All About Wood Stoves
It might make sense to heat your home with wood — even though firewood is the least convenient of all common fuels
If you’ve been heating your house with wood for years, you probably don’t need to read this article. By now, you know all about the disadvantages and inconveniences that accompany wood heat, and yet you still heat with wood — either because you genuinely love wood heat, or because you love the low cost of the fuel. If you haven’t burned down your house by now, you may even have figured out how to install and operate your stove safely.
This article is addressed to a different audience: those who are thinking about buying their first wood stove.
Forget about the warm glow — and get ready for lots of schlepping
For some homeowners, especially those who haven’t lived with a wood stove, wood heat has romantic associations. Veteran wood burners know better, however. As a document from the Cornell University Cooperative Extension points out, firewood “is one of the least convenient sources of heat, … requiring time and considerable effort to fell and split trees, move wood into dry outdoor storage for at least a year, transport wood indoors, maintain an effective woodstove fire, and keep the system cleaned for safety and efficiency.” I'll add another disadvantage: if you heat with wood, you'll be tethered to your house all winter. You won't be able to go away for the weekend unless your house has a backup heating system to keep your plumbing pipes from freezing.
Wood heat makes the most sense for:
- Homes in rural rather than suburban or urban locations;
- Homes equipped with a woodshed or outbuilding where several cords of firewood can be stored;
- Homes located in areas where firewood is cheap or can be cut for free;
- Compact two-story homes rather than stretched-out single-story homes;
- Homes with an open floor plan rather than homes with many small rooms and many closed doors.
Where does a wood stove go?
Don’t install an outdoor wood-fired boiler (sometimes mistakenly referred to as an “outdoor furnace”). These devices are notoriously polluting and inefficient. They require lots of electricity to run and they waste much of the energy value of the firewood they burn. Moreover, they are so smoky that they will drive your neighbors crazy. In many areas of the country, irate neighbors have sued owners of outdoor wood boilers, alleging that the devices represent a neighborhood nuisance.
The best place to install a wood stove is indoors, not outdoors. After all, that’s where you want the heat. Ideally, a stove should be installed in a large room near the center of your house. It needs to be installed on a fireproof hearth. Commercially available hearths are made of steel sandwiched to a thin layer of insulation. Other hearth options include brick, stone, or concrete.
The stove should be located at least 36 inches away from unprotected walls, furniture, or other flammable items. You can reduce the 36-inch-clearance rule (at least for walls) to a minimum of 12 inches by installing a metal heat shield on the wall you need to protect. There must be a 1-inch air gap between the sheet metal and the wall. The sheet metal should be attached to the wall with long screws; one-inch spacers are used to maintain the necessary air gap. (I favor using 1/2 inch by 1 inch brass or steel plumbing nipples to maintain this space; it’s also possible to use ceramic insulators sold for electric fences.) Remember that the sheet metal has to start 1 inch off the floor to allow a gap for air to enter at the bottom.
Adjacent to the hearth, you’ll need a chimney.
All about chimneys
Until the 1970s, all chimneys were made of brick or stone. Since then, however, metal chimneys have become common.
Older brick or stone chimneys were often unlined. Unlined chimneys cannot be safely used to vent a wood stove, however. If the chimney has clay flue tiles in good condition, or a stainless-steel flue, it can be safely used. When in doubt, hire a chimney specialist to inspect your chimney before you hook up a wood stove to the chimney.
If you are building your own brick chimney, don’t make the usual owner/builder errors. These include forgetting to install the clay flue tiles, and forgetting to install a cleanout door at the bottom of the chimney.
Metal chimneys are not the same as stovepipe. You only need about 3 or 4 feet of stovepipe; that’s the pipe that connects the stove with the chimney. Metal chimneys that are rated for wood stoves are double-walled. The inner pipe is stainless steel, while the outer pipe is either stainless steel or galvanized steel. Follow the manufacturer’s installation instructions when installing a metal chimney.
When it comes to elbows — either stovepipe elbows or elbows in metal chimneys — there is just one rule: you want as few of them as possible.
Whether your chimney is made of bricks or metal, it should be located in the interior of your house, not on the exterior. Exterior chimneys are cold, while interior chimneys are warm. Cold flues cause two problems: they don’t draw as well as warm flues, and they encourage the formation of creosote.
Creosote is a tar-like substance that accumulates on the interior surface of a wood-stove flue. It accumulates when flue gas components condense. There are two forms of creosote: liquid creosote and solid creosote. Liquid creosote is newly formed; you sometimes see it leaking through stovepipe joints, especially if the installer mistakenly pointed the male ends of the stovepipe segments toward the chimney thimble rather than toward the stove. Solid creosote is what you are left with when the liquid creosote dries up.
Solid creosote is flammable. If it ignites, you’ll get a roaring fire in your flue. This event is called a chimney fire. If you have a chimney fire, close all the dampers and air intakes on your wood stove, call the fire department, and get out of the house. When you’re standing in the snow looking at the top of your chimney, waiting for the fire trucks to arrive, you’ll see tongues of fire shooting skyward like the flames of a blast furnace.
Take the following steps to prevent chimney fires:
- Only burn seasoned firewood, not green firewood. Green firewood doesn’t burn as hot as seasoned firewood. Slow, smoldering fires encourage the creosote formation.
- Don't overload your stove with large pieces of firewood. A small, hot fire is the best type of fire for your flue.
- Inspect your flue regularly. You can do this by going to your basement, opening the chimney cleanout door, and sticking a small mirror into the opening. Adjust the mirror so that you see the sky. You want to see a clean flue, along with the sharp, geometrical shape of your flue — usually round or rectangular — without any jagged edges revealing the existence of thick creosote. If your flue looks like it has a reduced diameter, it’s definitely time to clean your chimney.
- Clean your flue and stovepipe regularly (at least once a year). If you plan to do your own chimney cleaning, you’ll need to buy a chimney brush that matches the size of your flue, along with some fiberglass extension poles that can be screwed together. You’ll need enough fiberglass poles to match the height of your flue. Cleaning a chimney is usually a two-person job. It’s best to have one person on the roof operating the brush, and one person in the basement shoveling out the creosote.
If you don’t feel confident enough to clean your own chimney, you’ll have to hire a chimney sweep to clean your chimney every year. The work is expensive but essential. Include the cost of annual chimney cleaning when you are trying to decide whether firewood is an inexpensive or expensive way to heat your house.
Locate your chimney near the center of the house. Ideally, the chimney will penetrate the roof at the ridge or very near the ridge. The farther the chimney is from the ridge, the more likely it is that wind turbulence associated with airflow over the ridge will cause the flue to downdraft. The top of the chimney should be at least 3 feet higher than the roofing at the chimney penetration, and should be at least 2 feet higher than the ridge.
What kind of stove should you buy?
To cut to the chase: I’m not going to tell you what brand of stove to buy. However, I’ll give you a few pointers.
Don’t try to heat your home with a traditional brick fireplace. Fireplaces are extremely inefficient; in many cases, if you light a wood fire in your fireplace, the net result is to cool rather than warm your house. (Fireplace chimneys encourage warm interior air to escape skyward; these air leaks rob more heat from your home than the burning wood supplies.)
You may be tempted to buy a pellet stove instead of a wood stove. If you like pellet stoves, go right ahead. But remember that virtually all pellet stoves require electricity to run, so they won’t work during a power outage.
Don’t be tempted to buy a used wood stove that was manufactured before 1990. Older stoves are much less efficient — in other words, they provide less heat per unit of firewood — than stoves that were manufactured in 1990 or later, after the Environmental Protection Agency (EPA) began requiring stove manufacturers to meet stricter efficiency requirements. Compared to a pre-1990 stove, an EPA-certified stove uses one-third less firewood for the same heat output.
EPA-certified stoves are also less polluting than pre-1990 stoves. The smoke emission limit for an EPA-certified stove is 7.5 grams of smoke per hour for non-catalytic stoves and 4.1 grams of smoke per hour for catalytic stoves. Pre-1990 stoves were much dirtier; they released between 15 and 30 grams of smoke per hour.
Buy a simple stove with as few bells and whistles as possible. A wood stove doesn’t need an electric fan. My favorite quote about electric fans on wood stoves comes from John Gulland, a writer for WoodHeat.org. Gulland wrote, “A retailer friend of mine likes to tell customers who ask about efficiency boosts from fans that ‘you’ll get the same effect if you let me tape your $200 to the back of the stove.’”
Catalytic or non-catalytic?
There are two types of wood stoves: catalytic and non-catalytic stoves. I would advise anyone who is buying a new stove to choose a non-catalytic model, because catalytic stoves are fussy to operate. Catalytic stoves have another major disadvantage: their catalytic converters have a limited lifespan and are expensive to replace when they wear out.
A document on an EPA web site notes, “All catalytic stoves have a lever-operated catalyst bypass damper which is opened for starting and reloading. The catalytic honeycomb degrades over time and must be replaced, but its durability is largely in the hands of the stove user. The catalyst can last more than six seasons if the stove is used properly; but if the stove is over-fired, inappropriate fuel (like garbage and treated wood) is burned, and if regular cleaning and maintenance are not done, the catalyst may break down in as little as two years.”
Which brand of stove is most efficient?
There is only one thing to remember about wood stove efficiency: buy an EPA-certified stove. Beyond that, the efficiency depends more on the behavior of the person operating the stove than it does on the equipment.
Again, John Gulland provides good advice. Gulland explains, “All EPA-certified stoves run within the same band of overall efficiency (combustion and heat transfer) — about 70 percent plus or minus 10 percent. … The stove operator has a lot of control over the net efficiency it delivers. For example, wet firewood cuts net delivered efficiency … Also, firewood pieces that are too large don’t burn well at low firing rates and, again, demand large air settings that reduce heat transfer. When I buy a new wood stove (which is way more often than most normal people do), efficiency is not one of my criteria because I know they are all pretty much the same and that I’m the guy in control of net delivered efficiency.”
Unless you plan to cut your own firewood, you’ll be negotiating with a firewood dealer. Firewood is sold by the cord. A cord is a volume of neatly stacked firewood measuring 128 cubic feet. The traditional way to stack a cord is to make the pile 8 feet long, 4 feet high, and 4 feet deep. If you are ordering 24-inch wood, such a cord is two stacks deep. If you are ordering 16-inch wood, such a cord is three stacks deep.
If you order a cord of wood, and the wood is delivered in an ordinary pickup truck, you're being taken for a ride. A cord of wood won't fit in a pickup truck bed. Moreover, a cord of seasoned hardwood weighs 3,700 pounds — more than twice the capacity of a 3/4-ton truck.
Before you order any firewood, ask what species are being offered. You want the densest available local species of firewood. Here in northern new England, that means sugar maple, beech, or yellow birch — definitely not white birch or poplar. Further south, hickory or oak may be available. In Alaska, your choice is probably limited to species that are considered junk elsewhere — species like spruce or aspen.
It’s not a good idea to burn green firewood. Firewood needs to be seasoned for at least one year; two years is better. To dry your firewood, stack it off the ground in a woodshed with a good roof. If you must stack it outdoors, keep it off the ground by stacking it on long poles or scrap pieces of lumber, and cover the top of your piles with old pieces of metal roofing weighted down by rocks.
If you want your wood to dry quickly, don’t restrict air flow through your wood pile. Don’t cover the pile with a tarp. If you are building a woodshed, you don’t necessarily need four walls. The smaller you split your firewood, the faster it will dry.
It’s hard to buy dry firewood. Almost any firewood dealer can claim that their firewood is dry, but it’s hard to verify whether the dealer is telling the truth. The best approach is to assume that you are buying green firewood, and season it yourself. By early September, I like to have two winters’ worth of firewood stacked and under cover. Half of the wood is between 12 and 18 months old, and will be burned soon. The rest of the wood is being seasoned for the following winter.
If you are heating with wood, there are lots of ways you can burn down your house. Fortunately, the number of house fires caused by wood stoves and other heating appliances is on the decline. According to FEMA, the number of “heating fires” (defined as fires “caused by functioning or malfunctioning central heating units, fixed or portable local heating units, fireplaces, heating stoves, chimneys, and water heaters”) in the U.S. declined from 200,000 in 1983 to 49,000 in 2004.
A few safety tips:
- Make sure that your stove is safely installed. It never hurts to call your local building department, fire department, or insurance agent to ask advice or to request an inspection if you have any doubts about the safety of your installation.
- Keep flammable objects away from your stove, especially clothing, drapes, furniture, newspaper, and kindling.
- Never put ashes in a cardboard box. Ashes should only be placed in a metal ash bucket. Once you have cleaned the ashes from your stove, the best thing to do with the ashes is to spread them above the snow that covers your vegetable garden. (Wood ashes contain potassium, a plant nutrient; ashes also help neutralize acidic soil. Never put wood ashes near blueberries, since blueberries prefer acidic soil.) If you can’t spread ashes in your garden immediately, place the ash bucket outdoors, away from the house, on a patch of bare ground, on a concrete slab, or in the snow — never on a wooden porch.
- Don’t turn on your range hood fan while your wood stove is operating unless you’re sure that the exhaust fan won’t make your stove backdraft.
- Before you go to bed, shut down the air intake dampers on your wood stove to a low setting.
Can you put a wood stove in a Passivhaus?
Passivhaus buildings are very tight — so tight that wood stoves may not operate well. There are two potential problems: in a very tight house, a wood stove might be starved for combustion air; and the heat output of a wood stove might be more than a superinsulated house requires.
Opinions differ on whether it’s possible to install a wood stove in a Passivhaus. At least two European stove manufacturers advertise wood stoves that are touted as appropriate for Passivhaus buildings:
- Rika, an Austrian stove manufacturer, sells the Vitra Passive House Stove. The stove uses ducted outdoor combustion air.
- Normatherm-Stahlheizkesselbau GmbH, a German stove manufacturer, sells a $7,100 stove called the Atago Design NK 20 (also known as the Normatherm KKH 20) that is said to be appropriate for a Passivhaus.
According to Mark Sidall, a Passivhaus consultant and designer in Newcastle Upon Tyne, England, Dr. Wolfang Feist advises against the installation of a wood stove in a Passivhaus. Sidall explained that in 2005, Feist wrote a technical paper on the topic: “Feist reported that there were no suitable wood burners for Passivhaus projects.”
A Swedish document (“Passive Houses in Sweden”) answers the question with a “maybe”: “Standard solutions for the integration of wood stoves into passive houses do not yet exist and need to be carefully planned in each project. Heat radiated and convected from the stove can quickly exceed the heating demand of the room where it is located, causing overheating. The heating power in the stove needs to be low (i.e. 1 - 3 kW) so as not to cause too high indoor temperatures. Only a few stoves with this required low heating power are available on the market. If a wood stove is desired, it is important to balance the ventilation system correctly to avoid the case of an underpressure in the room drawing out flue gases. The wood stove must have a separate air supply and the heat should be extracted from the stove via a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.. The heat can be used for heating other rooms or for heating domestic hot water.”
A British web site answers the question in the affirmative: “You can install a very small wood burner in a Passivhaus; however, using it in all but the coldest winter days could risk overheating unless one was very careful about how much wood was added.”
I know of at least five Passivhaus residences that include a wood stove:
- A Passivhaus built in Reims, France, in 2007 is equipped with “a very efficient wood stove.”
- Glenn Haupt, a certified Passive House Consultant in Bend, Oregon, installed a wood stove in a Passivhaus in Walla Walla, Washington. Haupt wrote, “I used a simple, small Vermont Castings wood stove 7,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. min output ($800 retail) … They burn a few scraps of wood on a cold morning and it carries them through to the next morning, most of the time. … Combustion air is coming directly into the box from outside, no smoke or drafting issues.”
- The primary heat source for a Passivhaus in Lancaster, New Hampshire (built by Garland Mill Timberframes) includes an “EPA-certified wood stove with dedicated combustion oxygen.”
- The heating system for a Passivhaus in Shelburne, Vermont (designed by Carol Stenberg and built by Tom Moore) is described this way: “The house has a wood stove as well as solar hot water panels and resistance electrical hot water for heating.”
- The space heat for a Passivhaus in Nova Scotia (the Solterre Concept Cottage) is “supplied by solar energy, both passive and active, and a small, high-efficiency wood fireplace.”
Rachel Wagner, an architect at Wagner Zaun Architecture in Duluth, Minnesota, has designed several tight superinsulated homes that include wood stoves. One of these homes (the home of Gail Olson and Erik Peterson in Esko, Minnesota) includes a Hearthstone Tribute wood stove. “I have had no difficulties putting a wood stove in such a tight house,” said Wagner. “I’ve done it more than half a dozen times, all in houses testing at less than 1 ach50. In the houses that also have a range hood, we caution the homeowner to pay attention before turning on the fan. Two homeowners report having to sometimes crack a window when the wood stove is used at the same time as an exhaust appliance like the range hood or the clothes dryer. We’ve provided dedicated combustion air routes in a couple of the houses, but not all.”
Another example of a Wagner Zaun house with a wood stove is a Duluth home that tested out at 0.7 ach50. The home includes an EPA-certified wood stove with ducted outdoor combustion air.
In spite of these examples of (apparently) successful wood stove installations in Passivhaus buildings, a few builders have reported problems. Jesper Kruse, a Passivhaus consultant in Greenwood, Maine, helped design a Passivhaus building for Ken Hotopp and Robin Gorrell in Newry, Maine. Kruse reported, “The house was designed to be heated by a small wood stove. We chose the Morso 3142 stove, since its heat output is a close match to the design heat load, and since the stove has a ducted outdoor air kit. We ran the outdoor air duct under the slab. When it came time to hook up the stove, however, we learned that experts have mixed thoughts on ducted outdoor air for wood stoves. We decided to hook up the stove without the ducted outdoor air supply and see what happened. (First, though, we had a discussion with the owners on the importance of being extremely vigilant while the stove was burning, to watch for back-puffing). … They had to crack a window to get the fire going, there was the smell of smoke at one point. I think the fact that their firewood wasn’t fully seasoned made the problem worse, but regardless of the cause, the stove just wasn’t working right. So we decided to hook up the air supply. It turned that the duct is not directly connected to the firebox; it’s just kind of close. … Although the wood stove is not a success so far, the owners really like the idea of being able to heat the house when there is no power, and so we’ll keep working on it.”
Another example of a Passivhaus project that encountered wood stove problems is the Jung Haus in Oakland County, Michigan. According to an author who identifies himself as G. Kragler, “The Jung Haus is … designed and certified to the German ‘passive house’ standard … The Jungs had originally insisted on a small wood stove in the first floor living room. After much deliberation, it was eliminated since we could not figure out a way to meet the air tightness and thermal insulation standards required for Passive House certification with a leaky stove damper.”
Wood heat resources
Here are links to two valuable online resources for anyone seeking more information on heating with wood:
- WoodHeat.org posts reliable information on wood heating topics.
- The Chimney Safety Institute of America is a useful resource for anyone looking for a certified chimney sweep.
Martin Holladay’s previous blog: “Weatherization Funding Has Been Slashed.”
- Erica Breetoe
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