Monday, June 18, 2018

Meet the Teams: An aerospace university department tackles an earthly challenge: electricity from a wood stove

This post is the fourth in a series of blogs introducing the 12 teams participating in the 2018 Wood Stove Design Challenge in November.

By Ken Adler and Shoshana Rybeck, Alliance for Green Heat 

Alexus, Connor, and Jack
with the Englander-30 NC stove. 

With the 2018 Wood Stove Design Challenge taking place on the National Mall this November, it is only natural that one teams is only a couple metro stops away. Students from George Washington University School of Engineering and Applied Science’s Department of Mechanical & Aerospace Engineering (GWU SEAS MAE) have been hard at work  developing a thermoelectric stove. Students from this department are more likely to work on nanotechnology in solar panels or landing spacecraft on Mars, but it turns out, getting a steady flow of reliable electricity from a wood stove is just as challenging.

What started as a class assignment quickly became  an imperative learning experience for team members, Alexus Camero (‘18), Connor Itani (‘18), and Jack Eaton (‘19). As students in the Department of Mechanical & Aerospace Engineering, Alexus and Connor both took a thermo-systems design class taught by Professor Saniya LeBlanc.  With Connor’s interest in green electricity and Alexus’ major relating to heat transfer, the duo took on the task of creating a thermoelectric stove to bring to the competition this November. With the necessary grants in place, and a stove donated by England’s Stove Works the Team began designing their thermoelectric generator (TEG) stove  in the Fall semester of 2017.
Dr. Saniya LeBlanc in her lab 
at George Washington University.

Later, their Team grew a little larger, when Jack Eaton joined Alexus and Conner working in   Professor LeBlanc’s lab. Jack says that he was personally drawn to this project for two reasons, one being that “working in his advisor’s lab gives him a lot of autonomy”, something that is invaluable for an innovative college student, and second that he “spent every winter in middle school and high school in New Hampshire and knows many people that use wood stoves and struggle to pay their heating bill.” Jack, Alexus, and Connor all  have personal, academic, and innovative drives for this challenge, which has kept them determined throughout the development process.

The team has been working with Professor  Saniya LeBlanc to design and create the most efficient thermoelectric stove  possible. As of now, their model is using thermoelectric modules by TEGMART that are designed to produce a maximum of 200 watts of electricity under optimal heating and cooling conditions, which is 86° F (30° C) for the cold side and 572° F (300° C) for the hot side of the module. Achieving these optimal temperatures in real life applications is a major challenge so actual power output is expected to be substantially less than the rated power.  

The Englander 30-NC in the lab.
Launching their model

Not all their challenges are technical. Operating a wood stove in downtown Washington, DC required the  Team to maneuver through a number of school and local government hoops to get approval for testing their stove.  Nevertheless, the team has recently finished this taxing process and is on to the testing phase. But, the testing phase has its own challenges as well. The team  recognizes that their stove’s heating and cooling system will require substantial improvements if it is going to maximize the TEG’s power output. As of now, the TEGs  are located on the steel stove top. Steel is not as good a conductor of heat as aluminum, so the Team will be conducting tests to determine if there is sufficient heat flow through the steel stove top.  The cold side of the TEG is cooled by water that is pumped through a 3 foot long baseboard heater.  However, the team is exploring the option of adding a fan to improve cooling. In a home setting, the water could be pumped through baseboards located in multiple rooms, which would allow for a much greater release of heat to cool the return water.  
View of the Englancer-30 NC
from the top. 
The trio is excited to showcase their model at the challenge to show how they retrofitted the EPA certified Englander 30-NC stove  to make electricity, and “limit the amount of heat that is lost” to improve the overall efficiency of the stove.  However, this competition also has personal messages for them. With an academic focus on heat transfer, one of the team’s recent graduate’s, Alexus, has been especially motivated to learn about how thermoelectric modules can convert heat into electricity.  For Connor, the team’s other recent GW graduate, this project has been all about “taking a concept from the drawing board to reality, from conception to completion, and how to deal with unforeseen challenges along the way”. This summer is expected to be an important time for their final developments. With testing commencing within the next month, Alexus, Connor, and Jack are looking forward to being in the lab this summer, working to get their model ready for competition day.  

Contact the team
Alexus Camero

Connor Itani

Jack Eaton

Wednesday, June 13, 2018

Meet the Teams: A father-son forge an entirely new breed of hybrid wood/pellet stoves

This post is the third in a series introducing the 12 teams participating in the 2018 Wood Stove Design Challenge in November.

By John Ackerly and Shoshana Rybeck, Alliance for Green Heat 

Dusty and his father with their stove. 
The stove is so novel that the EPA can’t figure out whether to call it a wood stove or a pellet stove. That’s because it’s neither—or both—depending on how you look at it.

The stove burns logs that are pressed out of sawdust. The logs are automatically fed by a gravity system one after the other. There is no auger, which all pellet stoves have, but the stove achieves consistently high efficiency and low emissions that no wood stove can get. It's the first one of its kind on the market, and the EPA and the entire stove industry doesn’t quite know what to call it yet.

This father-son run business team got into the industry after years of admiring the work of Frank Reed of Hayden, ID, a long time friend of the family. Reed first designed the stove on a dare in 2010 to prove that you can run a stove by burning a single log at a time. The stove had been heating his shop for 7 years. The son of the team, Dusty Henderson, kept asking Reed what he was planning on doing with the innovative design. Reed responded “nothing.” Dusty and his father then bought and patented the design, convinced that it was marketable and could fill an important gap in the residential wood and pellet stove market. 

With an educational background in criminal justice, Dusty would seem to be an unlikely entrepreneur to bring a novel stove to market. However, Dusty recalls being “mechanically inclined” from a young age and he always wanted to bring a product of his hard work to the marketplace. Dusty’s entrepreneurial drive along with his shared hobbies with his father first brought them to work together right after Dusty graduated college, when they began a contracting business together. Now, more than twenty five years later, Dusty and his father have developed Reed’s design, of which they have complete ownership.

Dusty says that they are committed to building such a high quality product that it should be a once in a lifetime purchase. They built the stove with premium parts, which are coated in quality paint with “no burn off whatsoever, smells, or off gasses.”

A different type of animal

509 Fabrication’s stove is unlike any other on the market in that it runs on compressed logs rather than cordwood or pellets. The stove, called the Optimum, continues to be classified as a pellet stove by the EPA, a strange designation since consumers are not allowed to burn pellets in it.  The EPA is increasingly focused on ensuring that stoves be advertised only to burn the fuel that they were tested with, as part of a renewed education effort. “Listing us as a pellet stove is not good for our business and we don’t want the EPA creating confusion about our product—which is already unique enough,” Dusty said. 

Final model of the stove heating
the showroom.

The Optimum burns one compressed log at a time, seamlessly from one end to another, and can be loaded with 3 logs at a time. Unlike a pellet stove, where the operator can adjust the amount of fuel going into the burn pot, the Optimum is adjusted solely by the amount of air getting to the fuel. Heat settings are controlled through the simple opening and closing of the air valve. 

Dusty and his father had to experiment with feed tubes and the early models used feed tubes at an angle, instead of vertically as they do now. The vertical feed tube allows for “more room to add heat exchangers” and ensures consistently high efficiencies at all burn rates. Getting the right amount of combustion air at each of the 3 heat settings was perhaps the biggest challenge. “A big issue was  finding the right fuel to air ratio, and then figuring out how to get that air to the right places,” Dusty said.  

This is one of the stoves competing in the 2018 Wood Stove Design Challenge that is already certified and on the market and thus has third party testing data available. The Optimum was certified at 1.5 grams of PM per hour and 78.8% efficiency and they post their lab report (pdf) on their website as required by the EPA, unlike some companies that still don't do that.  The report shows emission and efficiency levels at different heat setting and the Optimum may be the first wood or pellet stove to have such a high, steady efficiency, ranging between 77.2% and 79.5% on 6 test runs.

However, Dusty and his father went through many different prototypes until they landed on the final model shown above. The team constantly altered the models pre- and post-testing to improve efficiency, reliability, and overall quality. Below are a couple of the old prototypes that were ultimately changed to create the Optimum we see today. 

Early prototypes 
The Fuel

Dusty has seen the compressed log industry grow throughout the years. It continues to spread, even to far flung places like Alaska where cities and communities are battling excessive wood smoke and pressed logs could be a valuable fuel to help reduce emissions. The stove can be used with pressed logs made by a variety of companies across the US, including North Idaho Energy LogsLignetics in Sandpoint, ID, Home Fired Logs in Ferndale, WA, and Superior Pellet Fuels in North Pole, AK. What cannot be used in the stove are logs made with wax or coffee grounds. Those products typically emit far greater emissions and will clog up the Optimum and are often far more expensive than natural pressed logs, which like pellets, have no additives whatsoever.  Pressed logs and wood bricks are the same product in a different shape and they often cost about as much as cordwood when measured by BTUs that go into the house. Cordwood is much cheaper by volume but is much less dense, and cord wood stoves usually only get between 60-70% efficiency.

From the Challenge to the market 

509 Fabrications is currently looking to expand their market penetration, a common goal for most stove manufacturers. Many stove dealers have non-compete agreements where they cannot sell other brands of pellet stoves, but these would not apply to the Optimum. The company is also looking for more recognition from change out and incentive programs, as their hybrid stove may appeal to people who have burned cord wood, while putting out consistently low PM like a pellet stove.
Final model of the Optimum stove
 ready for shipment.

At the Design Challenge in November the stove will be tested again to see if it can meet—or exceed—the emissions and efficiency of other stoves in the competition. All of its other features and controls will also be assessed by a team of independent experts. Only a few stoves will win awards at the event and all will be ranked against one another.  

Click here to learn more about 509 Fabrications and the Optimum. 

Connect with the team

Dusty Henderson 

Monday, June 11, 2018

Meet the Teams: Canada’s largest stove company looks to the future in automated stoves

This post is the second in a series introducing the 12 teams participating in the 2018 Wood Stove Design Challenge in November.

By John Ackerly and Shoshana Rybeck, Alliance for Green Heat 

The SBI team 
Stove Builder International (SBI) built the innovative Osburn Volta pellet stove in 2012 and made important strides in understanding how sensors can control combustion. The Volta is unique in that it runs on direct current (DC) when all other pellet stoves run on the normal household AC current. The Volta has its own battery pack and can be easily integrated with solar power.

After seeing the announcement for the first Wood Stove Design Challenge, SBI knew that it could convert skills and lessons from the Volta to an automated cord wood stove. Their prototype was not ready for that competition, but lead engineer Nicolas Gagnon attended the 2013 event and was excited by what he saw.

Guillaume Thibodeau-Fortin, an SBI mechanical engineer, believes that wood stove automation is likely to be one of the main trends in stove manufacturing in the future. But finding the time and resources for the R&D was tricky. Guillaume started working at SBI in 2013 and knows that stove buyers are price conscious and they need to see far more value in an innovative stove than its ability to burn clean.

SBI’s location in Quebec, the French speaking part of Canada, gives them more insight and interaction with the European stove market where small, vertical stoves are more popular and where more companies are experimenting with automation. SBI’s headquarters is also not far from Montreal, North America’s largest urban area experiencing severe wood smoke issues. If there is any major North American city that needs a cord wood stove that can run reliably clean in the hands of consumers, it's Montreal.

“So we designed this for multiple reasons—not just to build a stove that operates cleanly in our customer's home. Automation also serves to improve safety, efficiency and ease of use,” Guillaume said.

The stove is built on two radically new ideas. The first is that the stove automatically finds the optimum combustion conditions. That is a core goal of all modern combustion, whether it’s in a car, a power plant, or now, a wood stove. The second is connecting that stove to the operator to engage, inform and educate the operator to better perform his or her roles.
Final model of the stove

A key problem with appliances that need periodic operating adjustments is educating the owner. Printed owner's manuals are often not good education tools, even when they are read and reread by the operator. The SBI stove is connected via Wi-Fi to a smart phone, and Guillaume’s vision is to transfer the old fashioned owner’s manual into a far more appealing, usable and effective interface on the phone. The stove can alert the operator when it’s time to reload for example, and instructions will appear that can include a link to a video about how to tell if your wood is dry.

To automate the stove, the first step was attaching a temperature sensor, but the key was to correlate data from the thermocouple with a static pressure sensor. Utilizing an oxygen sensor could have been better but the pressure of keeping down costs steered the team away from that strategy. The team then turned to computational fluid dynamics to track and predict gas activity in the firebox. 

Efficiency and particulate matter emissions are focal points of progress for wood stove design. With better efficiency, PM is usually reduced as combustion is more complete and therefore cleaner by nature. However, the opposite condition is not necessarily true, as emissions can be reduced by adding oxygen which ultimately penalizes efficiency. SBI is paying closer attention to efficiency, which many stove manufacturers did not do in the past, but ultimately, it’s the PM level that determines if a stove can go to market.  

Electronic board in the stove
SBI’s model for increased efficiency relies on sophisticated control of the primary air inlet and a digital user interface that is connected through Wi-Fi on the user’s mobile device. The stove includes three burn rates, low, medium, and high, and is set to each level via a mobile application. To achieve the user-set burn rate, the automatic stove then takes over, using temperature and static pressure sensors to open and close the air inlets and run a motor that adds primary air in or blocks it out depending on how high combustion is and what level the user would like. While the user has control over the desired heat output, the user does not actually have the ability to change the stove's air flow to raise or lower the burn rate mechanically. The inlet for the primary air cannot be opened or closed by the operator, unless the stove is running cleanly, and if so, the stove will respond in real time to the operator’s changes. If the stove is not burning cleanly, it will first get to a cleaner burn before getting to the desired heat output set by the operator.

Setting the heat level from the comfort of your couch via their mobile device should appeal to many customers. Knowing that you don’t have to periodically check to see whether you have given the stove too much or too little air should give everyone in the house more peace of mind. If the main person who operates the stove is out, the person who adjusts the stove doesn’t have to worry about over firing, making stove smolder, or putting out the fire. The internal temperature and static pressure sensors will bring the stove to the desired heat level in the safest and cleanest manner. 

When we spoke to the President of SBI, Marc-Antoine Cantin, about the development of an automated stove at the HPBA Expo in Nashville, we were struck with how hands off he was with the R&D process on the automated stove. He gave Guillaume and his team the autonomy and support to go through the multi-year process of developing a stove that the company can stand behind. To us, this indicated a company that is big enough to support engineers working on tangential, forward looking projects and an engineering team that has built the trust of management.  

While introducing an efficient, and user-friendly automated wood stove to the market is important, SBI understands that people are first and foremost looking for an affordable and reliable form of renewable heat. When designing and creating their model, the concept of affordability was always key. One of the three head engineers on the project said, “keeping it simple is best for efficiency and the bank.” For them, the 2018 Wood Stove Challenge is giving them the platform to challenge their engineering skills in order to prepare for a cleaner wood burning future.

Contact the team

Guillaume Thibodeau-Fortin

Maxime Legros

Nicolas Gagnon

Wednesday, June 6, 2018

Meet the Teams: A New Zealand entrepreneur automates the wood stove - without using electricity

This post is the first in a series of technology and innovation blogs introducing the 12 teams participating in the 2018 Wood Stove Design Challenge in November.

By John Ackerly and Shoshana Rybeck, Alliance for Green Heat 

Alistair Gauld with the VcV stove. 
Variable Choke Venturi (VcV) technology has come a long way from New Zealand, where it was invented over a decade ago. It is a clever, simple device to improve combustion in wood stoves that is reliable and does not require electricity. Brian Gauld, an accountant in New Zealand, met the VcV inventor and bought the rights to it, convinced that it held the secret to cleaning up manually operated wood stoves.  

The VcV is a valve controlled by the draft generated by the stove when burning, but making it work effectively on a wood stove and then getting that stove certified was a much longer journey than Brian initially expected. There is a huge need for an automated, “idiot proof” stove in New Zealand, but the market is not big enough to justify the costs so Brian set his sights on the North American market. In 2008, he hired Ben Myren to help integrate the VcV valve into a stove. So began one of the most innovative and promising new stove technologies on the US market.

The Promise of the Technology

Simply put, the valves increase or decrease the amount of combustion air entering the stove in response to static pressure changes at any given time, depending on how much combustion air the stove needs. Ben and Brian realized that by combining two VcV valvesone for primary air and one for secondary air—the VcVs could reduce particulate matter (PM) emissions even more, hoping that would be the key to reliably meeting the EPA’s 2020 standards. Brian and Ben were well on their way to building an automated stove without electricity, whereas other automated stove designers were trying to do the same thing with sensors, electronics and computer chips. The marketplace for wood stoves has a lot of folks who dislike the thought of a wood stove that needs electricity to operate. The simplicity of the Flamekeepers’ technology could go a long way to ensuring that the stove would not just operate well in the lab, but also in the hands of the average consumer. And that has been the great challenge of the wood stove since it was invented years ago.
Labeled diagram of on a VCV valve

From the moment the consumer lights the stove, they can shut the door, sit back, and enjoy the heat until it needs to be reloaded, according to Ben Myren. At the time of ignition, the second VcV is engaged, as explained in an illustrative informational video on the model. The Secondary VcV (S VcV), a disc attached to the secondary air inlet, rises and falls relative to static pressure, which increases and decreases with the amount of combustion taking place in the firebox. The S VcV functions all the time and supplies just enough secondary air during a burn to maximize combustion efficiency. Once the static pressure decreases at the end of the burn and more oxygen is needed to increase combustion, the P VcV disc goes down again. 

While most automated stoves and stove prototypes include a temperature sensor to help regulate air flow, thermometers require electronics and a control board which can be to be deceptively complicated and unhelpful to control combustion. Instead, static pressure can be a more reliable way to control what goes on inside the stove and what goes up the stove pipe. Ben says that temperature measurements can often be misleading when determining a stove’s combustion level and oxygen needs. 

When people hear about how the VcV works they think that it is very similar to a bi-metalic coil, which responds to heat, and closes down an air inlet as the stove gets hotter. Bi-metalic coils have been used for decades and a few manufacturers still use them even though they can be unpredictable during the lab certification process. Bi-metalic coils respond to heat, whereas the VcV responds to static pressure. While heat must move from the firebox to the coils, any change in combustion will automatically change the airflow, thus inciting an immediate VcV response. Therefore, Ben credits VcV technology with being far more efficient and simply better than a bi-metalic alternative. 

Advantages of Simplicity 

EPA test stove on scale with test filters 
being preheated in the background.
The simplicity of the VcV comes from the concept of letting the stove’s combustion determine its air flow needs at any given moment, which inherently reduces the operational errors. While the user may set the stove to a burn setting that is too low, the disc technology will not lift until the combustion, as determined by the static pressure, is at a high enough level to safely cut off the air flow. The stove will eventually go to that low setting, but not until the static pressure indicates the time is right. Therefore, consumers have the autonomy to choose the burn level they want, but the stove will only reach that level when it is able to do so cleanly. Ben also added a catalyst to the stove to ensure that it would operate well under 0.5 grams an hour and meet the 2020 EPA cordwood emission limits.

Brian and Ben also recognized and addressed one inherent problem in stove installations all over the US: varying heights of chimneys. The engine of a stove is its chimney, which creates a natural draft to pull both the primary and secondary air into the stove.  But homes can have chimneys anywhere from 10 to 30 feet high, which dramatically impacts combustion, and stove manufacturers have no way to address this as they build to the height of the chimney in the test lab—which is 14 to 16 feet. Brian and Ben realized that they could fit the VcV with heavier or lighter discs so that the stove could work well with any chimney height.  

At least one North America company has applied for licensing of the VcV technology and a stove with the VcV may also be licensed in New Zealand.  

Team Goals

Brian and Ben are no strangers to the Wood Stove Design Challenge. An early prototype of the stove competed in 2014. These prototypes showed great promise and led to the stove being the very first North American wood stove to be tested and certified with cordwood.
Ben Myren lighting a stove at the
2013 Wood Stove Design Challenge

Brian’s son Alister Gauld is also a part owner of the company and will be in Washington in November when the stove will be put through its paces by professional stove technicians to see whether it performs as designed. While Ben and his lab tech Eric Schaefer were just hired by Brian on a daily basis and have no financial stake in the company, they are proud of helping to develop the first non-electric automated stove in North America that can help clean up our air-sheds far better than stoves that can be left to smolder by their owners. The beauty of the VcV is that the owner doesn’t even have to know how it works inside or that it is a groundbreaking stove. From the outside it will look exactly like a traditional stove that will keep the house warm in a power outage.

Contact the team

Brian Gauld
Alistair Gauld

Eric Schaefer

Friday, June 1, 2018

Households heating with wood or pellets declined by nearly 10% between 2011 and 2016

By Melissa Bollman, Alliance for Green Heat

In 2011, the Alliance for Green Heat reported a significant rise in the number of U.S. homes using wood or pellets as a primary source of heat. According to the U.S. Census’ American Community Survey, residential wood and pellet heat grew 34% between 2000 and 2010, faster than any other heating fuel. Propane and oil heat experienced the greatest declines during the time period, leading the Alliance to speculate that the rise of wood and pellet heating was driven by the economic recession, rising oil prices, and support for renewable energy.

Wood and pellet heating continued to rise during the next few years (see AGH analyses for 2011 and 2012) at an average rate of about 1-2% per year with some states experiencing higher growth. In 2014, the Census reported that an estimated 2.5 million U.S. households used wood or pellets as a primary heating fuel. Since then, however, the number and percentage of U.S. homes primarily heating with wood and pellets leveled off and saw a modest decline. According to the newest American Community Survey, around 2.2 million homes of U.S. homes primarily heated with wood and pellets in 2016—a nearly 12% decline from 2014 and a 9.4% drop from 2011.

Census figures suggest that wood and pellet use has fallen the most in the Southern states of North Carolina (down 17% from 2011), South Carolina (-25%), and Louisiana (-20%). Wood and pellet heat also appears to be on the decline in the Northeastern states of Maine, Connecticut, New Hampshire, and Massachusetts, which may be the result of more homes installing electric heat pumps. While nearly all states reported fewer homes heating with wood and pellets between 2011 and 2016, the largest increases were reported in Delaware (+20% between 2011 and 2016), New Jersey (14%) Utah (11%), and North Dakota (7%).

While wood and pellet use has declined in recent years, electric heat pumps and solar thermal systems are on the rise. American Community Survey results show that U.S. electric heating rose 10.4% between 2011 and 2016, with the largest increases occurring in Utah (+48%), Maine (+36%), Nebraska (+30%), and Vermont (+30%). About 39% of U.S. homes used electricity as a primary heating source in 2016, making it the second-most common source behind utility gas, which is used in nearly half of U.S. homes.

Warmer winters likely contribute to a longer-term decline in wood and pellet stove sales. According to annual data from NOAA, average U.S. winter temperatures increased between 2014 and 2016, which coincided with a decrease in the number of homes heating mainly with wood or pellets. But the big event during the years of this analysis was the economic recession, which lasted from 2007 to 2009. During these years, the number of U.S. homes heating with wood and pellets rose 15%. The use of stoves as primary heater usually goes up when the economy slows down, as people turn on their fossil fuel central heaters less and rely more on cheap wood. When economy is good, families tend to burn more fossil fuels and less wood. Pellet usage may also follow that trend, but tends to be even more volatile than cord wood.

While wood and pellet use has declined in recent years, electric heat pumps and solar thermal systems are on the rise. American Community Survey results show that U.S. electric heating rose 10.4% between 2011 and 2016, with the largest increases occurring in Utah (+48%), Maine (+36%), Nebraska (+30%), and Vermont (+30%). About 39% of U.S. homes used electricity as a primary heating source in 2016, making it the second-most common source behind utility gas, which heats nearly half of U.S. homes.

Less than 0.5% of U.S. households obtain their heat from solar energy, but the number of American homes using solar thermal technologies more than doubled between 2011 and 2016. The Census estimated that over 150,000 U.S. homes used solar energy as a primary heat source in 2016, while only approximately 122,728 households (50% of which are in Pennsylvania) used coal. This marks the first time solar surpassed coal as a primary U.S. residential heating source in Census estimates. Maryland, Oklahoma, and Louisiana had the largest increases in solar heating between 2011 and 2016. However, solar thermal remains the most popular in California, Arizona, and Hawaii.
The percentage of U.S. homes using other heating sources, namely, utility gas, propane, and fuel oil, has remained fairly constant over the past few years. Although the number of U.S. homes using wood or pellets as a primary heat source grew significantly during the 2000s and early 2010s, the overall percentage of U.S. homes remained fairly constant at around 2%.
Since the U.S. Census Bureau started tracking heating data in 1950, wood heating has had wide swings. Starting at 10% of the population in 1950, it dropped to 1.3% in 1970, an all-time low. By 1990, wood had climbed back to 3.9%, only to drop back to 1.6% in 2000. The recent decline in wood and pellets as a primary heat source could indicate that wood use is starting to level off. However, this is complicated by the enduring popularity of wood and pellets as a supplementary heating source. The newest Residential Energy Consumption Survey by the EIA estimates that an additional 9.3 million U.S. households used wood as a secondary heat source in 2015.

This analysis is based on 1-year estimates from the American Community Survey. The decennial Census, which is used primarily for redistricting, provides the most accurate statistics. The 2017 American Community Survey results will be released in September.