James P. Danky*
Outside air vents in the firebox have nothing to do with a Rumford fireplace and we do not recommend them. See our comments at http://www.rumford.com/tech7.html where we include "fire hazard" among the problems with these vents.
I recommend that you replace the vent with a firebrick and solid masonry. Make sure, while you are at it, that the firebox is at least 8" thick and 4" clear of combustible material behind the firebox as required by code.
I'd like to know more about this incident. Do you think the vent just got so hot it set the siding on fire or was it back-drafting? Do you have pictures? I have been trying to get rid of the misguided code requirements for combustion air and this incident may help.
Hoping to hear from you soon,
Thanks for being in touch. I spoke with our chimney/fireplace installer and he had already spoken with the veteran who helped him to do ours, his first. Of course he was in a panic about getting this right. He and that he would propose, not having been to our place just yet, that he mortar in firebrick to fill in the vent space; and that he go down to the basement where the concrete blocks support the whole and knock a hole down there so that we could draw air from the basement through the ash vent in the fireplace?
I did notice the photo on the General Instructions page which has a caption, "Note: That's a "cowbell" combustion air intake in the fireback." That should have tipped him/us off not to install it in our unit if I am reading this correctly. I am at a loss to understand why we did it this way unless it was Code, and we need to change our unit and that code I guess.
So, how does that sound for a fix? Thank you.
James P. Danky
That sounds reasonable. Any chance you could send me a picture of the vent - inside and out - and tell me if you think it was back drafting or just conducting heat through to the siding? I'd still like to use the situation as an example in my effort to change the code.
The air through the ash dump is probably safer and more common but, unless you have to install combustion air, I would just close off the one you have and not put any combustion air directly into the firebox. It will just lead to bad smells and blowing ashes. See http://www.rumford.com/tech7.html
I think I will photoshop that "cowbell" out of that picture on the Instructions.
Best,
4/7/06
Thanks for your thorough discussion of the modern Rumford fireplace.
I am planning on building a 48" Rumford into my new well sealed home.
Ventilation has been a prime consideration in the design of the house and
I'll be using a heat recovery ventilation system. I live in a very cold
climate and will be relying on a wood boiler as my primary heat source (with
a propane backup boiler). I am using the Rumford primarily for ambience but
in keeping with my conservation ethic, I want it to be efficient (thus the
Rumford design).
It seemed quite counter intuitive to bring make up air through the house,
thus cooling the air. When I first moved into the cabin that I have lived in
for the last 18 years, I opened the doors of the wood stove, picked up my
book and settled in for a quiet night by the fire. The outside air
temperature was about -10 F and while my feet and the front of my body were
roasting, my back kept getting colder and colder until it was unbearable. I
realized that the air that I was drawing in to the stove was coming out of
the room and being replaced through the poorly sealed front door resulting
in a wall of ice cold air moving across the room. The problem was solved by
closing the woodstove doors, but it isn't an experience I will ever forget!
Clearly the radiance of the open woodstove was small compared to a Rumford,
but radiant heat is unidierctional, warming only one side of the receiving
body, where convective heat is more even.
So, I was quite surprised when I saw that you recommended not bringing
makeup air directly to the fireplace. It seemed to me that while it would
reduce the radiant heat input, it would reduce the heat losses at distance
from the fireplace (radiant heat transfer being subject to the inverse
square law, while convective heat transfer is more linear) and the net heat
transfer should be equal (or less if the cold combustion air yielded a lower
exhaust air temperature).
Any thoughts?
Thanks!
Robin McClellan
Well, you've hit on the Achilles heel of any open radiant-heating fireplace. In very cold weather a heating appliance with a closed combustion chamber (your stove with the door closed) needs less dilution air and is therefore more efficient. It's not quite that simple, however. A radiant heater makes you more comfortable at cooler air temperatures so, even in really cold weather, your insulation works better and you pay less of a penalty for healthy ventilation.
My comments at http://www.rumford.com/tech7.html about combustion air in the firebox basically add up to it doesn't work and it comes with four pretty bad side effects.
My recommendation is that you find that compromise involving efficiency, ventilation and comfort that suits you and your home best. Here are some suggestions.
1) You might consider a smaller Rumford. A 30" Rumford would radiate far more heat than the open door of your stove but require about a third as much dilution air that a 48" Rumford needs.
2) You have a heat recovery ventilation system. Does it bring in as much fresh air as it exhausts or does it have a net exhaust? Can it be adjusted to bring in more fresh air than it exhausts to help make up for other appliances and fans and the fireplace that exhaust air?
3) You have to bring in as much air as you leak or pump out. If you don't plan for it, it will leak in anyway. Comfort depends largely on where you bring in the fresh air. Bringing it in somewhere remote or where it can be tempered, filtered and humidified such as in a cold air return or behind the refrigerator would be the most comfortable. Introducing fresh air near the fireplace such as an open window next to the fireplace so the cold air will flow from the window to the fireplace without you being in the draft would be next most comfortable. Opening a window behind you so that you are in the draft would be the least comfortable. All would be equally efficient - or inefficient.
4) Radiation is, as you say, directional. Maybe we could invent a rotating spit-like easy chair. But in practice, after a degree of equilibrium has been established, the wall behind you gets warmer and the whole room is more evenly comfortable.
All this adds up to taking a whole house view of your heating and ventilation system. Obviously you need some ventilation for good health even if it does make your heating system less efficient. Instead of pumping even more stale air out of your house and creating a situation where all your appliances are competing with each other, consider pumping fresh air in and balancing your ventilation system. In a time before mechanical ventilation, Rumford was pleased than his fireplaces could be used to ventilate as well as heat. He suggested cutting a vent into the flue near the ceiling to clear stale air out of the room and argued that radiant heat was compatible with good ventilation. Of course he lived in Bavaria and England - not Siberia.
Warm regards,
>I am in the process of planning a house to be built. It includes a
tom wright
Your Rumford fireplace will be an efficient radiant heater. Like the sun, it will heat people and surfaces - not the air. It'll make you feel comfortable at cooler air temperatures. That's why they use commercial radiant heaters in UPS garages, aircraft hangars and outdoor restaurants where it's not very practical to heat the air.
Most people these days rely on a furnace to heat the house and, if they have a Rumford, they use it to make a cozy and comfortable place to settle for the evening without having to turn up the thermostat.
Rumfords are pretty efficient heaters. If the thermostat for your furnace is in the room with the fireplace, however, you may have a calibration problem. The furnace thinks the house is warm and will not come on when you have a fire, so the parts of the house that the fireplace doesn't "see" and heat radiantly, get cooler. That doesn't mean the fireplace is inefficient. If you had an extra very efficient gas space heater or pellet stove in the same room with the thermostat, the rest of the house would also cool off. The fix is to plan for zone control heating, put the thermostat in a bedroom or crank it up higher to make up for the calibration problem.
The addition of outside combustion air has no effect on efficiency. It's called the first law of thermodynamics and is related to the conservation of energy and is commonly known as "there ain't no free lunch".
To provide enough combustion air for the fireplace to work you have two choices: Duct outside air directly into the firebox which cools off the firebox with the result that you get less radiant heat out of the fireplace. Or you can let air in through an open window or other vent remote from the fireplace which allows the fireplace to burn hotter and radiate more heat - exactly the same amount of extra heat that it takes to heat up the cold outside air coming in the house remotely.
The real question then becomes where to introduce outside air for ventilation and combustion and to make up for all the air being pumped out by various fans so that it won't cause draftiness, like an open window might, or bad smells and enough turbulence to make the fireplace smoke, like air vents in the firebox might. I prefer adding makeup air to the cold air return of the air handling HVAC equipment, or adding a vent into a mechanical or laundry room. Simply opening a window a crack somewhere near the fireplace isn't a bad solution either. Remember it's a radiant heater and doesn't heat the air, so the efficiency penalty for good ventilation isn't bad.
Best,
Hello! We have a beautiful Rumford that we had built five years ago. For the most part, it has been a fantastic feature in our living room and we love it. This evening, my wife noticed a draft coming into the room from the fireplace while a fire was going. I felt around and the wolf heads were emitting cold air into the room. We thought this was odd so looked into the wolf head installation instructions on your website. Turns out they were installed wrong! See attached photos. Is this a big deal? We have had some quirky issues over the years with the fireplace and wonder if this is related.
Thank you,
Since the Wolfheads don't work when the doors are open, I would just block them off so they don't leak cold air into your room. You do need make-up exterior air but its better to proved the make-up air somewhere else like in the mechanical room. See Exterior Air. By the way the fact that you feel cold air coming through the Wolfhead air intake blocks must mean the indoor air pressure in you house is pretty negative meaning that you need more make-up air to balance the ventilation system. Once you’ve done that the Rumford and all your other appliances will work better. See Balancing the Ventilation System.
You might wonder why we have the Wolfhead air intakes if they don't work. They do work but, as I said, only when the doors are closed which blocks most of the radiant heat. When we tested for emissions the regulators said that we had to test with the doors closed because the Washington State Indoor Air Quality Code required fireplace doors. That made no sense to us since Rumfords are open radiant heaters so we tested the fireplaces again open and passed the emissions test both ways. So block off the Wolfhead air intakes, open the doors and enjoy your Rumford. I use my Wolfheads as book ends. I do think they look interesting.
Best,
Emissions
Heard that about 100 ICC Rumfords were sold in Bay Area at county level last year. Also that Isokern may have sold at least that many. [Hard to believe]
Not opposed to publicity - "bureaucrats being obstructive" is a good issue but better part of wisdom is to keep it quite as long as we can get permits at the county level.
___________
On Thu, Feb 17, 2011 at 9:27 AM, Eric Pop
Hello Jim,
I hope your meeting went well.
I wanted to make sure you had information on the where the air districts authority. Here is a link to CA air pollution control laws, specifically some language which defines the jurisdiction of the BAAQMD: http://www.arb.ca.gov/bluebook/bb10/hea/hea-40200.htm
Let me know if this is what you were looking for. Also, a voicemail from you was passed along to me so let me know if there is anything else with which I can assist you.
Sincerely,
Eric Pop, Air Quality Specialist
___________
Wed, Feb 16, 2011
Jim
Fireplaces are not going to be passed if the Air Quality has anything to do with it. Novato has adapted an attitude that only gas fireplaces or approved units (not site built) are OK.
Mike
___________
2/10/11
City of Belvedere made a client tear down outdoor Rumford, according to his landscape designer. What rule are they following? City won't even let client put in a gas fireplace, outdoors, unless it's a dedicated gas fireplace. He's going to put in a fire pit instead.
Interesting Firebrick Discussion
(From The Masonry Heater Association
The following is a slightly edited version of a discussion on the Yahoo
MHAmembers list, about firebricks:
To: MHAmembers(at)yahoogroups.com
Will high-duty or low-duty brick last longer in a firebox? It
would be really nice if there were some more or less definitive
answer to this question, yet it seems that there is not. On the one
hand, we have Albie and Heikki saying (20 years ago, in their book)
that the commonly available low-alumina, low-duty brick will better
withstand thermal cycling/thermal shock better than high-duty, and a
conversation I had with the sales manager for Mt. Savage seemed to
confirm this idea. And there are no doubt many 30 year-old heaters
out there built of low-duty brick that are still doing fine.
On the other hand, a number of experienced heater masons seem
to be using high-duty brick for fireboxes lately, and that is what
Albie uses now too, as a 2" replaceable liner (though that may be
because you can only get a 2" brick in high-duty). Have people been
using high-duty brick long enough to know how long they will last?
What do the Austrians use?
Perhaps the answer is "nobody knows which is better", or
perhaps the answer is "it doesn't matter much, especially if you make
your firebox and throat replaceable". In any case, I would welcome
anyone's thoughts or experiences with this question.
PS "Low-duty" in my case means Whittaker-Greers.
--Steve Paisley
This topic will undoubtedly have many opinions.
Duty rating is based on temperature limits. These don't have
much applicability in masonry heaters, since temperatures are low by
normal refractory standards.
Of most interest is spalling resistance. One way to determine it is
from manufacturer's data sheets, which gives spalling loss in a standardized
ASTM test. You can do your own version of an accelerated spalling test by heating a brick red hot and dunking it in water, and seeing how many cycles it
can withstand.
Here is some quick info from Google:
http://heatkit.com/research/fbrick01.jpg
(snip)
Best ....... Norbert
Interesting topic.
We sell a lot of Whitacre-Greer firebrick - as well as Alsey and I-XL. In our Western markets Mutual, Muddox and Pacific Clay firebrick is available so we are familiar with them too.
I use the word "firebrick" loosely. The old ASTM C27 standard that differentiated between low, medium and high duty firebrick was withdrawn by its sponsoring committee in the early 1990's. In it's place was established C 1261 Standard Specification for Firebox Brick for Residential Fireplaces which is a less stringent standard than C 27 was. C 1261 only requires a modulus of rupture of 500 psi and a pyrometric cone equivalent (PCE) of 13.
Alsey makes the best firebrick available. What they sell for fireplaces meets the old C 27 standard for medium duty firebrick. They don't even make a low duty brick or a "Firebox Brick for Residential Fireplaces". Whitacre-Greer and I-XL firebrick meet the C 1261 standard. Mutual, Muddox and Pacific Clay firebrick don't even meet C 1261 but maybe that isn't even important. Interestingly I-XL used to sell a red firebrick that we found performed better than Whitacre-Greer firebrick but the I-XL red brick only had a PCE of about 8 and so I-XL stopped selling it as firebrick.
I agree with Norbert that PCE - or the temperature at which the brick melts - is irrelevant. The practical tests he suggests sound reasonable but I would suggest that how much a brick expands when heated is the only relevant or important characteristic. Residential firebox brick fails by cracking or spalling due to thermal shock. It doesn't have to be particularly strong and it doesn't have to withstand temperatures of more than 1800 degrees F. What's needed is a limit to the linear expansion when heated in order to describe brick that actually performs well as residential firebox brick. And that is my answer to the question about whether low duty or high duty firebrick performs better in a heater. The difference between low and high duty firebrick is basically the PCE which I think is irrelevant. The best brick for a firebox will the the brick that expands the least when heated up to 1800 degrees F.
Another interesting tidbit is the difference between dry pressed and extruded brick. Most firebrick is dry pressed but Muddox makes an extruded firebrick that performs differently. Muddox firebrick seems to do okay when laid as a shiner with the grain perpendicular to the firebox wall but it tends to spall when laid as a stretcher with the grain parallel to the firebox wall.
In summation, firebrick is promoted but not actually required by code but we are all interested in which brick or stone performs well in a firebox. I'd like to see C 1261 revised to include linear expansion and lower or eliminate the PCE requirement but, until then, maybe we can come up with a list of brick that, in our collective experience, does perform well for firebox construction.
Best,
Any brick, including firebrick, is fired carefully, varying the rate of temperature increase through several quartz inversion levels, to a specific temperature, usually over 2,000 degrees F, determined to be just right for the particular mix of clay and/or shale the brick is made with. Brick manufacturers spend a lot of time and money buying computer controlled kilns and testing their clays to get the brick to have the optimum characteristics while using the least fuel.
Using green, unfired brick, to build a heater and count on firing it in situ is not a good idea. Very likely you'll never get it hot enough - firebrick is fired to about 2,300 degrees F - or you will get it too hot too fast through one of the quartz inversion levels and it will explode.
Modern refractories are a good thing. Build your heater fireboxes with firebrick or castible refractories, perhaps with an insulated backing, and then build the enclosing heater body walls with any stone, brick, rammed earthen block or concrete you like.
Jim Buckley
Jim Buckley
Fluid Dynamics
Date; 2/20/05
Comment on the venturi effect discussion. I think the round concept question ignores a difference in objectives. In the fireplace you don't want the streams to mix. In a carburetor you want to maximize mixing effects.
Follow up question on the air space question. In the firebox, do you mortar the firebrick to the back up brick? Is so, what type mortar?
New question: Should the chimney be topped with a rain cap? If not, where does the rain water go? Do you leave weep holes in the outer shell?
Charles,
Answer 2/20/05
I suppose that a carburetor is not an adequate analogy - maybe not even a very good one. I hoped that it would be something familiar to someone who never heard of Bernoulli and couldn't spell venturi.
But I think your comment about objectives is just a matter of focus. Where exactly - before or after the nozzle - you are focusing? The air intake of a carburetor is designed to keep the flow of air laminar and to maximize it's volume and speed through the narrow part of the venturi because it's main function is to draw in fuel by the reduction in pressure created in the venturi. Then, as the mixture of air and fuel exits the carburetor into the manifold and certainly by the time the mixture is drawn into the cylinders, you want the streams to mix turbulently for good combustion - especially in the cylinder - which is one reason why the Chrysler Hemi engine was so good.
Like a carburetor, the flow of air into the curved Rumford throat is laminar. That's the main purpose of the Rumford throat so we focus on it. The intake to a carburetor does the same thing but it's not the main objective and not where we focus. Does flow through the Rumford throat create a partial vacuum that helps to draw the smoke through the throat? I don't know, but it's not where we have focused. The analogy may be lacking since the main function of the Rumford throat is to keep the flow laminar until the mix is shot into the smoke chamber while the main function of a carburetor is to create a reduction of pressure to draw in fuel. However, in both a carburetor and a Rumford throat, the flow into the venturi is laminar and the flow out of the venturi is turbulent.
You don't say so but I suspect you know a lot about fluid dynamics. Maybe you'd be interested in a related issue and tell me what you think.
Most combustion engineers argue that they want turbulence in the firebox for better combustion. In an open fireplace, too much turbulence in the firebox will make the fireplace smoke. We do want turbulence in the neighborhood of the fire but then we want those turbulent products of combustion to stop being turbulent - or at least be confined - so that they flow into the throat without mixing with the dilution air flowing into the Rumford throat. We think we know that the flow into the throat is laminar but the devil is in the details. Is the layer of combustion products turbulent but overwhelmed by the sheet of laminar flowing dilution air? Can we make the throat smoother to reduce turbulence with the result that we can make the throat smaller and reduce the amount of dilution air? Or do we want to add spoilers in the firebox to increase the local turbulence within the fire for better combustion efficiency?
I think Rumford got it right - or at least pretty good - and that modern science (the part that hasn't moved on) probably can only expect to make minor improvements in the best solid fuel radiant heater ever developed.
On to the mundane........
Yes, I like to build a composite firebrick/backup masonry firebox. The firebrick I lay fast with weak corner-to corner joints between the fireback and the covings without taking the time and effort to cut the firebrick and weave it in the corners. Then I back the firebox up with at least 4" of solid masonry laid with Type S mortar, taking care to weave the back up units in the corners to make it strong if not pretty, and slush the half inch or so between the firebrick and the back up masonry solid with mortar. I leave an air space or bond break between the composite firebox and outer masonry shell of the fireplace. Other masons take time to make the firebox stronger and leave the air space or bond break between the firebrick and the rest of the masonry but I find that takes more time, isn't as strong and often the vertical joints between the fireback and covings don't look as good.
The vast majority of American chimneys are wide open at the top. It's certainly better to keep rain out of the chimney and to protect the top of the chimney from weather. If you don't it will lead to more maintenance. Where does the rain go if you have no rain cap? Generally it's absorbed in the masonry which, by ASTM standard can absorb up to about 12 % of its weight in water and still meet the "severe weather" (SW) standard. Clay flue liners are only allowed to absorb 8% of their weight but that's still a lot of water. Masonry "breathes".
Best,
11/29/15
Hi Jim,
I happened upon a letter dated 2/20/05 from Charles Riggs entitled "fluid dynamics" in which he compares the Rumford venturi to that of a carburetor. You very eloquently explained that both designs allow for a laminar flow entrance and a turbulent exit. Later, you asked three questions which it appears went unanswered and as a retired chemical engineer who was quite good at fluid dynamics, I would offer these comments now, in case they haven't already been answered:
1."Is the combustion layer turbulent but overwhelmed by the sheet of dilution air?"
2."Can we make the throat smoother to reduce turbulence with the result that we can make it smaller and reduce the amount of dilution air?"
3."Do we want to add spoilers in the firebox to increase the local turbulence within the fire for better combustion efficiency?"
I hope this helps make your fireplaces even better than they currently are.
Best regards,
Jon,
Thank you so much, Jon. Wow, that was ten years ago and I had all but forgotten that discussion. But, no, Charles Riggs didn't write back and no one else has attempted an answer. So your comments are timely, to the point and appreciated. I will add them to the discussion.
Warm regards,
Comment on Orton Rumfords
Dear Jim Buckley,
... I had built a large chimney at a house of mine in Redding, Connecticut (1974), with a Vrest Orton version of a Rumford fireplace. (It worked fine, but was lacking in rounded breast and straight fireback). I got ahold of your research, observations and geometry last November, and built a Jim Buckley Rumford, and what a beauty! Have yet to finish firebrick, but just had to burn it. I allowed every excuse for problems with the draft; the windows and doors wide open and winds blowing thru the house. Not a wisp of smoke that did not race straight up the stack. My neighbors, who have borne witness to my labors, and were witness to the first fire, kept saying "that's AMAZING, that's amazing."!! Thanks so much for your setting me on the true Rumford. I will send a couple of other photos if I can find them.
All Best, JMSJ
You bet. I always love to learn something new. Orton's book was loaned to
me by a client about 12 years ago. They wanted the Rumford in their house.
WE built it exactly by the Orton book. It was very difficult to build with
the fire back pulled forward as he illustrates. Keeping the center of the
throat over the center of the firebox is damned near impossible. IT has
smoked ever since. Since then I have modified them in our details to counter
the smoking problem. This was before we learned about Superior. We are on
the same page. Thanks for coming in. I am flattered by your visit and I
appreciate your kind comments about what we are doing.
Maybe I should put a link on our website to you!
See you soon.
JJ
From: Jim Buckley [mailto:buckley@rumford.com]
John,
It was a pleasure to meet you last week once Al Trustdorf and I found
your offices. I've also enjoyed looking over your website - lots of
traditional homes with proper chimneys and masonry fireplaces.
I hope I wasn't too hard on you about proper Rumford design but at
the same time I hope I irritated you enough to read Rumford and see
why the Orton version is wrong even though it has been picted up by
Graphic Standards and the BIA.
Start with the article at http://www.rumford.com/articleOrton.html
and move on to Rumford's original essays available at any good
academic library or on line at
http://www.rumford.com/chimneyfireplacesa.html if you don't believe
me - or need a little help getting to sleep.
Keep in touch. I really like your work.
Warm regards,
Jim,
Well, another dose of true enlightenment; thanks again.
My first "Orton" is over twenty years old and really does work beautifully. the only smoking we get is when something is just too far forward. The other is about six years old, and is further deviant in that I was forced to angle the chimney backwards to miss a beam. but that one works beautifully also.
My family owns a children's summer camp in upstate new york, and we have over fifteen other fireplaces. The "Ortons" work far better than the others, which range from Heatilators on the really low end to the odd ideas of thoroughly uneducated stone masons. But some of the stone work is beautiful.
Really looking forward to building a real Rumford, now more than ever.
Thanks again,
.........
Richard,
We've had some anecdotal testimony that "real" Rumfords perform better than "Ortons" but maybe not from people who have two Ortons and 15 other fireplaces. I'll be interested in your evaluation. We may have something of a statistically significant sample and an objective comparative analysis going here.
Best,
...........
Jim,
Thanks for your quick response; we hope to be pouring the foundation in about two weeks, so I need to have the details ready.
I appreciate all you've done in forwarding the "Rumford" cause. I've built two previous rumfords, but neither with your system. I built them based on the specifications in Vrest Orton's book, and both are terrific, but I've gotten older and my back tells me that using the superior clay items you designed will be a lot easier and faster.
And your website is a goldmine. So thanks for everything.
Richard
............
Richard,
Thanks for the kind words. And, you might add, our system is more authentic and will work even better than the Orton Rumfords. See our evaluation of Orton's book at http://www.rumford.com/articleOrton.html and read Rumford in the original so you don't have to take my word for it at http://www.rumford.com/chimneyfireplacesa.html
Happy building and thanks again,
On Thu, Feb 24, 2011, Marco Resendes
Hi,
Do I need throat and damper for outdoor fireplace?
Thank you
Not a damper, which does nothing but prevent warm indoor air from going up the chimney when the fireplace is not in use.
But you should use the Rumford throat so that the fireplace will burn cleaner, heat more efficiently and draft well. See the diagram showing how a Rumford works and imagine the quenching turbulence there would be if not for the rounded throat directing and keeping separate the smoke and dilution air.
Jim
7/14/09
I posted a blog on outdoor Rumfords at
http://www.greatbearoutdoors.com/index.php/2009/07/115115/
Thanks!
Geoff
Back to Outdoor Fireplaces
Ovens
Radiant Heat
Conversation with Gerald Maxwell - PhD with a Rumford
Refractory Mortar
A few guys fooled around with Redart and sand quite a few years back.
Dale Hisler used to use it, and Tom Trout has done some stuff with it.
I think the consensus was that it is not as good as Sairset, etc., in
a firebox in the long term.
Aside from the hydraulic mortar that Jim describes, which is basically
calcium aluminate cement, clay and sand, traditional refractory
mortars were usually clay, and were "air set" or "heat set". Heat set
was just fireclay. It would take a ceramic bond like Jim describes,
because it was used in high heat areas of industrial furnaces like
smelters, etc.
Air set has sodium silicate (waterglass) added. So, it is just clay
and waterglass. The waterglass is refractory, sets when it dries, and
theoretically will dissolve again with water (unless you get some on
your jeans and put them through the laundry ;-) Tulikivi uses
waterglass and soapstone dust for mortar.
Redart is a superplastic, earthenware (low fire) clay. It has high
shrinkage, and potters use it as an additive to other clays, but not
by itself. It has a lot of iron in it. I've used it for brick facings
(4-5 parts sand, maybe 10% portland), but wouldn't trust it for a
firebox. I agree with you that it makes a lovely mortar.
In the "old days" you likely didn't have nice quality-controlled
commercial products available, and masons got to know their local
materials real well. They'd know which clay was "good" and which wasn't.
Check out the Russian poster in MHA News on how to test and prepare
local clay. Marcus Flynn picked it up in Belarus. It's now in the archive section: at http://mha-net.org/docs/v8n2/v8n2-arc.htm
Kiko Denzer's earth oven book also has great info on testing and
preparing clay.
Have a good one ...... Norbert
*********
On Dec 6, 2010 Brian L. Snell, P.E.
Jim,
Excellent article on refractory mortar!!
I am a structural engineer and really didn't have sufficient knowledge of fire bricks and refractory mortar to make suggestions to a Longboat Key, FL condominium that wishes to construct a BBQ pit.
I have now performed as much of a study as I feel is needed to make recommendations, except for their primary question:
They asked how long the mortar should be allowed to set before firing it up for the first time. I will be recommending hydraulic setting refractory mortar. In your opinion, how long should it be allowed to set before using?
Sincerely,
By the way I would like to be listed as one of your Engineers for the State of Florida. The firm has been in existence since 1993. Our web address (still under construction) is www.snellengineering.com
Most refractories were developed for the steel and glass manufacturing industries. They just aren't going to shut down Anchor Hocking or US Steel for very long just to build or repair a kiln. The instructions on the bags usually read, "Cure for 24 hours. Then bring up to operating temperature at 50 degrees per hour." The old guys say, "We just used to build a wood fire in the kiln." So I would wait 24 hours and then build a slow break-in fire.
Thank for the kind words. Glad you found what you needed. We'll get your firm listed at http://www.rumford.com/arch.html tomorrow.
Best,
Rosin Fireplaces
Fred is clearly a believer in his Rosin and, like me, one of the few interested in open fireplaces. Among his admonisions are:
1) Fireplaces should be small. (I agree)
2) The entire fireplace and flue should be insulated. (I agree.)
3) The parabolic fireback of the Rosin directs heat back into the fire to concentrate the heat, burn cleaner and radiate more heat. He criticises the Rumford for having a smoke stained straight fireback indicating that it doesn't get hot enough. (Sounds good except there are plenty of pictures of Rosins with smoke stained firebacks too.)
4) Advocates top-down fire starting and doesn't like tipi fires. (Logical consequence of the shape of the firebox.)
5) The Bellfires stainless steel throat is a transition from the Rumford throat he modeled after ours directly into the round SS flue with no smoke shelf or smoke chamber. Claims small unit can be vented by 5" flue. (I agree with Rumford that the abrupt temrmiation of the throat and the smoke chamber volume will discourage sudden downdrafts. If, in fact the Bellfires can be vented by a 5" flue I think it might be more likely due to the smoothness of the SS throat.)
It would be interesting to do some comparative testing.
Jim Buckley, 4/07
Tipi Fires
Jim,
I have been interested in Rumford fireplaces for some time now. A little
over a year ago I experimented by setting tipi fires in my ugly inefficient
zero clearance fire place. I noticed a significant increase in the amount of
heat radiated into the room with the tipi! The fireplace output (into the
room!) increased from "almost not worth the effort" to "drive you out of the
room"! I introduced the tipi to several friends, and if they want to heat
the room rather than just create a mood, they always lay a tipi.
I am a bit concerned with the possible additional heat against the back of
the metal fire box, but I figure if it burns through, I'll be able to
install a proper fireplace!
Has anyone quantified the efficiency gains contributed by the basic design
vs. the tipi fire method?
Regards,
Gary Prideaux.
PS I still want to install a Rumford in place of my existing boxes!
See "Building a Fire in Your Rumford"
See-Through Fireplaces
Nancy Kelleher
We recently built a new cottage and have a beautiful large two sided Rumford fireplace. The opening is approximately 36 x 36. We are having a very hard time with it smoking. We have lit fires with grate, with no grate. We have installed an Exhausto Fan. We have installed heavier screens. We have experimented with opening windows. We have adjusted the damper. We are considering getting one side glassed in although we have heard concerns about potential problems with glass breaking from heat. It is our main feature and we are desperate to solve this problem in order to have a functional fireplace.
Your thoughts?
I'm sorry. We try our best to discourage customers from building see-through fireplaces. See our sarcastic disclaimers. I don't know how to fix them. You have already tried some of the things I would try. I do have considerable vicarious experience, however, so let me make some more suggestions:
1) Glass doors (or cheaper) a fixed glass panel on one side will probably stop it from smoking but the glass will probably smoke up immediately so you can't see through it. You might as well block in the one side with firebrick and make it into a single faced fireplace. You can get neo-ceram or high temperature glass which will not break from the heat of the fire. And you might experiment with an air wash inside the glass to keep the smoke off the glass but that would be tricky and probably not work. See the design for an air wash.
2) With as little trouble and less expense you might build a common fireback in the middle of the fireplace and convert it to back-to-back Rumfords. See the See Through fix.
Let me help you and let me know if you have any questions. I would love to help you fix your see-through.
Best,
2/1/12
Jim,
A contractor came to us with a problem and asked us to help him if we could. This might make an interesting story for fine homebuilding ..
Given: Homeowner approaches contractor to convert a single sided fireplace to a double sided fireplace. Mason says no problem and reconfigures the firebox with two 36"x36" openings, installs a new single sloped one piece terra cotta smoke chamber (36" wide at bottom, 18" wide at top) with a single blade 12" butterfly (?) damper at the top which begins 4" above the top of the fireplace opening instead of the code required minimum of 8". This ties into an existing chimney flue which measures 8"x12". No provisions were made for introducing outside air as the code also requires. When homeowner attempts to start a wood fire, house fills up with smoke. Original architect won't return phone calls, contractor stops trying and contacts us for advice.
My answer was to rip out what he has and build a new fireplace that we can design for him but I told him I would pose the question for some advice. We told him that for the size of the openings, he needs a larger damper (36"), larger smoke chamber (16x42x30), larger flue (16x20) and caution him that even with these modifications, fireplace will still smoke (though hopefully not as bad) and we will still make a recommendation for including glass doors on one side; I see your website also calls for a chimney top fan (though we are concerned that someone will want to use fireplace as a heat source in a power failure which we still get for time to time).
His Question: if the fireplace is converted to gas with a gas log, is the flue large enough to support a +/-90,000 BTU log set? I wanted you advice before consulting a mechanical engineer; we feel that this is a bad idea only because some day, this homeowner (or the next) will attempt to burn wood and converting to gas logs will only defer a potentially deadly problem.
Any advice you can offer would be most helpful.
Best,
Well, it's worse than that. Never install gas logs in a fireplace that
doesn't draw. You will just spill gas fumes, full of CO, instead of
smoke. A much more insidious situation. You could kill your client
instead of just irritating him.
We do not ever recommend see-through or multi sided fireplaces as
maybe we don't say strongly enough at
http://www.rumford.com/disclaimer.html Back-to-back Rumford
fireplaces are always a better solution. The fans are expensive and
don't always work, doors get so smokey in minutes that you can't see
through them. The details about throat configuration and damper size
are immaterial. No see-through fireplace works and would certainly not
be efficient enough to be useful in an emergency.
Now, for a minute, back to the gas logs. If you fitted the fireplace
with two fixed or semi-fixed glass panels or doors (which would block
nearly all of the radiant heat, but also block a lot of the air loss)
and you arranged the gas logs so that the flame did not impinge on the
glass to make it smokey, maybe you could make the fireplace look
presentable. But it wouldn't heat and you should hope the client
doesn't open the doors. I think a TV screen would be better. Then at
least you could switch it to a fish tank in summer.
Best,
Best,
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