Hayden Critiqued

Fireplaces: Studies in Contrasts--
by A. C. S. Hayden
Click here to read Hayden's anti fireplace article.

Critiqued--
by Jim Buckley 5/18/96

Mr. (Skip) Hayden's article on fireplaces, published originally in Home Energy Magazine and promoted on the Internet by two different industry websites, is a scurrilous, piece of propaganda unworthy of anyone purporting to be objective, let alone one who holds an important position in a publicly supported research laboratory.

The article is deficient in two different ways:

The science is flimsy and unsubstantiated.
Techniques of propaganda are used in an attempt to lead the reader to conclusions not supported by the science.

The "science" in Hayden's article is based on a flawed field study and some circular assumptions about excess air and flue gas temperatures. With much repetition and a few charts and numbers, he has managed to expand this trivial scientific basis into a convincing piece of propaganda for the uncritical who share his anti fireplace bias. The article is full of examples comparing what Hayden likes in the most favorable light with what he doesn't like in the most unfavorable situation. This, combined with his use of subjective and prejudicial language, are the techniques of propaganda Hayden uses. If it were written by a junior and overly zealous advertising copywriter for one of the metal fireplace or stove manufacturers that would be one thing, but masquerading as a technical or scientific article Hayden's piece is most unworthy and unprofessional.

THE SCIENCE

Flawed Study:

Hayden refers to "field trials conducted by the Combustion and Carbonization Research Laboratory (CCRL)", the lab where he is the head of Energy Conservation Technology. The study evaluated fireplaces in Canadian homes, "in conjunction with other combustion equipment," and he says showed "that in all but one case, on cold winter days, use of conventional masonry fireplaces actually resulted in an increase in fossil-fuel consumption for heating." "The fireplaces actually had a negative energy efficiency during the tests," writes Hayden.

Hayden doesn't offer any other details about these field studies nor does he reference them so that we can read them for ourselves.

At first I wondered if Hayden understood the concept of radiant heat. I've heard of other similar "tests" ostensibly showing that fireplaces are negatively efficient. In these tests a fire is allowed to burn in the fireplace for about two hours in a house which is heated by a gas or oil furnace and after the fire goes out the damper is left open all night with the result that the furnace has to work harder to maintain the air temperature than it would if the fireplace were not used - hence the fireplace must be negatively efficient. I think, from his description, Hayden's test must be similar.

Such a test ignores the fact that open fireplaces don't heat air - they heat people and surfaces, radiantly. Such a test could be used to prove that the sun doesn't heat the earth. Certainly the same test would show that a radiant floor, masonry heater, or any radiant heater for that matter, would be "negatively efficient." Radiant heaters heat surfaces raising the mean radiant temperature so that people feel comfortable at cooler air temperatures.

Even Hayden's lone exceptional case involving the people from Great Britain is telling. I would make a different interpretation, however: The owners from Great Britain probably were very comfortable being heated directly and radiantly by their fireplace at lower air temperatures and probably were just as pleased as Hayden's stove customer (mentioned later in his article) that they saved fuel by not heating the bedrooms needlessly.

I don't really need to go into the concept of radiant heat any further here. There is plenty of information available from ASHRAE and the manufactures of various commercial and residential radiant heating appliances. My purpose here is to simply point out that open fireplaces are radiant heaters. They don't heat the air at all, except indirectly as the walls heated by the fireplace gradually heat the air that comes in contact with them. Hayden is not being fair, honest or scientific by measuring the ability of a radiant fireplace to heat air. He's counting on our ignorance.

Excess Air and Flue Gas Temperature - a Circular Assumption

Hayden's main hypothetical argument about fireplace efficiency is also hollow, deceptive and circular. It does not appear to be based on any actual testing.

"Conventional fireplaces," he writes, "operate at about 1500% excess air, 16 times the theoretical requirement or more than 10 times what a fossil fuel furnace needs. No evidence nor study is cited. The open fireplace tests we've done show excess air factors to be as low as 8 or 9 during very vigorous fires and much higher during the charcoal tail out stage and averaging about 15 (just what Hayden says) for a typical two or three hour burn, so no quarrel here yet.

Hayden then sets up a table to show the effect of excess air and, without any explanation at all, he assumes the flue gas temperature will be 300 degrees F - no matter what the excess air is. Actually, flue gas temperature has an inverse relationship with excess air. Excess air cools and dilutes the gasses in the flue so the more excess air there is, the cooler the flue gas temperature.

Hayden's "Table 1" shows the effect of excess air with the following "assumptions":

seasoned wood at 17% moisture
flue gas temperature of 300deg.F
no loss due to incomplete combustion products

Hayden says his table shows that "at 100% excess air, the sensible heat loss is 10% and the maximum possible efficiency the system can have is 78%" while "at 1500% excess air, the sensible heat loss up the chimney is a huge 73%." " The best efficiency a conventional fireplace can have is only 15%", according to Hayden.

Excess air and flue gas temperature. These are the only two variables that count in determining overall efficiency. By assuming all the variables, the conclusion is forgone and his argument is circular. Hayden must know there is an inverse relationship between excess air and flue gas temperature. It's a little like arguing that if we assume that the maximum speed of an automobile is one mile per minute, then science, as our little table shows, proves that a car can only go 60 mile per hour.

I took the spread sheet from an actual test of an open Rumford fireplace run at OMNI Labs in Portland, OR and substituted Hayden's excess air and flue gas temperatures to see what would happen. With 1,500% excess air and average flue gas temperatures of 300 degrees, the spreadsheet showed that the efficiency would be 26% - not 15%.

The overall efficiency depends on the combustion efficiency (which was 93%) and the heat transfer efficiency, which in turn depends on the boiling water loss (which was 11% and usually doesn't vary much) and the dry gas loss. The dry gas loss (the cost of heating all the excess air) is determined entirely by the dilution factor and the difference in temperature between the flue gas temperature and the ambient temperature.

We actually got an average flue gas temperature of 207 degrees on this test so I plugged that in instead of the arbitrary 300 degrees Hayden suggests. Now I get an overall efficiency of 49% and I still have 1,500% excess air.

What is going on here? Why are Hayden's numbers and ours so different? I suspect it's our different assumptions about ambient temperature. Our test was run in a lab which was about 70 degrees F. That was the temperature of the excess air flowing into the fireplace and being heated up to 207 degrees and that's what we used in our formula for the ambient temperature. I suspect Hayden used some especially cold outdoor temperature as his ambient temperature without telling us what it is. If I plug in 1,500% excess air and 300 degrees flue gas temperature and 0 degrees ambient outdoor temperature I do get an efficiency of about 9%.

Let's see if that's fair. On the surface it does seem fair to use outdoor ambient air temperature because it is replacing air being lost up the chimney. But how are other heating appliances measured? It's not fair nor is it good science to compare the efficiency of a fireplace under the most adverse set of assumptions with the efficiency of a stove determined under a different and most favorable set of assumptions.

All heating devises that depend on combustion of some fuel and exhaust flue gasses to the outside will be less efficient the colder it is outside. Fireplaces, because of their fairly high excess air needs are especially susceptible to this problem. But, how is the efficiency of a wood stove or gas furnace measured? Is the ambient temperature assumed to be zero degrees F? Let's compare apples to apples and measure the efficiency of a fireplace the same way we would measure the efficiency of a stove with which we would compare it.

A deception Hayden employs throughout his article is to compare fireplaces in worst case field conditions with stoves in ideal laboratory conditions. For example, he abuses the fact that a masonry fireplace is basically a supplemental, periodic heat source and uses this difference in the way fireplaces are used to their disadvantage. If the fireplaces in his field "tests" had been allowed to burn constantly - or to be used as periodic heaters with a succession of hot fires, instead of being fired for only two or three hours and then allowing the warm air in the house to escape up the chimney for the next eight or twelve hours, the fireplaces would have been more efficient.

"Real world," one might say, but do we take into account the "off" time cycles when we measure the efficiency of gas and oil furnaces? Do we measure the efficiency of a wood stove over a "real world" twenty-four hour period during which the fire is banked and the stove smoldering much of the time? The efficiency of metal wood stoves Hayden uses is determined during a laboratory test fire that is built in a hot stove on a hot bed of coals and ends when the weight of the fuel is back to starting conditions and the stove is still hot.

Let's compare apples to apples and measure the efficiency of a fireplace the same way we would measure the efficiency of a stove with which we would compare it.

THE PROPAGANDA

While Hayden's "science" is full of propaganda, there are more examples:

It's hard to believe Hayden is serious when he argues that a "well situated wood stove" is more efficient than a gas furnace because "the temperature of the rest of the house can be allowed to fall somewhat, resulting in a reduced overall heat demand". Isn't that one of the arguments he used against the fireplace that "fooled the thermostat into thinking the house temperature was satisfied, while allowing the rest of the house to become quite cold"? Maybe it was the fireplace that was "well situated" and the stove that "fooled the thermostat". Colorful and situational language is the stuff of propaganda not science.
What is an "advanced-combustion wood fireplace"? Sounds like a metal one built pretty much like a stove. I don't think he's talking about masonry heaters or Rumford fireplaces here.
In his description of "how wood burns" Hayden describes incomplete combustion more typical of a banked fire ina wood stove than of a fire in an open fireplace. It's a poor fire-builder who would let such a fire burn in an open fireplace where he can see it - the sort of person, more comfortable in a car with automatic shift, who would lug along in fourth gear rather than bothering to downshift.
The fact is there are very few chimney fires in chimneys serving open masonry fireplaces because open masonry fireplaces burn inherently hot and clean. All you have to do is look at fire statistics after the late 1970's energy crisis which spawned and wood stove boom. It wasn't the fireplaces that caused chimney fires unless a stove or insert had been vented into them without a proper flue. Just because Hayden says fireplaces burn dirty doesn't make it so. This is just another example of Hayden comparing the worst case for a fireplace in the field with the best case for a wood stove in a test lab.
Hayden does have faint praise for masonry heaters:
"Masonry heaters are another type of fireplace that have long been common in Northern Europe, but are rarely seen in North America. Wood is burned (ideally cleanly) at a high rate for about a two-hour period in a masonry firebox...
"Ideally cleanly"? What about stoves? They always burn cleanly? Heaters are much cleaner than stoves. There is absolutely no comparison between a masonry heater (with hot intense periodic fires and then no fire at all while it continues to heat) and a stove smoldering away all day because it's oversized - even if it is theoretically capable of burning "ideally cleanly" in the lab. And this gem:
"Recent work indicates that underfire air (in a masonry heater) leads to poor combustion, inefficiencies and fairly high emissions; also, significant heat loss can occur unless the heater is only installed on inside walls."
Compared to what? Masonry heaters with underfire air are still cleaner than stoves and the over fire versions are much cleaner. The language here is not wrong, just well chosen to give the impression heaters are not very clean.

There are a lot more missleading assumptions, snide remarks and examples of value laden language in Hayden's article:

He says fireplaces are twice as dirty as conventional "dirty" wood stoves, but he cites a rate (grams per hour) which is meaningless unless you know the burn rate and heating cycle and is prejudicial against fireplaces and masonry heaters which are periodic heaters and typically have higher burn rates than stoves when they are being fired.
He doesn't assume anything can be done to limit excess air except perhaps glass doors, which are "truly air-tight" and "gasketed" when used with "advanced-combustion wood-burning fireplaces" but "ineffective" on a regular fireplace.
Dampers too are "quite ineffective, if they are even used," snipes Hayden.
At one point, talking about CO emissions during the tail end charcoal stage of a fire, Hayden gets almost hysterical: "Who would put a charcoal barbecue in their living room?" he writes, and "...there is a potential for CO poisoning. People have died this way." Actually CO emissions, an indication of incomplete combustion due to lack of combustion air, is more typical of air tight stoves than of open fireplaces. And gas logs, set to burn too rich so they have a "realistic" yellow flame, are also high in the CO department. But to listen to Hayden the worst that can happen with a stove is that a catalyst which provides resistance to flue gas flow, can result in "flue gas spillage or poor combustion performance under marginal draft conditions." Don't people die from the CO stoves spill?

One after another, the litany of pejorative traits and terms ascribed to fireplaces does less to convince than to reveal the writer's biases.

The heat transfer of a fireplace is poor. (Only if you ignore radiation - the way a fireplaces heats.)
Fireplaces burn dirty and spill smoke into the house. (Flat out not tru e. Precisely because of their high excess air requirements, fireplaces help to ventilate as well as heat. And a masonry chimney will stay warm longer than a metal chimney and continue to draw better during the tail-out, CO stage. It must be a strangely built fireplace that causes 1.4 air changes per hour, according to Hayden's estimate, but can't manage to exhaust the smoke.)

There is no new science in the rest of the article - just invectives and snotty comments, comparing "advanced technology" stoves and metal fireplaces in the test lab with masonry fireplaces incompetently built, poorly situated and mismanaged in the field, until Hayden degenerates into sappy adulation of the latest new metal gismo:

"The new fireplace has truly air-tight, gasketed doors, a special glass window made from a pyro-ceramic to transmit the infrared radiation from the flame to the room and a hot air "sweeping" of the window to allow clear viewing. With the two combustion zones in plain sight, the result is a unique, riveting, chaotic flame which is far more attractive and hypnotically interesting than any flame burning in a traditional fireplace."

GIVE ME A BREAK - AND A TRADITIONAL FIREPLACE - PLEASE

A stove or masonry heater with a closed combustion chamber is more efficient than an open fireplace in really cold weather albeit often at the expense of good indoor air quality and adequate ventilation. In not so cold weather, as exists in Seattle or North Carolina, or even in Manitoba in the spring and fall, an efficient high intensity radiant heating open fireplace like a Rumford can be more efficient, cleaner and more effective. Perhaps this is why preferences are somewhat cultural. Fireplaces were developed and remain most popular in temperate climates like those of England, France and most of the US while masonry heaters and stoves come out of a northern European tradition.

At temperatures typical in the Seattle area the high intensity radiant heat from a fireplace is very effective especially in large open areas which would be difficult to heat with a stove or a furnace that heats the air. The radiant heat from the fireplace doesn't rise and escape into the rafters nor out an open window. Radiant heating open fireplaces, like the commercial radiant heaters used in aircraft hangers and outdoor restaurants, are consistent with comfort and good ventilation. If the outdoor temperature isn't too cold an open fireplace can save on fuel too because you can be comfortable with the furnace set lower or off - even with the windows open. And if the windows are open the excess air the fireplace uses is not much of an issue.

Fireplaces can be used, as they were 200 years ago, as a primary heat source, but today, (mainly because they are interactive and require tending) they are usually used as supplemental heaters. The heat from the fireplace is transmitted to the walls of the room and to the mass of the fireplace itself to continue heating the room even after the fire has died down or gone out. If it's not very cold outside the masonry fireplace or masonry heater owner still burns a hot intense fire but less often or for a shorter period of time.

By comparison metal stoves that are used for heating are usually sized for the coldest expected conditions, which means they are too big and put out too much heat most of the time when it isn't so cold. Since metal stoves don't have much mass and don't store heat very long their owners typically bank the fire, cut the air supply and let their stoves smolder, which is both inefficient and polluting. This is why there is such disparity between lab and field results when testing metal stoves for emissions.

In the real world substituting masonry heaters in cold climates and masonry fireplaces in temperate climates (and to supplement masonry heaters in cold climates) would reduce pollution and use our scarce renewable resource (wood) more wisely. To thumb our noses back at our friends in the metal stove industry, "Let's outlaw metal stoves!"

CONCLUSION

Hayden makes his point that an "advanced technology" wood stove or metal fireplace operated in a test lab by an expert in a hot-to-hot test is more efficient than a poorly built open masonry fireplace operated by a bozo who doesn't know how to build a fire measured over a testing period on a very cold day during which there is no fire in the fireplace for most of the time. I think Hayden could prove that the moon is made of green cheese if he'd just use a different table.

Hayden gets one thing right: "Fireplaces have long been a staple of North American households. Builders find it difficult to sell a new house without one."

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