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Wood Stove Overfiring:
What is it, and how do we avoid it?

Q: We just bought a new, EPA approved woodstove from our local dealer, and while reading the owner's manual, we noticed a precaution against 'overfiring'. Our dealer says he has no idea what this might mean. Do you suppose the term 'overfire' means to get the stove too hot? How can that be? Doesn't a woodstove have to get hot in order to heat the house? If the stove can be damaged by over-heating, please give us some input from your experience as to what are the indicators it is getting too hot, and what sort of damage can result. Finally, is overfiring an issue with the woodstoves you sell? We hate to think we bought a woodstove that must be babied for fear it might self-destruct!

SweepyA: Ever seen a Blacksmith's forge in action? The concept is simple: hit a big, hot fire with enough oxygen and you can melt iron and steel like butter. Since all woodstoves have iron or steel components, they can all be damaged if simultaneously given too much fuel and too much air (the situation referred to in your manual as overfiring).

This doesn't mean you have to 'baby' your stove:
Today's non-catalytic woodstoves burn hotter than the stoves of yesteryear, so they incorporate high-refractory and stainless steel components not found in older models. Nonetheless, even though your new stove is built to withstand the occasional temperature "spikes" that might occur during a given burn, it could be damaged if you leave the draft control in the High Burn range for extended periods of time. This damage could take any of several forms, including discolored paint, warped or burned out internal components and, in extreme cases, cracked welds and body parts.

Here's a pretty good analogy:
If you've ever driven a car with a manual transmission, you may have noticed there's a "red zone" on the tachometer dial. When the needle approaches the red zone, it is time to shift to a higher gear, or you risk damaging your engine. You can wander into the red zone on occasion, say when you're accelerating extra hard to merge with high-speed traffic, but for everyday cruising, automotive experts agree that your engine speed should be kept well below the "redline".

Driving a wood stove presents a similar situation:
The draft control that supplies air to your fire has a High setting, to be used when needed to provide extra combustion air when kindling a fresh load, or to bring a cold house up to temperature quickly.
Care must be taken that these occasional forays into the High Burn range are of reasonably short duration. As the load commences burning, or the house begins to warm up, the draft control should be turned down in corresponding increments to maintain a "controlled burn" situation. Once you've determined that your fire has settled in for the long burn, leave your draft control at a low-to-medium setting until it is time to refuel.

Be mindful of the octane level of your fuel:
To further ensure that you stay in control of your fire, you should avoid loading the firebox with large quantities of super-hot burning materials like kiln-dried mill ends, cardboard or paper, and you should never burn treated lumber, rubber, styrofoam or plastic in any quantity. You'll find some version of this cautionary info in the owner's manuals of most of today's woodstoves.

Put a "tachometer" on your wood stove:
A stovetop thermometer is an invaluable tool for monitoring the progression of your fire, but it must be located on the critical "hot spot" at the center of the top plate to give you a meaningful reading. If you can't position a stovetop thermometer in a readable position on the top plate of the firebox, as is often the case with fireplace inserts, consider investing in a point-and-read infrared thermometer. Use the thermometer to monitor your fire until you're familiar with how your stove responds during various stages of the burn. With experience, you'll eventually learn to "read" your fire through the viewing window, and instinctively make the appropriate draft adjustments.

Plan your trips in advance:
Wood stoves are not as responsive as car engines, so a little anticipation on the part of the driver is required. Early indications of a wood stove that is approaching overfire temperatures include simultaneously raging primary and secondary flames, or surface temperatures (taken at the center of the top plate) rising past 600° F. When either of these signs are observed, your fire is heading toward the red zone, and you need to adjust the draft control to a lower setting. The top plate temperature may continue to rise for a short while after you adjust the draft control, and may even spike as high as 750-800°F when the secondary flames light off, but should soon stabilize and drift down to cruising temp (around 500° - 650°F for most models).

With practice, proper driving habits become automatic:
The fact that there is a bit of a learning curve to the art of tending the fire needn't be daunting: a little sensible diligence is all that is required: get in the habit of monitoring your stove for 1/2 hour or so every time you add wood, and as the hot spot approaches the red zone, turn the draft control down gradually, a little bit at a time, until you bring your fire under control. You'll soon discover that burning in the cruise zone has many advantages, such as more constant and comfortable room temperatures, less frequent need to refuel, a better night's sleep, and longer-lasting stove components.

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Q: I read your article about overfiring a wood stove, and it seems to make perfect sense, except for one factor you seem to have conveniently overlooked: when I turn my stove down so it's burning in the 500 - 650° "cruise" range you recommend, it won't heat my house! I'm interested to hear what your comment might be about that!

- Cold in Ontario

SweepyA: Actually, this one's easy: if your stove isn't capable of maintaining a comfortable room temperature while operating in the cruise zone, you need a larger stove, a second stove, or better insulation.

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  Q: My dad had several wood stoves while I was growing up, and he operated them all by the same rule:
"Wait til she starts to glow, then turn 'er down to low."
Seems to me dad's little rhyme conveys the same information as your several-paragraph article on Over-Firing, eh?

- Simon Berger

SweepyA: Almost. By the time those stoves began to glow, they had reached at least 900°F. Which is deep in overfire country, especially when you consider the total time spent in the red zone during the journey there and back. If ol' Dad made it a regular habit to wait until he saw his stoves glow before adjusting the draft control, he was subjecting the metal to systematic torture. Might explain why the family went through several stoves in such a short period of time, eh?

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Q: About your comment that 900° is "deep in overfire country", it has been reported on an online wood heat forum that at least one manufacturer has OK'd that temperature (for their brand, anyway). What say you about that?

- No Name, Please

SweepyA: A mixture of misunderstanding and sloppy reporting, I'd say. It simply defies the imagination that anyone qualified to be a spokesperson for any wood stove manufacturer would OK stoking the stove until it glows (which is what happens at 900 degrees) as a general practice. I happen to be familiar with the post you mention, and the fact is, the manufacturer in question actually states in their manuals that if any part of your stove is glowing, you are overfiring.

Most likely, the real scenario was something like this:

Frantic Stove Owner: "I got distracted and left the room while I was kindling a fresh load with the draft control wide open, and by the time I came to my senses and ran back in, my stovetop thermometer had reached 900 Degrees! Did I destroy my new Reactor 2000 wood stove?"

Nuclear Stove Co. Customer Service Rep: "No need to panic. We build extremely stout stoves here at Nuclear, and one short trip to 900° most likely did your Reactor 2000 no harm."

No-Longer-Frantic Stove Owner's Forum Post: "Hey, I called Nuclear Stove Company, and they said running the Reactor 2000 at 900° is no big deal."

Inevitable Result: An unusually large percentage of forum members subsequently complain of heat-damaged stoves.

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Miscellaneous Comments, heavily condensed from the original E-mails:

Regardless the gear, if you're redlining, taking your foot off the gas (reducing airflow by closing down the damper) always works.
~Dan

The Honest Abe Analogy: You should replace your tires if you can see Lincoln's head when you insert a penny into the treads, and you should replace your door gasket when the latched door won't hold a $5.00 bill!
~William Rodland

Household garbage in a wood stove is like leaded gas in a car: don't burn it.
~Jim Moser

Here's one for your auto-stove comparison page, unless, as I suspect, you write all the letters yourselves:
"Make sure there's nothing blocking your exhaust pipe!"
~Roland the Headless Thompson Gunner

Sweepy Roland, as evidenced by your contribution, the letters we publish are from real visitors to our website.  Now, go tell Warren Zevon you actually exist.

       

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