A Perfect Stove for Each User: Evaluating Catalytic and Secondary Combustion Technologies
A Perfect Stove for Each User: Evaluating Catalytic and Secondary Combustion Technologies
Chris Neufeld
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The world of heating our homes with wood has changed greatly over the past 20 or so years. Education of consumers purchasing our products, engagement of industry with regulators, and the wood stoves themselves have all vastly improved. Of course, anytime you remind regulators that wood heaters have gone from 70 g/hr to 2 g/hr or less, they respond, “You’re welcome.” There is no doubt that regulation has influenced many of the changes that have been observed. Grasping the influence of these changes is essential when discussing the topic of wood heating. In a recent article in the Fire Time Magazine, Noel Putaansuu made several observations and expressed his opinions. Those observations and opinions encouraged me to respond with this article.
The world of heating our homes with wood has changed greatly over the past 20 or so years.
On the topic of one technology of combustion being better for cold climates, it should be known that most homes built after the 1980s in these cold climate areas are built to address extreme climates. As a person who has spent a considerable amount of time in ultra-cold climate communities for the past 28 years, it should be noted that homes in these communities are mostly smaller, and the home designs employ unique construction methods. With vast improvements in home construction, high BTU production isn’t always necessary. In fact, some of the absolute coldest communities in the U.S. and Canada rely upon the low turn down heat production ability of catalytic stoves. Of course, older homes or cabins in these remote locations, which often have little R-value, might benefit from wood stoves that produce higher BTUs, whether that be a catalytic model or a secondary combustion model.
With the introduction of the 2015 NSPS, manufacturers have had, until recently, three methods of testing when bringing a new wood heater to market. First and easiest is M28R, also known as the crib fuel method. The second option was, until mid-2023, ASTM3053 (cordwood method), and finally, a process called Alternative Test Method (ATM). Each of these methods has unique characteristics that make comparing test results difficult. For example, let’s say we take two identical wood stoves, and one is tested to M28R and the other to ASTM3053. When each stove is loaded with 40lbs of fuel, the scale that registers the fuel load weight will show the 40lbs. In the M28R tested stove, the test is considered “over” when the scale reads 0lbs of fuel left. This can result in what is called a long tail on the burn, as it can go on for hours and hours. The ASTM3053 stove test, with the exact same amount of fuel, is considered “over” when 90% of the fuel load is consumed.
Of course, this is an oversimplification, but the point is easy to understand: The shorter test run will show higher BTUs, when in fact both stoves were able to reach a comparable level of BTUs on the “peak” of the test. It is true that some secondary stoves produce higher BTUs than some catalytic stoves, but this is not 100% the case based solely upon combustion technology or what is printed on a brochure. As wood heaters cannot be touched once a test begins, that “peak” BTU is only a very brief period and cannot be maintained unless a user continually adds more fuel.
The shorter test run will show higher BTUs, when in fact both stoves were able to reach a comparable level of BTUs on the “peak” of the test.
As to reliability and cost of maintaining a catalytic wood stove, here are some facts. Catalysts do degrade in efficacy over time. How much time is the important factor. Prior to the 2015 NSPS, catalytic wood stove manufacturers contracted with OMNI Test Labs in Oregon to run some tests to answer that question. OMNI located multiple catalytic wood stoves that had unmolested, original combustors still in use and had them shipped to their facility in Portland, Oregon. Each of the combustors to be tested had to have burned at least 3 cords of fuel each winter. The results indicated that after nine years—that is, nine winters of use—the catalyst burned approximately 1 g/hr higher than the original certification value. All the while producing higher efficiencies, meaning burning less fuel to heat a given space.
This study and the data from the study influenced the EPA to terminate bifurcated standards for wood heaters. This means all wood heaters are required to pass the same emissions thresholds set by the EPA. It should be noted that after many years of use, yes, combustors can cost $150 to $400, depending upon the model of stove. It should also be noted that the essential elements for clean combustion (tubes, baffles, and refractories) can and do fail at times in secondary combustion stoves. These parts can be as costly or more than a new combustor.
Catalytic wood stoves are no more finicky about the fuel they burn than secondary combustion stoves. In fact, a recent study by Climate Change Canada showed that catalytic stoves performed brilliantly on a wider range of moisture content than secondary combustion stoves. Of course, we all want consumers to burn well-seasoned wood. Catalytic wood stoves do offer a wider range of operating temperatures, with some having the ability to produce as little as 9,000 BTUs and run as high as 40,000+.
Of course, there are exceptions to every rule, but studying wood stove test reports yields some significant trends. There appears to be an inverse relationship in emissions tests between secondary combustion and catalytic wood stoves. Secondary wood heaters trend toward cleanest emission tests on the high and medium-high tests. This should come as no surprise, as thermal destruction of particulate matter allows these stoves to burn exceptionally clean, especially on the upper end of operating temperatures. Catalytic wood stoves burn exceptionally clean on the low and medium-low burn rates. This is the result of increased residence time of particulate matter being converted to heat by the combustor. Most consumers—verified by many studies, surveys, and even manufacturer literature—spend 70% or more of their time on low and medium-low. Weighting in the EPA test method M28R reflects this understanding.
Yes, there are more catalytic wood stoves on the market today than in the recent past. This is not because catalytic wood stoves are easier to engineer, but rather because wood stove manufacturers have had 30 years to refine the technology, and, in some instances, by adding dedicated room air, created hybrid wood stoves. With the higher efficiencies of catalytic wood stoves, both manufacturers and consumers also want models that qualify for tax credits and stove changeout programs.
Both catalytic and secondary combustion stoves are easy to operate and are simple in design. Plenty of catalytic wood stoves are operated by young family members due to their ease of use. As for durability, all stoves require regular maintenance and inspection on an annual basis. Today’s wood heaters, of various combustion designs, are capable of great lifespans so long as regular maintenance intervals are followed. There is a perfect stove for each unique user and application.
To sum it up, over the past 30 years, the wood heater market has been refined and improved to the point that consumers, regardless of technology, can expect repeatable performance from their investment in an EPA-approved wood stove.
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Chris Neufeld
Click here to read this month's Contributor Spotlight, which is about Chris and his contributions to the hearth industry.
