At what point in the burn cycle does wood stop making cresote?

So, Ill fill my stove and let it get nice and warm. Say 475 on my outside pipe temp. At that temp, I go ahead and start closing it off. On my stove that means closing the ash door but I do leave the over and underfire drafts wide open. That 475 temp will settle to around 400. Once all of the wood is involved (usually another 10 min) ill close the underfire draft 95% and the overfire draft 50% and then it settles into a nice fire at about 350-375 pipe temp.

During the next 2 hours, it will stay in the 300's but the 3rd hour it will dip into the 2's and then hour 4 the 1's and usually by the 5th hour its down to coals.

My question is, am I making creosote still in that 3rd hour at 250*? The wood is well burned and heading toward ashes. To me its Impossible to keep it up high for that long. After a cold snap and a 2 week burn, my stove in the bottom rear, under the fire bricks in the corners has some wet creosote. Not a lot, just its there. I thought I was burning hot enough for sure and my wood is checking under 20* on the mm, most times, under 15%.

Thanks for the help.

 

The gasses given off by the wood during the combustion process will do 1 of 3 things. Burn, coat your appliance/chimney with creosote or exit the top of the chimney unburned as smoke. High moisture content makes it difficult to have all of those gasses end up in the first category and puts a lot in the second and third.. With dry wood, the determining factors of what category the gasses end up in are presence of oxygen and the temperature at the time the gasses and oxygen mix... if above combustion temp, they will burn. If below, they go up the chimney and either provide creosote or exit the top. IMHO, you aren’t getting enough oxygen to the fire, Try running a bit hotter temps and with air open a bit more... (I was just doing the same thing recently) while you will sacrifice burn times, you will extract more heat from the wood in the stove. Many factors at play and you’ll need to experiment to find the right combination of air flow, fuel type/amount/size, outdoor conditions, etc, etc.... good luck.

 

Thanks a lot, more air it is.

 

just a note: checking moisture with a meter needs to be done on a fresh face of a split at Room temperature. ( wood needs to be a room temperature all the way through).

 

yep, all from fresh splits in my basement. I'm into a year old load of Cherry and its pretty dry. 8-18% or so.

 

Cherry bark holds a lot of moisture right underneath it. If it is dry wood don't be fooled by the pops and hisses.

 

Awesome thanks guys. She’s at 400 now. Trying to keep it there is another issue. Still learning!

 

They way you are burning, and if you have good dry wood, I would say after the first two hours most of the creosote causing compounds are burnt off...definitely after 3 hrs.
If you cut the air way back (not off) on a firebox that is full of a pile of red hot coals, no creosote (that's been my experience anyways)

 

Assuming you are measuring the outside smoke pipe temps., those numbers look high enough to me.

The wood will stop making creosote once it is all fully charred and has stopped producing any yellow flame at all. During the coals burn- down part of the burn, when any flame is blue, there is no longer any creosote being produced.

The firebox temp. required will be dependent on what type of stove you have, an older 'smoke dragon' (basically a big container with a damper to let air in), a modern tube- based secondary stove or catalytic type.

As several others have mentioned, burning dry wood is the first and easiest step toward burning clean. The second one is to either burn hot enough to force the creosote up and out of the chimney while still hot enough so the creosote does not condense, or run it through a cat. (catalytic combustor) to consume the creosote while still in the stove.

The easiest way to tell if you are burning clean is to look at the exhaust from the chimney: clear exhaust means you are burning clean. Pure white exhaust that dissipates pretty close to the chimney and disappears entirely also means you are burning clean as that is just steam (pure water). Darker smoke, and any smoke that trails off into the distance means you are producing solids in the stove's exhaust, and at that point you need to keep the entire stove top (inside) and chimney hot enough to prevent condensation, at least 250F at the chimney exit; this can be hard- to- impossible if using a masonry chimney.

Brian

 

That seems to be correct in my experience as well but only if you use the tester's probes (Easy Boys!) alone. I find I can test any split, even through the bark, using a pair of nails driven into the wood about 3/4" apart. Some time ago I used a small piece of belting material, drove two roofing nails through it and keep that 'set up' with my moisture meter; when I want to test a piece of wood, I just drive the nails into the wood using the strap to set the spacing and then touch the probe's tips to the nail heads. I find the readings correlate very well to splitting a piece of wood and driving the probes in as far as I can with the meter, which is not very far at all. If the nails are not driven in all the way, it is also very easy to pull them using either a cat's paw or a claw hammer.

It is not that either way is better or more accurate IME, just that driving nails into a split is usually a lot easier than splitting it and several different pieces of wood can be tested with nothing more than the nails and a claw hammer (and the moisture meter, of course ).

Brian

 

the volatiles and moisture that support the formation of creosote are pretty much gone by the charcoal phase of a burn.
the last phase of a burn is mostly carbon and usually no flame and is primarily carbon.
it's unfortunately a cooler phase of a burn but a necessary one.

Because my stove is a little undersized for the coldest days of Winter I'll have a pine fire on top of oak coals that seem to take forever to finish burning. Trying to keep the stove up to a higher temp or a phase one and two burn going with oak I end up building up coals. So I have a couple of pine fires when it is convenient for me to tend the fire more often, get the coal bed burned down so I can go back to burning oak again, not giving the coals enough time to burn down. It's just what I have to do short cycling a undersized stove. Usually we don't have the kind of weather where I wish we had a bigger stove.

 

You don't mention your CHIMNEY.

It's ll about the temperature of flue gasses within the flue.
2 things must be present to form creosote. Smoke particles and condensing water vapor.

1). The more smoke particles that are burned by installing a baffle, correct burning procedures or an efficient stove design the less smoke exiting the firebox.

2). The hotter the inside of the flue (above 250* to the top is required) the less creosote forms.

Pipe or flue larger than the stove outlet allows expansion and cooling of gasses. Uninsulated liners, and exterior masonry chimneys take much more heat to keep above the 250* condensing point.

It is the chimney flue temperature that allows smoke particles to stick. So a hot stove dumping into a large 10 X 10 flue is going to expand and be below the condensing point. If there is smoke, particles stick forming creosote. Internal firebox temps reduce smoke particles (firebrick lined stoves reflect heat into firebox) but keeping it hot enough to burn exceptionally clean is not as efficient as insulating your chimney flue. Same as the same hot stove using black exposed pipe outdoors calling it a chimney. Flue gas cools, condenses and forms creosote rapidly. Make your chimney more efficient and you will decrease creosote formation.

Moisture content in fuel adds to the water vapor, but most is from combustion of any fossil fuel. (burning hydrogen is the reason so hydrogen content in fuel is directly related to water vapor formed) High moisture content creates steam, cooling the flue gasses as well as adding water vapor that can condense at the lower temperature.
For example; Propane when burned produces about a gallon of water vapor for every gallon burned. Notice how much water vapor is visible from your vehicle exhaust before the exhaust system is hot enough to support the moisture in the air. (your exhaust system is wet inside just like your chimney) Or a turbine engine going through cold air that doesn't support the water vapor emitted from burning kerosene. There is no moisture content in those fuels, just water vapor from combustion. So even oven dry wood produces a LOT of water vapor. I'll clip and paste an article I found on water vapor created by combustion when I find it. (OK, I wrote it......) but it gives you an idea just how much water vapor must exit the chimney before condensing.

 

Information condensed from Woodburners Encyclopedia;

Water vapor is a by product of combustion when hydrogen is present. Oven dry wood contains 6% hydrogen. One pound oven dry wood produces .54 pounds of water vapor. Wood with moisture content of 25% contains another 1/4 pound of water vapor. When entire vent system is above condensing point of 250* flue gasses rarely condense.
**The formula for calculating water vapor formed burning any fuel is 9 being the ratio of the molecular weight of water to hydrogen. Every pound of hydrogen becomes 9 pounds of water. So a fuel containing approx. 6% hydrogen such as dry wood produces .54 pounds water for every pound burned.**

No matter how you burn it, 1/2 pound of water for every pound of oven dry wood you put in your stove must make it out of your chimney without condensing on the flue walls allowing smoke particles to stick. That is without moisture content figured in!

 

Awesome thanks and thanks to everyone. I welcome all the info I can get. As far as my CHIMNEY, its 20 feet masonry with 6x6 clay tiles. It also has a witches bend in it. Ive ran the gopro down It several times to make sure its ok. Its 40 years old but not cracked except for the top tile and I think the cold weather caused that maybe with some water. There are points here and there where the cement in between the tiles have fallin out but its fairly minimal and it also has the chimney blocks around the tiles. It drafts pretty hard. I was gonna take out the 90* elbow and put in 2 45's but I think it will be ok, as it has a lot of draft.

 

You know, the more I learn about chimneys, the more I realize I didn't know.
I think most people have no clue, just think its a place for the smoke to go, and hot air rises, so thats how it works. And most also have no clue that the chimney actually makes the stove work and not vice versa.
I remember you posting about a good book(s?) on chimneys (design, function) somewhere here in the past Coaly ...I'd like to look that up, do you remember what that might have been?

 

id like to see It as well. Thanks!

 

Read some of this last night. Interesting...

Creosote - Wikipedia