Buckley Rumford Fireplace Home Page
Rumford Technical Discussion
How efficient is the Rumford?

Rumford fireplaces are high intensity radiant heaters as described by ASHRAE. (see footnote) They work best in large, open, rooms with high ceilings - even outdoors. Rumford fireplaces provide an efficient means of delivering thermal comfort directly to people and surfaces such as floors and walls without having to condition the entire space. Highly efficient thermal comfort can be delivered because radiant heaters focus thermal energy, and therefore thermal comfort, directly on the occupants, rather than controlling room temperature as do other heating systems.

Determination of the amount of heat required to accomplish a desired heating level for any space relies upon an estimation of the space heat loss. The desired inside temperature when using a radiant heating system can be about 10 deg.F lower than one for a conventional warm-air heating system to achieve the same level of thermal comfort. With lower indoor air temperatures, the home's insulation works more efficiently and there is less heat loss through the building envelope. Adequate ventilation is also less expensive in a radiantly heated space because the fresh outside make-up air doesn't have to be heated up as much.

For both reasons - 1. efficient means of delivering thermal comfort and 2. equal confort at lower air temperature - radiant heating Rumford fireplaces are effective and efficient heaters compatable with good ventilation.

For the calculations and more detail see the Physics of Radiant Heat

Discussion

Have a Technical Question or want to make a Comment? Click here

Comparitive Efficiency - graph from 2000 OMNI Report
Do cast iron firebacks improve the radiant heat output?
Discussion with Hoffman about how even high temperature glass blocks radiant heat
Efficiency Gain from interior chimney
Flue gas loss method - wrong way to measure radiant efficiency.
Efficiency Is in the Eye of the Beholder by James E. Houck and Paul Tiegs
Review of literature on fireplace use, emissions and efficiency by Jim Houck & John Crouch
Dilution Air - a Matter of Perspective
Kansas State Research on Radiant Heaters and Thermal Comfort Calculations 6/1/03
Paul Tiegs offers a radiant equivalency factor 7/7/03
John Gulland - another expert 4/21/03
Jeff Lockhart does some testing 3/03
David Didion - a real expert 10/2/00
Gary Rust asks how efficient in 1998
Skip Hayden's anti fireplace article - Critiqued 5/18/96

2/11/08

Cast Iron Firebacks Not Recommended

From: "cherner"
To: "'Jim Buckley'"
Subject: back plates
Date: Mon, 11 Feb 2008

Do cast iron back plates or other materials increase the radiant power of the fireplace, how about highly reflective glazed firebricks?

Thanks,
Ben Cherner
Cherner Design LLC

    From: Jim Buckley
    Sent: Monday, February 11, 2008
    To: cherner
    Subject: Re: back plates

    Ben,

    Cast iron firebacks absorb heat like the good black body radiators they are but the only re-radiated heat that is useful is the heat radiated from the from the side you can see. The heat radiated from the back of the fireback only serves to heat the back of the fireplace and ultimately increase the heat lost up the chimney.

    Rumford railed against firebacks because he wanted to reflect the radiant heat into the room - not absorb the heat only to be lost up the chimney. Rumford was incorrect, however, and since Planck now we understand that firebrick is a pretty good black body radiator not much different than a cast iron fireback - except for the fact that the stand-alone fireback radiates heat off the back side which is lost. See http://www.rumford.com/radiant/index.htm So, if you built the iron fireback into the masonry it probably would be indistinguishable from firebrick. Standing in front of the firebrick an iron fireback would reduce the radiant heat.

    The idea of reflecting the radiant heat as Rumford thought he was doing by plastering and whitewashing the firebox is still attractive. It's all in the surface and we know the emissivity of lots of materials. Gold and aluminum have low emissivity and would be good firebox coatings. Some glazes may be better than others but what reflects light does not necessarily have low emissivity in the infrared wave length.

    But the real problem is how do you keep your gold plated firebox clean and free of smoke? When it gets smoky it would just turn it into an expensive black body radiator with high emissivity pretty much like firebrick.

    So, gold plate your fireback, wire it so you can preheat it to 300 degrees to keep the smoke from condensing on it and build it into your fireback and I think you've got something.

    Best,
    Jim Buckley

Wow thanks, it looks like the black glazed fire bricks may be the best choice, as anything will eventually be mostly black after awhile.

Ben Cherner
Chernerdesign LLC

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2/27/07

Even high temperature glass blocks radiant heat

Applied Energy claims their Robax 1,400 deg. glass-ceramic material "makes an ideal cover plate for radiant heaters ... because of its high transmission properties in the near infrared range." They were even kind enough to fax me a graph (also on line) which shows that Schott Robax glass transmits in the near infrared range, wave lengths up to about 2.5 microns.

You put this glass in front of a fire, however, and it seems to block nearly all the heat. How can this be?

My friend, Kimo Hoffman, the space age infrared expert, makes three observations:

1) The flame and hottest part of the fireback might be around 2,000 deg F and emitting radiation in the 2 micron wave length range but it's a small part of the total infrared radiation from the cooler parts of the fireback and even parts of the fire obscured by smoke or logs and this radiation, with a lower temperature source, is mostly delivered in the 5 to 9 micron wave length range where the Schott Robax glass, like ordinary glass, is opaque. The Schott Robax glass, even though it does transmit some near-infrared radiation in the 2 micron range, still blocks most of the thermal infrared radiation.

2) No one really knows what the emissivity of flame is. Some estimate it to be only around 0.3 rather than the 0.9 assumed for most materials. So the power of the radiation in the 2 micron wave length range from the high temperature flame may be greatly reduced by multiplying it by the lower emissivity. See Stephan-Boltzman Law. Again, even a bigger portion of the radiant heat produced by a wood fire may be in the longer 5 to 9 micron wave length range.

3) There are problems with the instrumentation and measurement. Assumptions are made about emissivity and it's unclear if the instrument can measure the radiation being transmitted or only the surface temperature of the glass. Therefore the best plan is to use the methods spelled out in ANSI Z83.19-2001 to measure the BTU output directly by placing thermocouples on black painted plywood placed at specific angels and distances from the fireplace. It would be interesting to measure the radiant heat output of a fireplace without glass doors, with ordinary glass doors and with the Schott Robax glass doors. Back to Fireplace Doors.

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3/6/06

Efficiency Gain from interior chimney

I've read through most of your writings about rumford efficiency and I'm wondering what kind of chimneys were used in the tests and if they were considered. My reason for this is that I am planning on building a rumford in my new home with a masonry chimney, about 15 feet of which will be within the house. Presumably having this much chimney in the house would give an efficiency boost. Do you have any idea how much?

My clues are this: The Book of Masonry Stoves says that flue gases lose 4-8d celsius per meter of chimney, suggesting that the flue gases in my scenario would lose ~ 40 - 80 degrees fahrenheit during their ascent inside the house. It seems like this could have a substantial effect on efficiency, but I have no idea how to quantify it. Let's say it meant that flue gases would leave the house at 250 instead of 300 because of the interior chimney. If ambient air temperature is 0 (outside temp), have I gained 17% efficiency?

Brad*

Brad,

I think you are on the right track but I can't quantify it. All the tests are run inside the lab and flue gas samples taken at eight feet off the floor. Remember, the main point is to measure emissions and the thinking is that all the combustion is over by then. Your observation that the hot gasses may continue to heat the chimney as they rise, I think, is true but is not of current EPA interest. At least the tests are consistent so that when we measure fireplace efficiency and stove efficiency we are not considering any extra heat might be extracted by either farther up the chimney.

My guess is that, after flue gas losses are considered, we "lose" about 20% of the available heat into the brick mass while radiating about 80% directly out of the fireplace opening. Masonry heaters are probably at the other end of that continuum "losing" about 20% out through the loading door while capturing about 80% into the mass. Keeping the mass of the chimney inside the house would enable you to re-capture a little of that lost heat - and a little of the heat presumed to be lost in our flue gas loss calculations. It might be substantial but my guess is that we would quickly run into diminishing returns. I think you are on the right track but that 17% is a lot to get back - maybe 5%. Then again it's all guesswork in the absence of any real science.

I think your question is valid and interesting. I will think about a way we might test it the next time we have an opportunity. We could at least measure the flue gas temperature at the top of the chimney to verify your and the Book of Masonry Stoves assertion that the temperature will drop 40 to 80 degrees F in fifteen feet.

Thanks,
Jim Buckley

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6/12/04

Flue gas loss method - wrong way to measure radiant efficiency.

Efficiency for most wood-burning appliances like stoves and masonry heaters is determined by the "flue gas loss" method. See the article by James E. Houck and Paul Tiegs on efficiency. The energy wasted up the chimney can easily be measured by determining the volume, rate of flow and temperature of the flue gasses. The amount of energy contained in the wood (or fuel) is well known (see BTU output of a Rumford) so, after a couple of minor adjustments for combustion efficiency, etc, the flue gas losses are measured and all the rest of the energy is presumed to be available for heat. It's a pretty simple idea that works pretty well - for air heaters.

Radiant heaters, however, like masonry fireplaces and those infrared heaters you see in garages and outdoor restaurants, heat differently and the flue gas loss method for measuring their efficiency is pretty meaningless. Radiant heaters are very efficient at providing heat directly to people without having to heat a lot of air to heat people indirectly. But the flue gas loss method of measuring efficiency doesn't tell the story when it comes to heating radiantly. That's why commercial radiant heater manufacturers have developed a way to measure radiant heat directly. A commercial radiant heater that has a radiant heating of efficiency of only 35% heats a warehouse much more efficiently and at less cost than an air heating furnace that might have an efficiency rating of around 90% as measured by the flue gas loss method. Of course that's not an apples to apples comparison which is why the ANSI Z83.19-2001 standard for gas-fired high intensity infrared heaters call it a "radiant coefficient of at least 0.35" rather than call it "efficiency". If we measured the actual heat in BTUs delivered to a person by a 90% efficient air-heating furnace it might also have an air heating coefficient of about 0.35, depending on the size of the room and the ventilation.

Back to stoves and fireplaces. The EPA stopped requiring that stoves be measured for efficiency when it was discovered that all stoves that were clean-burning had thermal efficiencies of about 63%. Now we are discussing the merits of measuring the efficiency of fireplaces and masonry heaters. (See comparative test results at http://www.rumford.com/emissions/images/report17.gif) Masonry heaters, which are basically masonry stoves that heat air, seem to have efficiencies of (surprize) almost 63%.

Fireplaces don't do so well though and they don't do so well in a bazar and interesting way. Tested with glass doors closed so that nearly all of the radiant heat is blocked, fireplace efficiencies approached the masonry heater and stove norms but when the doors were removed so the fireplace could heat radiantly, the way they were designed to heat, fireplace efficiencies, using the flue gas loss method, dropped to around 30%.

How could fireplaces be less efficient when they were obviously delivering much more heat? Clearly radiant heating fireplaces should be tested for radiant efficiency like commercial radiant heaters and not, like air heaters, using the flue gas loss method.

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4/6/04

Dilution Air - a Matter of Perspective

The reason an open fireplace isn't more efficient when measured by the inappropriate flue gas loss method is because so much extra air - dilution air - is wasted up the chimney to carry away the smoke. The small efficient Rumford throat is able to reduce this loss somewhat but the only real way found so far to reduce the amount of dilution air is to put doors on the fireplace to control the intake air. That does improve the efficiency numbers measured by the flue gas loss method but the door blocks nearly all of the radiant heat - the primary way a fireplace heats - so how can that be a solution? Most of this section deals with the nature of radiant heat, how it's measured and why it's much more effective than would be indicated by the flue gas loss method of testing.

There's another way to look at it though. Step away, look at the whole house system, not just the heating appliance. Imagine a very tight well-built house with a very efficient, air tight stove that takes hardly any air to burn. That house, by code, will need to have a ventilation system to insure a healthy indoor air quality. Consider the stove PLUS the ventilation system. How efficient is the stove now if you take into account the air required to ventilate the house because the stove is so air tight? Now consider the Rumford fireplace in conjunction with the ventilation system and you will find that the fireplace actually ventilates as well as heats and you can eliminate or reduce the extra ventilation.

We'll work on this idea with some numbers, codes, and historical references to the "salubriousness" of living with a Rumford in the days before electricity when the fireplace both heated and ventilated. - Jim Buckley

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Paul Tiegs offers insight and a radiant equivalency factor

7/7/03

Jim,

The amount of heat transferred by infra-red radiation versus heat transferred by convection and/or conduction is small. The actual amount of matter heated by infra-red (ie, what you feel on your skin) is very small but because it gets delivered directly to your skin it is many times more effective. It gets heat exactly where it is needed and most easily absorbed and sensed by humans. It takes a lot of very hot (ie, high temperature) material to produce infra-red radiation. It consists of electro-magnetic waves that are just a little longer and lower energy from the visible light spectrum. Infra-red or any other kind of electro-magnetic energy radiation like visible or ultra violet light or x-rays is a function of temperature not just Btus; the higher the temperature, the higher the radiated energy level. As you know about the sun, it is very, very hot and puts out a very wide spectrum of radiation including all of those I've mentioned. Wood fires on the other hand are quite cool in comparison and only have small amounts of infra-red type of radiation.

In advancing the development of a radiation vs convected/conducted efficiency factor we could also develop a universal radiated versus convected/conducted equivalency factor. This would be a universal factor that indicated how much convected/conducted heat would have to be produced in order to equal the radiated amount of heat from a fireplace. For example if we developed the radiation vs convected/conducted equivalency factor to be "x", the measured radiation from a fireplace would then be multiplied by x to get its equivalent amount of convected/conducted heat. For example, if the equivalency factor was 3.5, a measured radiant heat output of 1000 Btus/hour would be equivalent to 3.5 x 1000 or 3500 Btus.

I certainly do not know if that is even a close equivalency factor; it could be lower or higher but the fact is, it would definitely produce a larger number.

Have a good one,

Paul Tiegs, PE
paultiegs@omni-test.com


7/7/03

Paul,

This is starting to make some sense. ANSI Z83.19-2001, and CSA 2.35-2001, the national standards for gas-fired high intensity infrared heaters, require infrared heaters to have a radiant coefficient of at least 0.35. If that's what you mean by "universal radiated vs. convected/conducted heat factor" then you're right on the money. The test method is spelled out in the standard. It deals mostly with a standard way by specifying angles and distances to measure "effective radiant output". The radiant output (RO) divided by the input in BTUs (I) is the radiant coefficient (RC). If you don't have a copy of the ANSI standard I can fax it to you.

I never quite understood this. I had heard that high intensity radiant heaters were required to have a radiant efficiency of 35% and I never could get an explanation of what "radiant efficiency" is. Your explanation makes more sense. Rather than efficiency maybe it's more a measure of "effectiveness".

I know the high intensity (glowing ceramic burner) heater guys argue with the low intensity (black tube) guys. Apparently the low intensity radiant heaters don't have a radiant coefficient of at least 0.35 so they poo poo that and claim to be 80% or 90% efficient based on a flue gas loss method like we use. The glowing burner guys, of course, say that's bogus because all the air the tubes heat is lost in the rafters and what counts is the radiant heat delivered to people and the floor.

Thanks for taking the time to explain.

Warm regards,
Jim

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John Gulland writing an article on fireplace efficiency

4/21/03
Hi Jim,

Sorry I missed you. I'm doing a short article for Mother Earth News on fireplaces. Specifically, is a fireplace a practical device for home heating? As others have mentioned to me in interviews, there is a spectrum of practicality in fireplaces, from purely decorative to serious heaters. One of the questions to be asked and answered in the article is, what does a heating fireplace look like and in what ways does it differ from a purely decorative one? Where along the spectrum do you place the Rumfords?

Those are the questions I wanted to ask you. I could call you at an arranged time, or you could just respond to this email.

Thanks, John


Hi, John. Nice to hear from you.

I suppose "heating fireplaces" could include fireplaces with doors that primarily heat air as well as radiant heating Rumfords. They would look quite different from each other.

As for an objective line that separates "purely decorative" from "serious heaters" the ASTM E6.54 committee has been working on an efficiency standard. While I'm not in complete agreement with the need for an efficiency standard - at least as part of an emissions standard - the line the committee has suggested would be drawn at 25% efficient.

Mostly this was suggested by Paul Tiegs. You will see from one of his charts which I have on line at http://www.rumford.com/emissions/images/report16.gif based on recent comparative testing that only the metal zeros and one masonry fireplace came in under 25% efficient while the Rumford and Jerry's Rosin and even a conventional fireplace with an insulated firebox and tuned air system came in between 25% and 60% efficient.

You'll notice that there were no fireplaces testing any where near zero or negatively efficient and there is no clear dividing line separating the "purely decorative" from "serious heaters". In fact I don't trust the science well enough to judge whether or not the differences are within the margin of error of the test. Also I still am not satisfied with an efficiency measurement which seems to favor closed air-heating systems over radiant systems and am looking into the arguments about the meaning of "radiant efficiency" between various manufacturers of gas fired commercial radiant heaters.

Having said all that, I am really only interested in radiant heating Rumfords. I worked hard on my argument when Skip Hayden got my blood boiling a few years ago and that response is still on line at http://www.rumford.com/articleHayden.html Suffice it to say Rumfords are effective radiant heaters. They heat people and surfaces rather than the air and make people feel comfortable at cooler air temperatures. Because the temperature difference between inside and outside is less, the home's insulation works better. And radiant heat is more compatible with good ventilation than air-heating. In certain situations and certain climates, such as a great room with 25 foot ceilings in Seattle with the windows open, a Rumford will be more effective than a stove or other air-heating appliance.

So, I would put the Rumford clear at the extreme "serious heater" end of your scale (probably along with efficient air heating fireplaces) with the caveat that radiant heat works better in some situations and heating the air works better in others.

I'll be in tomorrow if you'd like to talk by phone.

Regards,
Jim Buckley

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To: Jim Buckley "buckley@rumford.com"
From: David Didion "david.didion@nist.gov"
Subject: Re: new home fireplace

Jim

Thank you for the materials' advice. I will digest it, call Todd, and get back to you. My wife has her heart set on having the fireplace in by Christmas. (Christmas eve is a big deal in our house with our three children being there with their significant others and eight grandchildren.) So schedule will influence decisions.

As to my background, it turns out that I have had experience with testing of thermal heating devices; although not fireplaces, per se. For the past 30 years I have worked in the Building and Fire Research Laboratory of the National Institute of Standards and Technology (the old National Bureau of Standards) which is a Federal Government Laboratory. In the '70s I headed up the team that developed all the performance test methods for boilers, furnances, heat pumps, and air conditioners. These methods have been adopted by ASHRAE and are used throughout the Industry as mandated by the U.S. Department of Energy. I'll look into what standards exist on fireplaces.

Sometime back I had seen your web page comments on the performance tests and if I remember correctly you expected an efficiency increase due to an increase in radiant heat. This surprised me because the only way this could be so is if the wall temperature was increased. The radiant surface area is still the frontal surface area; which is the same for Rumford's fireplace as it is for the modern fireplace. The fact that Rumford sloped his walls means that that there is less total interior wall area for the same heat and thus the walls may be abit hotter but my guess is that would be a difficult comparison. Perhaps it might be done with an IR camera moments after the fire was extinguished.In any case a simple model might be used to predict the interior wall temperature differences. My guess is that Rumford's efficiency increase would be due to the reduction in excess air up the chimney as compared to that day's fireplace but not necessarily today's, with its shelf and damper. As to your stack loss method of measure, it is true we used the same in our boiler/furnace tests but these devices were of the orderof 60% to 90% efficient and thus we were measuring the lesser of energy flows. This means that if you are using a flow meter, whose accuracy is a function of the mass flow rate then the error is small compared to the difference between the input and the stack loss, that is the useful heat. However in the fireplace low efficiency case, the opposite is true and so it is not recommended. However since the useful heat is virtually all radiant, which is often a difficult and expensive measurement, stack measurement may be your only choice. Also I'm not sure how well you can know the input heat value since wood fuel values are probably rather moisture dependent. My guess is the an "electric fire" source would be a better method for evaluating the stack flow, at least. Well these are just random thoughts, tommorrow I might feel differently.

In any case, don't pay anybody if you are not in a hurry. I will be glad to look at your data as a labor of love. However I am very busy until Christmas break because I teach thermal system design at Hopkins in addition to my NIST job. In the meantime we can speculate! And I would like to ask an old NIST associate of mine to join in if you would like. He is John Lyons a retired Director of NIST, who happens to be a Rumford fan and an owner of an 18th century house with lots of firelaces. More importantly he is an author of a Scientific American book called "FIRE". I recommend you get a copy through your library.

By the way there were two Bernoullis, father and son, who grew to hate each other over jealousy of each's work. My son who is also a thermal engineer works at NASA on outer space problems whereas mine are all terrestrial, so no danger of that happening to us. Of course we aren't in the Bernoulli class anyway.

David Didion

*********

David,

Thanks for the suggestions. I can certainly wait until after Christmas. I'll get a copy of John Lyons' book "Fire" and probably go ahead with the article about our recent round of emissions testing for The Masonry Society. It may do as a way to get me up to speed for a more interesting article with you on efficiency.

Best,

Jim Buckley

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4/6/98

Jim,

How efficient are Rumford fireplaces? The question keeps coming up and I can't find anything on the website about the subject.

Gary Rust

4/8/98

Gary,

We know from our emissions testing that Rumfords have an overall efficiency of 63% - give or take 50 percent.

We tried to get good numbers but the problem was Washington didn't require efficiency and the lab wasn't very careful.

Years ago the stove industry found that all the EPA certified stoves huddled so closely around 63% efficient it wasn't worth measuring differences so they don't measure efficiency anymore because it's fairly complex and expensive to measure accurately.

To aggravate the problem for fireplaces our "equivalency" test protocol calls for testing from a cold start to cold finish rather than hot to hot on a scale as the stoves are tested. Efficiency is derived from the calculated Oxygen readings and in the long cold to cold test our average Oxygen levels are very close to ambient - something like 19.7% when ambient it's about 20%. The equipment is only accurate to about 1/10 of a percent - in other words we are near the vertical end of the graph - and the difference between 19.7% and 19.8% translates to overall efficiency differences of around 50% - the fireplace is 70%, or maybe 20%.

My intuitive feeling is that Rumfords are about three times as efficient as regular fireplaces because they have an opening about 50% taller and a throat opening about half as big - ergo they radiate more heat and take less heated room air to carry away the smoke. We have anecdotal evidence that Rumfords heat better than heatalators and inserts but that may be subjective and due to the fact that radiant heat works better than heating the air in some situations. Since our Rumfords were as clean or cleaner than EPA stoves, I think it's reasonable to assume they, like all EPA certified stoves and masonry heaters, are about 63% efficient. Give or take 50%.

If you can wade through it, Skip Hayden's anti fireplace article and my rebuttal (both on the website) can give you an idea how slippery efficiency numbers can be.

Hope this helps.

Best,

Jim

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Footnote:

The ASHRAE Handbook of Fundamentals (1993) outlines engineers' widely-accepted strategies for modeling systems and estimating energy usage of various heating system designs. Following the ASHRAE standardized methods, a heating design guidebook published by Solaronics, Inc. (1994a/b), a radiant heater manufacture, summarizes radiant heater system design. Based on the first law of thermodynamics, determination of the amount of heat required to accomplish a desired heating level for any space relies upon an estimation of the space heat loss. Estimation of heat loss involves obtaining the total loss through the walls, roof, and floor of the space to be heated, and the amount of air passing through the space per unit time (Solaronics, 1994a/b). To accomplish this, a complete survey of the space to be heated is necessary. The survey includes desired inside temperature, outside design temperature, building construction, and anything that affects the rate of air change per hour. To determine the U-value for each wall, roof, and floor the building construction must be inspected. The U-value is the inverse of thermal resistance of the material. Hence, a better insulating material results in a smaller U-value. Lists of U-values for different materials are available in the ASHRAE Handbook of Fundamentals (2001). The desired inside temperature depends on usage of the building and the customer's preference. According to Solaronics (1994 a/b), the desired inside temperature when using a radiant heating system can be about 10F lower than one for a conventional warm-air heating system to achieve the same level of thermal comfort. This decrease is because the comfort level measurement should be based on the operative temperature rather than warm-air temperature in the case of radiant heating. The outside design temperature for different cities can be found in the ASHRAE Handbook of Fundamentals (2001). The total transmission loss for the building then can be calculated.

Excerpted from 2.2.1 of the Kansas State Research on Radiant Heaters and Thermal Comfort Calculations 6/1/03

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12/18/02

Review of literature on fireplace use, emissions and efficiency
by Jim Houck & John Crouch

Updated Emissions Data for Revision of AP-42 Section 1.9, Residential Fireplaces (PDF)

The review of literature on fireplace use, emissions, and efficiency, prepared by James Houck, OMNI, and Joun Crouch, HPBA, for the EPA is interesting, mostly accurate and includes some 66 "resources" by the handful of people who have tested and written about fireplaces in the past 30 years.

The only comment about efficiency is this:

    The aesthetic warming and space heating effects from fireplaces are primarily from radiant heat. Little heat is ga ined from convection or conduction. The efficiency of a fireplace without a door is often cited as 7% and with a door it is in the 30% range. Both these efficiency values are quite variable depending on home and fireplace construction as well as operational practices and weather. The low efficiencies are primarily from the large amount of excess air heated and lost out the chimney (see Table 1.9-2 for chimney flow rates) which is made up by cold air infiltrating into the house. Efficiency also tends to be low due to the fact that fireplaces are generally on outside walls rather than in the center of a structure, hence radiant and convective heat transfers are not efficient.
I suppose it might be cumbersome to cite all the sources, but I wish the authors had identified where fireplacce efficiency "is often cited as 7% and with a door it is in the 30% range" - especially in light of the testing done only two years before at OMNI showing fireplace efficiencies to be in the 20% to 50% range whether or not they were tested with doors. Actually one fireplace tested about 7% efficient - both open and wieht doors - ant that onw was the metal fireplace. See graph from 2000 OMNI Report.

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