Efficient Heating with Wood: Page 3 of 4

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Efficient Heating with Wood
Efficient Heating with Wood
splitting wood
Wood warms you three times—once when you cut it; once when you split and stack it; and once when you burn it.
poorly burning fire
Besides being less efficient at heating a home, a poorly burning fire contributes to local air pollution, emitting particulate matter.
catalytic wood heater
This catalytic wood heater marries modern technology with a traditional aesthetic.
Wood heater
Wood heaters are available in a variety of styles, from traditional cast-iron models to conventional welded-plate steel versions.
pellet stove
Exhaust gases from some pellet stoves may be cool enough that hidden, through-wall flues, instead of interior stovepipe, can be used.
masonry heater
Extensive channels inside a masonry heater expose heated gases to the heater’s thermal mass, which absorbs and slowly radiates the heat into the room.
Masonry stove
Masonry stoves are also available as kit assemblies from companies like Tulikivi. Starting at about $5,000 (plus delivery), this can be a less expensive option compared to a custom-built heater.
Tulikivi stove
A serpentine flue path and high thermal mass capture a high percentage of the fire’s heat for gradual release into the living space.
Efficient Heating with Wood
splitting wood
poorly burning fire
catalytic wood heater
Wood heater
pellet stove
masonry heater
Masonry stove
Tulikivi stove

The second improvement is a metal channel adjacent to the firebox that heats secondary combustion air and feeds this preheated air into the top of the fire, providing fresh oxygen without reducing the fire’s temperature. This allows for volatile gases and particulates to be further combusted in the top of the firebox.

The third innovation is the catalytic converter, a metal honeycombed chamber that allows the wood smoke to combust at about half the temperature—about 500°F. The catalyst itself often reaches temperatures of 1,500°F. Catalytic converters work great when new (as they are when tested) but the catalytic element degrades with use. Ironically, by the time the catalyst is due for replacement, emissions are a lot higher than 7.5 grams per hour limit set for noncatalytic wood heaters, according to WoodHeat.org. An additional technological improvement, coming out of Northern Europe and especially Denmark, are stoves that regulate secondary air intake using oxygen sensors, which results in a reliably hot, and thus more efficient and low-emission, fire. The sensors work in conjunction with regulating the airflow. If the oxygen is low, more air is admitted, and vice versa.

The firebox size and the heater’s efficiency determine how often it will need to be stoked, known as its burn time. Because they are low-mass, these types of wood-heating appliances have a high heating efficiency—they rapidly transfer heat produced by the fire to the room, creating a very hot space around them. They also cool fairly quickly when not in use. Depending on your heating needs, this may be an advantage or disadvantage. It may be difficult to regulate the heat output so that it is comfortable, but restricting the air supply to regulate heat output reduces combustion efficiency, creating more emissions.

Some metal stoves may have enough horizontal surface to serve as a cooking surface. If the power goes out, you’ll still have space heating—and a place to cook meals.

Pellet heaters. Instead of using logs, pellet heaters burn little compressed nuggets of sawdust or wood shavings. Because their emissions are lower, wood pellet heaters may be a better choice in places where population densities are higher, such as in suburban areas, which average between 1,000 and 3,000 people per square mile. Because pellets burn hotter than cordwood and more completely, they produce fewer particulate emissions. The regularity of the shape of the pellets and their high surface area compared to cordwood lends to their burning more evenly.

Pellets are first loaded into a hopper and an electrically powered auger conveys them into the firebox, where they are burned. One fan moves air through the firebox to combust the pellets, and another fan blows heated air into the room. Compared to a typical wood heater, a pellet heater is easier to operate. It also burns longer without attention and burns more efficiently.

While pellet heaters cost more, this expense can often be recouped in lower installation costs since these heaters can often be vented through a sidewall instead of through the roof, although this is subject to model type and local building codes. Pellet heaters require annual maintenance, which tends to be more costly than wood heater maintenance because of their increased complexity, but require less-frequent ash removal during the heating season. If you want to have a secure source of heat during power outages, you’ll need a generator or battery backup for your pellet heater, since it relies on an electric screw conveyor and blower.

In many suburban areas, pellets are readily available. While obtaining pellets is usually not difficult in forested areas where there is a wood products industry, the sustainability of these products can be somewhat questionable, since they take energy to manufacture and transport. In some areas, Forest Stewardship Council-certified wood pellets may be available. If not, inquire from the retailer about their origin.

Comments (10)

Joseph Clarkson's picture

Aloha Richard Strachan,

You made a very good point about the potential problems with having a smoke plume adversely affecting the water falling on a roof that is part of a catchment system. We have been using catchment for 27 years here in Hawaii and we do also heat with wood since we live at 2300 ft.

We are able to deal with the conflict by simply bypassing any catchment stream away from the storage tank when we are burning wood (only 30 or so days a year). Of course someone who needs to burn wood 24 hours a day for months on end would find this procedure difficult.

On the other hand, if you are collecting rainwater, the outside air temperature is above freezing. It might be possible to shut down the wood stove overnight and allow rain to be collected without having the house get too cold, then go to catchment bypass when starting a fire in the morning.

It helps to have a large catchment tank. Ours is just under 40,000 gallons so we can go months without collecting rain if necessary.

adamj777's picture

In Europe, wood pellet burning has been used for decades as a source of central heating in the home. The technology is very advanced and efficiency ratings are very high. Why this technology has not taken root in the US, especially since the US one of the largest exporters of wood pellets to other countries, is a mystery to me.

In any case, some of the manufacturers of advanced wood pellet boilers, for instance, have been making inroads to the US market. This is a boiler that is fed pellets from a large capacity silo automatically and meets all US requirements for central heating.

We recently replaced our old oil-fueled boiler with an AutoPellet Boiler from MeSYS and we immediately saw savings even after financing a large part of the install. (Our monthly payments plus the average monthly cost of pellet delivery is less than the average monthly cost for oil alone.) No modifications to our existing plumbing or chimney were necessary. Pellets are delivered in bulk- tons at a time. The truck blows them in through a feed pipe into our basement storage tank which holds 3.5 tons. The pellets are then fed to a reserve hopper inside the boiler either by vacuum (as in our home) or by a flexible auger conduit.

In our 2000 sq ft home, we are estimated to go through 7-8 tons of pellets a year. So far, that estimate seems to be right on. (We installed 6 months ago.) The major factors to consider were 1. Is there a bulk supplier of pellets who will deliver to us? 2. Do we have room for a pellet storage tank either in our house or in a convenient location outside of our house?

In our case, there was really only one local supplier of bulk pellets but their prices were competitive- $215 per ton. And, since we had just moved into our house, we had not really used much of the basement. So we had plenty of space to spare. Due to our low ceiling height, however, we only had a few options available to us. Still, the outside silos look pretty nice and I would not have minded one of those but my wife did not want to risk being THAT neighbor. :)

Anyway, the system runs well and we have plenty of heat. The boiler has a lot of intelligence in it which we have barely tapped. There is internal intelligence which dynamically regulates burn rate to maximize efficiency no matter what kind of pellets you supply (hardwood/softwood/mixed.) It also has additional features which allow you to plug it into your home network where you can access information and statistics from a web browser. (This feature we have not yet employed but hope to do so in the near future.)

One inconvenience of wood pellet stoves is the need to clean them and remove ash on a regular basis. Well, this unit is self-cleaning so there is no getting yourself dirty and making a mess while you clean it. The boiler also has an ash collection system which deposits and compresses the ash into a container which is removable and has a handle. There is another receptacle which you can line with a plastic bag into which the removable receptacle can dump its ash without making a mess. Just a flip of a lever and you are done. You can then use the ash to fertilize your lawn or garden if you like.

There are also new government incentives available which cover pellet boilers. At the time we installed our system, there was a pilot program which granted us a total of $15k toward our project. This helped make the decision between wood pellet and natural gas an easy choice.

I suspect that, as people move away from oil and take the easy choice of natural gas, the price of natural gas will start to rise just as oil did because demand will be high. I am also not a fan of the fracking techniques used to extract oil and natural gas from the earth, which is gaining popularity. It seems an incredibly wasteful and irresponsible practice, injecting thousands of gallons of poisonous chemical compounds deep underground where they can seep into fresh water aquifers and poison our fresh water supplies. These were the factors that caused us to look for alternatives to both oil and gas. As it turns out, there was a simple, technologically mature, and environmentally responsible alternative right around the corner.

I highly recommend looking into options like these. The are "carbon neutral"; they run on locally produced, sustainable, renewable fuel, and they are simple to install and maintain.

Richard Strachan's picture

This is a good basic article as far as it goes. I'd like to add one small caveat that I'm sure many readers of HomePower "would" appreciate, and that is that I have learned the hard way that water catchment and wood burning don't mix. In other words, if you live in a region where water catchment is legal, and plan on catching your rainwater to provide just that little extra oomph of personal resilience and preparedness, do not !! vent your gasifying wood boiler (go for efficiency!) through the same roof plane that also drains water into your cistern, or you will have a big dissolved creosote problem.

This is one of those things that seems doh! obvious in retrospect, but I can also attest from painful experience that when you get a bunch of experts in the room, none of us is as dumb as all of us, and mistakes like this can creep into a design and be caught by no one. I'm currently pushing the envelope of flue design to find some workaround to this problem.

I guess what this points out is that "low emission" boilers or stoves do emit flue ejecta when the flue or boiler is cleaned, even on a daily basis. In other words, flues cleanings that fly out of your flue must apparently not be considered emissions in the advertising world. You need to keep these little particles in mind.

And that leads into one other little doh! factor and that is houses roofed with flammable materials are also not a good idea for serious wood burners. Just a safety thing. Local building codes are recognizing this more and more.

PRMaine's picture

I've been using a pellet stove for about seven years now and it's worked out well for me. It's more convenient than a wood stove, it is quite efficient (the exhaust is cool enough to hold your bare hand in front of it) and for me, the pellets are manufactured locally and are easy to source. For a while I burned corn in the stove and that worked well too. I also tried burning cherry pits; they smelled a bit and were only available for a short time but they did produce good heat.

When I first got the stove I saw samples of bio-mass pellets but I've never seen any for sale. There was the suggestion then that pellets could be made from various weeds and grasses and I've wondered whether this idea has ever gone somewhere. Perhaps in other parts of the country, but there's no sign of them here in Maine.

NorthPilot's picture

We bought a Tulikivi Soapstone stove about 7 years ago and it is the best thing that we have ever purchased. In a nutshell, it 'makes firewood work'. It burns very efficiently and does not require tending after the initial 1 hour to 1 1/2 hour burn cycle. It provides instant radiant heat and a cheery fireplace glow thru the ceramic glass door, and long lasting radiant heat from the soapstone mass. The genius of this design is that, by having this 2 to 3 ton thermal mass central to your living space absorbing intense heat during firing and then radiating that stored heat for the next 12 or 24 hours, you are maintaining an evenly heated house, all without the need for complicated water pumps, fans, thermostats, back-up generators, etc.

After the quick hot fire has consumed all the wood, we often scrape down the ashes thru the grate, and the hot firebox is then a perfect oven for bread, or a turkey, for instance (this works wonderfully -- you don't need the special bake oven models...).

For a season's heating here in the foothills of the Alaska Range, we burn about 3 cords of well split and 3-year-cycle-seasoned aspen/birch/spruce. I use about 2 gallons of mixed chain-saw gas, 2-3 gallons of tractor diesel for the splitter, and about 2 gallons of 4-wheeler gas to harvest the wood from our homestead. Harvesting wood locally is central to the economy -- if you have to burn all kinds of fossil fuel in a truck to harvest the wood from 30 miles away, you might as well just use fuel oil in a Toyo stove.

Instead of our old double-barreled stove roaring away in the basement, our home now has a warm soapstone heart. During -40 deg. cold snaps, we sleep downstairs near the stove on our fold-out couch, and generally just pull in to the downstairs living room/kitchen/dining area around the soapstone stove.

You'll spend about $12-15,000 dollars to have a Tulikivi stove built by the masons, and a couple of thou more to get set up with chainsaws and splitters, but the stove lasts pretty much forever, and you'll save big bucks after the initial investment while making your home a cozy fortress against economic cycles and fuel crises.

Anonymous _5208's picture

The outside wood burners that regulate temp by restricting airflow are killing people. One of these smug pots produces the pollution of 8,000 natural gas furnaces. I do not oppose wood heat - I have a Vermont Castings Encore and a 4 acre wood lot and I use the stove on days that are sub zero to augment my passive solar and geothermal furnace. I just don't like the outside wood burners that produce tremendous amounts of acrid air pollution and ruin the quality of a whole neighborhood's air because one uninformed person uses one. For God's sake if you have one get rid of it and buy a decent wood stove or efficient furnace. The government needs to regulate and, hopefully someday, ban these deadly devices.

pfreeman's picture

This was an informative article but contains some silvicultural misinformation. Clearcutting is a regeneration method and silvicultural system and is not synonymous with the high grade harvesting of 100 years ago. It is not intrinsically unsustainable and is in fact the most appropriate method to use when sustainably managing, for example, the aspen forest type in the Lake States. It may not look "nice" to some eyes, but nature has no concern for our aesthetic tastes. Just as we must plant our tomatoes and corn in full sunlight if we want to harvest crops from them, some forest types similarly need full sun to regenerate.

Chuck Lohre's picture

A recent blog post in POLLUTION ENGINEERING inspired us to plot a path to LEED Platinum for a Frank Lloyd Wright-designed home.

LEED Platinum HomeEditor Roy Bigham's piece was about new EPA clean air standards being considered for for wood stoves. We use a sawdust pellet stove in our office to demonstrate renewable energy. The U.S. Green Building Council considers the burning of sawdust a renewable energy source because sawdust would decay anyway and release its carbon. Since the production of lumber is considered a proper use of natural resources if it is a common species, its byproduct, sawdust, satisfies the renewable energy requirement for that credit.... So let's get back to Bigham. We disagree when he concludes, "These and many other changes are not driven by market pressures to improve anything. These are bureaucratic changes that the government tells us is better for us. Nobody in my circle of friends wants to be forced to spend up to $100K for their next car. My last purchase was around $18K."

U.S. citizens' tax dollars fund the EPA's work to develop air quality standards. I want to follow them for my and my neighbors' health. We paid $3500 for our EPA Certified sawdust pellet stove. You can get one like it in size at Walmart for $1500, which pollutes the atmosphere unnecessarily. We achieved LEED Platinum for $12 per square foot. We believe you can be environment-positive and economical by spending where it's needed and saving through reusing materials. My last car purchase was $7K 12-year-old Honda Insight. It gets 100 mpg once you learn how to drive efficiently. Now that's green, clean and inspiring!

Anonymous _3293's picture

As a wood burner I enjoyed your infomative article. But the most important question for me was not addressed. That is:
When fosil fuels stay in the ground, they don't normally contribute to green house gases or other air polution. But when dead wood rots on the forest floor, it does contribute. So - what is the comparison between the stuff emitted from a stove burning wood vs. that left on the ground to rot away?

Thanks
Power plus

pfreeman's picture

Yours is an astute and relevant observation. Carbon dioxide is the same molecule and produces the same greenhouse effect regardless of where it comes from. The difference between burning wood and burning fossil fuel is that when we burn wood, we are "borrowing" from the biological carbon cycle that operates on a time scale ranging from minutes or hours for microbial life to a few centuries for the longest-lived trees. As you note, trees that are not burned or otherwise used, end up eventually cycling their carbon back to the atmosphere through (mostly) bacterial respiration as they break down. Forests do, however, act as carbon "pools", storing carbon that would otherwise be released back to the atmosphere, but it is a dynamic process with older trees constantly dying and releasing carbon and younger trees taking up carbon. We could increase the size of that carbon pool if we reduced our population so that farmland could be converted back to forest.
In contrast, when we burn fossil fuel we are tapping in to the geological carbon cycle that operates on a scale of millions to hundreds of millions of years. We are releasing carbon that otherwise would remain geologically bound for such a long time that from our perspective it might as well be forever.

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