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, you mentioned putting a thermometer on the flue – what temperature range is good?

A: We have recorded our observations of how much creosote we take out of each chimney (as well as customer-supplied performance data) during our annual chimney inspection and cleaning.

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 fiber: 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 twelve-year-old baffled airtight can be presumed to operate at about 45% efficiency, while many of today’s approved woodstoves exceed 70% efficiency.

The big difference between the older woodstoves and today’s woodstoves can be found in the baffle area, where newer techniques have been incorporated to re-burn the exhaust gases.

When you add a 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, causing excessive creosote formation (creosote condenses out of wood smoke as it cools). Excessive creosote formation leads to chimney fires. So, when you burn wet wood, you dramatically DECREASE your heat output, while dramatically INCREASING the likelihood of chimney fires.

You don’t say why you operate your stove with the draft control wide open when you’re burning the dry 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″ above 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 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.