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After 20 Years U.S. EPA Revisits the Wood Stove Program

The U.S. Environmental Protection Agency has initiated a review of the New Source Performance Standards (NSPS) for wood heaters. That might seem like a desperately boring bureaucratic project, and in some ways it probably will be, but after all the hand-wringing and endless haggling is over, the result could be fundamental changes to wood heating technology that could shake the foundations of the manufacturers that make it and forever alter the wood heating experience of users.

If that sounds too dramatic, let me point out that it wouldn’t be the first time.

Back in the mid-1980s when the first wood heater NSPS was in development, there was much anguish among manufacturers, and right up to the first deadline in 1988 plenty of them said there was no way they could meet the new smoke emission limits, which, once the second phase-in period was over ended up being 7.5 grams per hour for non-catalytic appliances and 4.1 g/h for catalyst-equipped stoves.

By some estimates, the first round NSPS put three-quarters of the existing stove manufacturers right out of business. Cruel as it sounds, that was a good thing because a lot of them were just welding shops with no real knowledge of or commitment to wood heating. And even though the best of the manufacturers still standing did manage to meet the emission limits, there were tough times ahead as some of the new designs failed spectacularly from the extreme heat generated by good combustion, something most makers had little experience with.

The manufacturers did in the end come up with some great stoves so that now, twenty years later, a family can buy a wood stove that leaves them giddy with delight when the first fire is lit and produces a flame display so lush and dramatic it hardly seems real. The best of them are durable too, and even better, they can be rebuilt when they do deteriorate, as everything under that much heat stress will in time.

In this round of review, the manufacturers’ technical people, lab testing engineers, EPA officials and state air quality people will quibble over details like whether the test fuel load should continue to be built up from dimensional lumber like two-by-fours and four-by-fours, or should be changed to a more realistic fuel like cord wood. That particular argument could easily go on for weeks and get quite heated, so to speak. But I don’t want to bore you with that sort of thing, and besides, I don’t pretend to know enough of the details to explain the issues, even though I chaired the equivalent standards committee in Canada for seven years back in the 1980s. For a simple description of how wood stove tests are conducted, go here.

My main concern is what these changes could mean for cost, durability and, most of all, the day-to-day effectiveness and convenience of wood burning appliances in real peoples’ houses. Let me give you a few examples of why this NSPS review process makes me feel a little queasy.

As the manufacturers have tweaked their designs to drive their emissions ever lower, they have begun to push some limits that can show up as problems in peoples’ homes.

For example, most non-catalytic stoves (by far the majority) have a baffle that occupies the whole top of the firebox except for a narrow slot just above and behind the door. This slot is only about 1 1/2 inches wide and forces the exhaust to exit the top front of the firebox and then travel back through a flat chamber above the baffle to the flue collar where it leaves the stove. This works fine when the door is closed, but when it is opened to put more wood on the fire, the exhaust can spill out the top of the door opening if it isn’t all drawn through the narrow exhaust slot. The only thing to prevent open-door smoke spillage is very strong chimney draft, which is produced by tall chimneys that ideally run straight up from the stove flue collar. Lots of homeowners don’t have chimneys that produce enough draft to prevent smoke spillage, given the obstacles created by advanced non-catalytic combustion systems.

Looking up into the firebox of a non-catalytic wood stove

Manufacturers like to make wide, shallow stoves so they offer a panoramic glass door and don’t project far into the room, both of which sound great in the showroom. But logs can roll against the door if too many pieces are loaded in this so-called east-west configuration. A north-south configuration or a roughly square firebox floor are usually best for serious wood heating in cold weather.

A wide door opening, combined with the almost-universal six inch flue collar also make open door smoke spillage more likely because so much air must flow through that large opening to keep smoke from rolling out. Add these design elements to the narrow exhaust slot and you have to wonder how much further stove makers can push the design without making the stoves even more susceptible to smoke roll-out if they are not connected to perfect chimney systems.

The original test developed in the 1980s was designed to ensure that stoves wouldn’t allow wood to smoulder, even at very low burn rates. In effect, the test designers determined the average hourly heat loss of a house and said that a stove must burn that low without smoking. The result was a minimum burn rate of 0.8 of a kilogram (1.75 pounds) of wood per hour.

Anyone who heats with wood can see an immediate problem with that, considering that if you were to fill up a firebox and set the air control to burn at a rate of one and three-quarters pounds an hour, the fire would smolder or go out, regardless of how good the combustion system was. One of the ways stove manufacturers tap danced around that problem was by letting the stove leak by using a fixed minimum air supply. The rules say that if a stove just won’t burn as low as the minimum burn rate, then it must complete two runs at the next higher burn rate. This strategy works fine in lab testing and lets stoves pass the tests, but it allows stoves to over-fire when they are connected to good tall chimneys, especially in cold climates. This problem is described in detail in a report written few years ago called the Florida Bungalow Syndrome.

One of the main reasons the NSPS is being revisited (in addition to it being way overdue) is that some air quality regulators think the allowable emissions are too high and should be reduced to give manufacturers incentive to do research aimed at making stoves cleaner. The idea probably has merit, if for no other reason than to maintain the credibility of the standard, but driving down the limit too far could have unintended consequences that I’ll wager few of those regulators are aware of.

As it is, the best manufacturers have improved their designs over the years, so that now a good selection of mainstream non-cats are certified down in the 3 grams per hour range. These low rates have been achieved by using reflective, non-metallic materials in the firebox and by fine-tuning the air supply patterns.

It is much easier to get good results in emission tests using a catalyst to clean up the exhaust, so the real challenge is for the non-cat makers which is important because they account for by far the largest share of the market. But if the emission limit is driven down too low, say to 2 g/h, many more manufacturers could be forced to go back to catalytic combustion, just so they can stay in business. There are a few reasons why that would be a real shame.

Catalysts are expensive and they can fail fast if the user doesn’t follow the manufacturer’s instructions carefully. If a catalyst plugs up, as it can if misused, the user might decide that the $150 - $500 replacement cost is too much and just remove it and run the heater in a conventional, smoky, low-efficiency mode.

Second, the catalytic element is restrictive and shouldn’t have low temperature smoke going through it, so these stoves must have a bypass damper which is opened to light a fire or when the door is opened to reload. A bypass damper might seem simple enough, but lots of users have real trouble getting the hang of using them properly and get confused between the bypass damper and the combustion air damper. Most people who have used stoves with bypass dampers have mistakenly opened the door without first opening the damper and gotten a gush of smoke into the room.

An experienced stove retailer once said that catalytic stoves are fine for enthusiasts, the kind of guys who tinker with antique sports cars and the like. People who are keen about the technology can make it work extremely well, but if a person just wants to heat with wood, a cat might not be a good fit.

Some major manufacturers that had developed full lines of catalytic appliances when the EPA rules were first imposed, subsequently switched their entire lines to non-cats, which is an incredibly expensive process in terms of research and development costs and changing a company’s marketing strategy. My opinion is that non-cats are better for most people and over the long run deliver lower emissions, but it really is a matter of opinion. As we say in a article on choosing a wood stove: “Both options have their benefits and limitations, as well as legions of loyal users who swear that their (cat or non-cat) is far better than those silly (cats or non-cats).”

Wood burning appliances are not like most other heating systems because the quality of the outcome is so much in the hands of the user. People who don’t care about the impacts of their actions on neighbours and are content to remain ignorant of how to burn wood effectively will make a lot of smoke, regardless of the emissions rating of the appliance they choose. All those who want to see EPA’s emission limits ratcheted down to extremely low levels in the belief that the result will be cleaner air might want to consider the negative backwash that could result if they got their wish.

The NSPS review might produce an entirely positive outcome. A lower allowable emission rate would provide manufacturers with the incentive to refine their designs, producing incremental improvements in performance, while preserving the effectiveness and convenience of daily use. All appliance categories would be included, which would correct the egregious market distortion caused when the first version in 1988 exempted fireplaces, central heating systems and cooking ranges.

Considering the influence of user input to the amount of smoke a wood burning appliance emits, it is remarkable that so little effort has been devoted to supporting the public in using wood burning appliances effectively. While the industry and various government agencies have made some effort at public education, virtually no resources have been expended in researching how people learn, what techniques could produce lower emissions, and whether the messages that have been tried are effective. Our efforts at to fill this obvious gap have not attracted much comment or support from agencies responsible for energy use or air quality. The feedback we receive from visitors to and from participants at public workshops we have led suggest that users are receptive to carefully developed suggestions on operating technique. It is clear to us, however, that much more could be done if some resources were applied to research and development of public information materials.

As the NSPS process proceeds over the next several months, we will have more to say about how the process is evolving and what issues are dominating the discussions. In particular, we will revisit the issue of the human factor in wood heating and challenge the idea that an exclusive focus on emissions testing and test results is the best way to reduce emissions.

What do you think? Leave a comment below.

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