Fireplaces: Studies in Contrasts--
by A. C. S. Hayden
Click here to read Hayden's anti fireplace article.
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:
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":
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 miles 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.
While Hayden's "science" is full of propaganda, there are more examples:
"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!"
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."
I just looked over your
Fireplaces: Studies in Contrasts--
Thanks for balancing out Mr. Hayden's presentation. It was good to see
you seeking to bring scientific references into the discussion.........
I just looked over your
Fireplaces: Studies in Contrasts--
Thanks for balancing out Mr. Hayden's presentation. It was good to see you seeking to bring scientific references into the discussion.........
Buckley Rumford Fireplaces
Copyright 1996 - 2010 Jim Buckley
All rights reserved.