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Firewood Questions & Answers

Q: Am looking in the newspaper for firewood, and the terminology used in the ads has me confused. What is the difference between green wood and wet wood? What is the difference between seasoned wood and dry wood? Can wood be seasoned but not dry? Dry but not seasoned? If given the choice between dry wood, wet wood, green wood and seasoned wood, which should I choose?

Sweepy A: Firewood terminology is a bit confusing. Green wood is always too wet to use as fuel, but "seasoned" wood may or may not be dry enough to use as fuel. For proper burning and heat production, you want your wood to be both seasoned and dry.

Freshly cut wood is referred to as green, and is very wet, containing up to 80% moisture by weight. To dry out the wood to 20-25% moisture content so it can be used as fuel, you season it by cutting it up into short lengths and stacking it out of the rain in such a way that air can circulate freely to carry away the water as it evaporates out both ends of each piece. Remember Granddad's wood shed? The open slat walls allowed air to circulate, and the overhanging roof kept the rain off the wood. The seasoning process generally takes around 12 months, depending upon the species of wood and the airflow, humidity and temperature of the woodshed. Once the wood has evaporated down to 20-25% moisture content level, it is seasoned, and ready to burn.

Seasoned wood doesn't always stay that way: wood is cellular, and will reabsorb water like a sponge. Here in the rainy Pacific Northwest, if you take dry, seasoned fuel wood and stack it out in the rain, it can soak up enough water to revert to its original water content in just a matter of hours. When this happens, your seasoned wood needs to be reseasoned until it is once again dry enough to use for fuel, which can take several weeks.

You can read about the importance of avoiding burning green or wet fuelwood in an airtight woodstove here.

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Q: My husband took down some trees on the property last week, and we thought we'd dispose of them and help heat the house at the same time by burning the wood in our fireplace, which we have never tried to use before. The problem is, we can't seem to build a fire! We get the paper and kindling going, but the wood just won't catch. Any advice for a couple of city slickers who don't know how to make their fireplace work?

Sweepy A: Your freshly-cut wood is too green to burn. Try seasoning it for about 12 months, and I'll bet it burns just fine.

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Q: We recently moved to the Pacific Northwest, and bought a new airtight woodstove with an "airwash" design that is supposed to keep the viewing window clean. Well, it doesn't. If we don't scrape the thick coating of black crust off the inside of the window several times a day, we can't even see the fire! We have invested a lot of money in the stove, hearth and stainless steel chimney, not to mention the mountain of seasoned firewood we bought that is piled up in the back yard, and are really disappointed that we don't have the view of cheery flames we had counted on to help brighten up the gray and rainy Winter days here. What can we do?

Sweepy A: Stop storing your fuel wood in a pile in the rain. It is soaking up the rainwater, and your stove's airwash design can't hope to compete with the thick, black, creosote-laden smoke produced by burning wet wood.

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Q: We bought a woodstove from you people in June, and did everything you suggested (outside combustion air, properly sized chimney liner, etc.). We even built a woodshed, and cut and stacked our wood inside all Summer! Our problem is, even though we bought a stove rated to heat 1800 square feet, it won't heat our well insulated, 1400 sq.ft. house! It is only September, and we shudder to think what our electricity bill will be when it really gets cold outside. Can you send someone over and figure out what is wrong with this stove?

Sweepy A: The only thing wrong with your stove is, it can't extract heat from water. Our moisture meter test revealed that the wood you're burning is at 50% moisture content, which is too wet to allow proper heat extraction and transfer. All Summer may sound like a long time, but three months is just not enough time to allow firewood to properly season: the wood in your shed will be ready to burn next Fall.

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Q: There is black goo dripping off our woodstove chimney cap and onto our brand new light gray roof. The company we bought our stove from says this is our fault, that our wood is green or we're smoldering the fire. We never smolder the fire, and our wood is well seasoned: my husband bucked it up from some trees my Father cut down over two years ago! We think we were sold a bad stove, and want the company we bought it from to give us our money back and pay to have the tarry mess cleaned off our new roof. If you can help us document the fact that there must be something wrong with this stove, we promise to buy the replacement stove from you.

Sweepy A: Would love to sell you a stove, but I'm afraid I must agree with your retailer: your wood isn't properly seasoned. The black goo that is dripping off your cap is liquid creosote, and the source is water content in your fuel wood. Trees need to contain the water absorbed by the root system to deliver nutrients up the trunk and to the tips of each branch, so the outer surface (bark) is designed to prevent leakage and evaporation. When you cut down a tree, the water will start evaporating through the exposed endgrain at the cut, but might take many years to work its way out of the entire length of the tree. In the meantime, every time it rains, the endgrain reabsorbs fresh water, and the drying process starts over. In the rainy climate of the Pacific Northwest, a fallen tree will wet-rot before it ever dries out enough to use as fuel. This is why we cut our firewood into short lengths and put it into a ventilated woodshed to season. Cut wood presents two open endgrain surfaces to encourage evaporation, and the water in the center of each piece has a far shorter distance to travel to evaporate out the ends.

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Q: For some reason, my woodstove deposits TONS of creosote in the chimney. I have to clean it out every few weeks, and even so we've had several chimney fires. When the first chimney fire cracked the terra cotta flue liner in the chimney, I installed a stainless liner the same size as the vent collar on my stove and insulated around it with ceramic blanket, so I know the cause isn't a cold or oversized flue. Everyone I consult about this problem suggests the cause might be wet wood, but I stacked my wood on pallets to keep it off the wet ground and covered the entire stack with plastic tarps to keep the rain off! Can you explain to me in layman's terms what my problem might be?

Sweepy A: Make a cheese sandwich on some nice fresh bread. Cut it in half, wrap one half tightly in plastic wrap, and put both halves in your refrigerator for a few weeks. Compare the dry, crusty half with the bag of soggy goo, and you will be looking at a good demonstration of why your woodpile won't season properly if you cover it with plastic tarps.

This is also the reason woodsheds traditionally have open slat sidewalls: as moisture evaporates from the wood, air circulation is necessary to carry it away. Your tarps create a mini-environment where the evaporated moisture condenses on the underside and rains back down on the woodpile, where it is reabsorbed.

If a tarp is your only option to keep rain off your fuelwood, drive tall stakes a foot or two from the four corners of your woodpile and drape the tarp over the stakes so air can flow through the woodpile beneath it. Take a few pieces of wood and stack them on top of the pile at the center under the tarp so rainwater won't puddle, and fasten the tarp to the stakes so the wind won't blow it off.

Wet fuelwood accelerates creosote formation in the flue. You can read about this phenomenon here.

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Q: We moved my Aunt Alice out of the family homestead a few months ago, and found a couple of cords of alder in the woodshed that must have been there since Uncle George died ten years ago! Thinking I had a real find, I appropriated the wood and have been burning it for the past several weeks. Well, my experience has been disappointing, to say the least: short duration fires, and not much heat. Now don't tell me the problem is wet wood, because after ten years in the woodshed, this stuff is BONE dry. What gives?

Sweepy A: Sounds like Uncle George's stash has gotten too dry, a condition savvy woodburners in his day used to call "punky."

Wood that has been seasoned for 9-12 months still contains about 20-25% moisture, most of which is wood resins. These resins play an important part in the three stages of wood combustion. During Stage 1, the kindling fire warms up the fresh load of wood and any remaining water content is removed by evaporation and vaporization. As the wood reaches 500 degrees or so (Stage 2), the resins begin to break down chemically, and volatile gases are released which squirt out through the wood fiber and ignite, boosting the temperature of the fire to around 1,100 degrees and producing 50-60% of the heat value from that load of wood. As the gases burn away, the flames finally attack the wood fiber itself (Stage 3), and extract the remaining heat value through the process known as charcoaling.

If your firewood has dried to the point where it has lost its resin content, your fire will go directly from Stage 1 (warming up to combustion temperature) to Stage 3 (charcoaling), skipping Stage 2 and missing out on 50-60% of the heat (and burn time) you'd expect to get from that load of wood. Here's what the US Dept. of Energy website has to say about too-dry fuelwood:

"Some well-seasoned wood can in fact be too dry for today's airtight stoves. If you place wood that is too dry on a bed of coals, it will instantly give up its gases as smoke, wasting unburned smoke and producing creosote buildup."

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Q: Have been reading about firewood on your excellent site, and one thing has me confused: why doesn't the wood just keep on seasoning at the same rate? I mean, if the wood is 80% moisture content when it is fresh cut and dries to 25% moisture content after 12 months (dropping about 5% per month), wouldn't it drop to 19% after 13 months, 14% after 14 months, and so on? It seems to me that after a year and a half or so, the wood would have become BONE dry, which you indicate is not desirable.

Sweepy A: Wood actually contains two types of moisture, which evaporate at different rates.

Visualize a piece of fuel wood as a bark-wrapped bundle of tiny tubes made of wood fiber. These tubes deliver water and nutrients from the roots to the leaves, so they're always filled with moisture. This "free moisture" is mostly water, and evaporates readily out both ends of each cut piece of fuel wood.

The wood fiber surfaces of the tubes themselves are also saturated with moisture, in the form of wood resins, sometimes called "sap" or "pitch". This "bound moisture" is of heavier consistency than water, and much of it is soaked right into the wood fiber.

Most of the free moisture content of fuel wood evaporates away in 9-12 months, at which point the wood is "seasoned" and ready to burn: the 20% - 25% moisture that remains is mostly bound moisture, which evaporates much more slowly (think years instead of months).

Here's what happens to the bound moisture when you add a piece of seasoned wood to your fire: when the wood heats up to 212 degrees, the wood resins give up their water content in the form of steam, which exits through the end grains. At about 500 degrees, the remaining bound moisture turns into volatile gases, which ignite and burn. The combustion of these gases provides a significant portion of the heat value from each load of wood.

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Q: What difference does it make how wet the wood is if you're going to throw it onto a hot fire? It seems to me that if I already have a fire going and add a load of wet wood, the blazing hot temperatures inside my woodstove should dry the new load out almost instantly, causing little or no impact to my fire. No?

Sweepy A: No.

Let's say you bring an armload of wood into the house which contains a pound of water, in whatever form, and that the temperature of the wood [and the water it contains] is 32 degrees. The only way to get rid of the water in the wood is to heat it to the boiling point [212 degrees F], and turn it into steam so it can be carried away by the chimney updraft.

A btu is loosely defined as the amount of heat required to raise a pound of water one degree F. Thus, it takes 180 btus to get the wood up to the boiling point of the water (212-32). To convert a pound of water at 212 degrees to a pound of steam at 212 degrees requires another 970 btus: this is the latent heat of evaporization. This heat has to come from somewhere. It may come from the wood that contains the water or, failing that, it will come from the fire itself. The temperature of the wood cannot exceed 212 degrees until all the water has been removed, which may take quite awhile depending on the actual amount of water present and the intensity of the fire. In the meantime, the rising steam will have extinguished the secondary fire above, robbing you of that heat as well. The total effect on the fire can be quite dramatic: if the temperature in the firebox drops below the kindling point, the primary fire itself can actually be extinguished.

Many thanks to hearth product industry guru Dave Johnson for info used in the above answer

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Q: Hopefully you can help me out. Based on your website it would seem that virtually every wood is OK to burn in a stove as long as it is properly seasoned. Does the same thought apply to plain old fire places or are there different dynamics at work? I am building up a stock of firewood as I hope to purchase a stove or an insert in the next couple of years. In the meantime, I would like to burn some of what I have in my fireplace. Tree services will supply me with all the Willow I want free. A load of hardwood is harder to come by at least as a freebie. Any problem burning Willow?

Randall R Guse

Sweepy A: Burn it. All wood species contain the same ingredients: basically, a hodgepodge of wood fiber, wood resins and water in various percentages. Once you've seasoned out the water, all burn just fine, and emit the same products of combustion.

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Q: We've been heating our house with our woodstove for several years, and are always careful to season our wood to 20% - 25% moisture content (we have a fuelwood moisture meter). This year, for various reasons, we've gotten into some wood that reads more like 35% - 40% on the meter.

The problem is, we're going through this wood like crazy! Our wood pile is shrinking at an alarming rate, much faster than usual.

My question is, why does wet wood burn faster than dry wood?

I posted this question to an online wood heat forum, and one guy said this is impossible, labeling it "counterintuitive." Well, counterintuitive or not, it is happening, and I wonder if you can tell me why?

Sweepy A: Sure. Because the water content in the wood absorbs heat from the fire, while the steam it creates mixes with the gasified wood resins to interfere with secondary combustion, wet wood doesn't produce nearly as much heat as dry wood. To make matters worse, you've probably noticed you have to crank your draft control farther open than usual just to get the wet wood to burn at all, which reduces the duration of the burn.

What this boils down to is, to get the same amount of heat from wet wood as you do from dry wood, you have to burn more of it.

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Q: Dear Sweep:
I've been heating with wood for 25 years. Last year you talked me into placing all my cut wood in the wood shed. This year, with really dry wood for the first time ever, my stove is working great. I have yet to see smoke coming out my chimney, whether I'm double burning, or straight firing. I use a Vermont Castings Resolute. Could the lack of smoke be due to the dry wood? Will the dry wood increase my efficiency?

Thanks,
Maure Weigel

Sweepy A: Yep, and yep. Now that you no longer have a boatload of steam from the wet wood filling your firebox, your stove can reburn the wood exhaust like it was designed to do. This not only increases the heat output to the room, but decreases the outflow of unburned particulates by something like 90%. Prediction: you're going to burn a lot less wood from now on, and your chimney won't need cleaning nearly as often.

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