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Sweep's Library: Burning Wet Wood

Q: Is it better to burn unseasoned wood with the vents wide open, or use dry, seasoned wood with the vents shut down? If we burn the dry wood wide open, the stove gets too hot and the wood burns too quick. Maybe the best solution is to mix the two lots? I'd be interested to know your opinion. Also, our Sweep mentioned putting a thermometer on the flue pipe - what temperature range is good?

Sweepy  Dave Hughes of Master Sweep Chimney Service forwarded your letter to us for a second opinion. We're 25-year veteran Chimney Sweeps who also operate a hearth product retail business. We have burned over 60 different woodstove models on our test flues over the years, and have also kept a careful record of how our stoves perform in the field, recording our observations of how much creosote we take out of each chimney (as well as customer-supplied performance data) during our annual chimney service inspections.

Our experience shows beyond a doubt that wet fuelwood provides MUCH LESS heat, and causes MUCH MORE creosote to form in the chimney, regardless of the draft control setting. Here's why:

Airtight woodstoves extract heat from wood in two ways. The primary source of heat from a woodstove is the combustion of the wood itself: the secondary source is the combustion of the gasified resins and unburned wood particles that result from the primary fire. Unless yours is a very primitive model, you'll find a baffle plate of some kind near the top of your stove, between the fire chamber and the flue outlet. This is where the secondary burn occurs, and where your stove creates up to half the heat it delivers to you. The amount of secondary combustion that occurs varies widely from model to model, largely due to advances in heat extraction technology over the years; a twenty-year-old baffled airtight can be presumed to operate at about 45% efficiency, while many of today's EPA approved woodstoves exceed 75% efficiency. The big difference between the older woodstoves and today's woodstoves can be found in the baffle area, where newer, more sophisticated techniques have been incorporated to re-burn the exhaust gases.

When you add an unseasoned or wet piece of fuelwood to your fire, the water contained in the wood heats up and turns to steam, which mixes with the exhaust gases and extinguishes the secondary burn. Regardless of how sophisticated your baffle system is, this cuts your heat output by up to 50%, and results in cool, water-laden exhaust filled with unburned particles and exhaust gases. This wet, heavy, high-density smoke travels very slowly up the chimney, where it cools even further, condensing onto the walls of the flue and causing excessive creosote formation. So, when you burn unseasoned or wet wood, you dramatically DECREASE your heat output, while dramatically INCREASING the likelihood of chimney fires.

Another drawback to burning wet or unseasoned wood is creosote formation on the viewing window. No matter how good the airwash design that keeps the window clean, it won't work when the firebox is full of wet smoke. A blackened viewing window is one of the most reliable indicators that the fuelwood is improperly seasoned.

You don't say why you operate your stove with the draft control wide open when you're burning the dry, seasoned fuelwood, but you shouldn't have to; the draft control is there to enable you to control the combustion of your dry fuelwood, UP TO A POINT. Go ahead and turn the draft control down some to control heat output and burn time, but be careful not to smolder the fire. If your stove has a viewing window, you can easily see if you're starving the fire for air; the flames go out. If you don't have a viewing window, attach a flue gas thermometer to the stovepipe, 18" to 24" away from the stove, and keep the flue gas temperature above 325 degrees. Oxygen is required to ignite the gases in the secondary burn area, so if you take away too much air by adjusting your draft control too low, you'll lose the benefits of the secondary burn even if your fuelwood is dry. This will evidence itself on the flue gas thermometer, which will quickly fall into the creosote zone.

Today's EPA approved woodstoves provide pre-heated oxygen to the secondary burn chamber directly, through a separate intake controlled by the chimney updraft, enabling you to turn your draft control all the way down to control the primary fire without extinguishing the secondary burn. This technology results in fewer particulate emissions, longer burn times and cleaner chimneys, but it is important to note that steam is still not combustible: even these state-of-the-art stoves won't burn the gases in the secondary burn chamber if the fuelwood is wet.

Even when properly seasoned, some wood species don't make very good fuelwood. You can view a chart comparing the various species by clicking here.


12/9/10: Dry Wood, No Smoke!

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?

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.

To read more fuelwood questions and answers, click here.


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