Obvious inference? Wood burning science

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Status
Not open for further replies.

joefrompa

Minister of Fire
Sep 7, 2010
810
SE PA
Hi all,

Ok, this is gonna sound really obvious to some of you...

But I've been really learning the stove this year and found I can get the BEST burns by shoving the stove FULL and then shutting the air all the way.

If I fill it 2/3rd, shutting the air all the way will kill it under the same circumstances.

But if it's, say, 450 stove top and I shove it full, I can shut the air down and within 10 minutes it's roaring secondaries, spiking up to 650-700 with the fan on high the whole time, and burning for hours.

So I recently "figured out" the science behind this -

With the stove box full, there's alot less AIR in the box. So I can shut the air input down, and it doesn't need nearly as much oxygen in order to burn really efficiently. Whereas, if I leave another .5 cubic feet of space free in there - it requires more air input (oxygen) in order to burn.

Meanwhile, I'm getting less overall heat output & more wood consumption when I'm only loading in 1/2 to 2/3rds full.

And it's all a matter of airspace available in the stove - less airspace = less air input needed to burn efficiently = greater heat output and slower burns due to less overall air flow.

There's my equation for the night :)

Joe
 
Yeah..but the more flames you have the more oxygen it needs.
But I still concur.
 
Maybe it enables the smoke combustion phase more efficiently?
 
I think you are incorrectly attributing the volume of air rather than the distance from the secondaries to the fuel. The free space affects the dynamics of flow.
 
I think what he is saying is once .5 cf of wood burns up that the burn turns to crap. :-S
 
Chettt said:
So we should all fill our stoves half full with bricks and burn just a split or two at a time?
That makes no sense unless you are talking about biobricks. CMU bricks don't burn.
 
I'm going to guess that it's because the heat being thrown off of the burning material is in closer proximity to other combustibles, therefore the heat exponentially increases at a faster rate. Since we know that flames are relatively inefficient as far as burning goes, then the small amount of air is enough to get the chain reaction going. As far as the air being closed goes, the air will simply come through the intake at a faster rate, although smaller volume, but when the fire gets going enough, it will "automatically" determine how much air it needs. As long as it's closed enough to keep the fire from going out of control. Make sense or am I just babbling here?
 
Here's my science.
I think a larger firebox filled right up on a warm reload is the way to go.
Get her going and eventually you will have all 4 stages of burning going on at once.
While the wood on the bottom is hitting the stages first the rest of the load is above and waiting and going into stage one.
The longer this cycle can last..the better..for sure with a cat on low burn.

Got the presents I bought all wrapped!
Grandson that lives with us is in bed!
He told me I better go to bed too cause I'm getting up real early..lol.
 
Scott2373 said:
I'm going to guess that it's because the heat being thrown off of the burning material is in closer proximity to other combustibles, therefore the heat exponentially increases at a faster rate. Since we know that flames are relatively inefficient as far as burning goes, then the small amount of air is enough to get the chain reaction going. As far as the air being closed goes, the air will simply come through the intake at a faster rate, although smaller volume, but when the fire gets going enough, it will "automatically" determine how much air it needs. As long as it's closed enough to keep the fire from going out of control. Make sense or am I just babbling here?
Also in closer proximity to the baffle above the secondaries. The baffle reflects the heat back to the fuel and distance has a logarithmic heat loss.
 
LOGarithmic..now that seems like a fitting term.
 
I'll throw out some thoughts for consideration:

First: More wood use with a less-than-full load may be some what of an illusion. Consider if you're only loading the stove 1/2 full, then even if you are loading it twice as often, that is still break-even with burning full loads. Maybe the extra leg work only makes it 'seem' like burning more wood?

Second: With each load of wood, (at least for me) the general steps are: load the wood, open draft to char the wood/heat the stove (a low efficiency step), then close down the draft to light the secondaries and cruise in an efficient mode. So figure two small loads of wood, each with their own charring/heating step, vs one large load with it's single char/heat step. If you say the char/heat step consumes 1 pound of wood and you did that with two 10 pound loads, that would be 2 pounds charred and 18 burned efficiently. If you did that with with one 20 pound load, that would be 1 pound char and 19 burned efficiently. So a slight efficiency boost with the big load.

Three: When the entire stove is full of wood, All of that wood can absorb heat and start outgasing which drives the secondaries. Anytime the wood outgases, the light/volatile stuff comes off first, followed by heaver and heavier compounds. Assuming you need a certain volume of gas to light the secondaries, that volume is easier to obtain / will come earlier from 20 pounds of wood vs 10. So again a slight boost in efficiency to the larger load.

Last: Most all of this assumes 'all else being equal' - if a large load of wood cases you to open the windows and run around in shorts, then a slight boost in burning efficiency doesn't mean much!
 
Some folk think if the secondaries aren't firing that the stove is not burning at max efficiency. Not necessarily. The secondaries can provide superheated air to the flames without it looking like the flames are coming from the secondaries. As long as the stove is in the sweet spot, it matters not where the flames appear.
 
cozy heat said:
I'll throw out some thoughts for consideration:

First: More wood use with a less-than-full load may be some what of an illusion. Consider if you're only loading the stove 1/2 full, then even if you are loading it twice as often, that is still break-even with burning full loads. Maybe the extra leg work only makes it 'seem' like burning more wood?

Second: With each load of wood, (at least for me) the general steps are: load the wood, open draft to char the wood/heat the stove (a low efficiency step), then close down the draft to light the secondaries and cruise in an efficient mode. So figure two small loads of wood, each with their own charring/heating step, vs one large load with it's single char/heat step. If you say the char/heat step consumes 1 pound of wood and you did that with two 10 pound loads, that would be 2 pounds charred and 18 burned efficiently. If you did that with with one 20 pound load, that would be 1 pound char and 19 burned efficiently. So a slight efficiency boost with the big load.

Three: When the entire stove is full of wood, All of that wood can absorb heat and start outgasing which drives the secondaries. Anytime the wood outgases, the light/volatile stuff comes off first, followed by heaver and heavier compounds. Assuming you need a certain volume of gas to light the secondaries, that volume is easier to obtain / will come earlier from 20 pounds of wood vs 10. So again a slight boost in efficiency to the larger load.

Last: Most all of this assumes 'all else being equal' - if a large load of wood cases you to open the windows and run around in shorts, then a slight boost in burning efficiency doesn't mean much!

I do think the full load deal pays off more on cat stoves.
 
HotCoals, we have a cat stove and still have not filled it this year. We are burning usually 3 or 4 splits simply because that is all the heat we require right now. We won't fill the stove until winter gets here, if it ever does this year.
 
LLigetfa said:
Some folk think if the secondaries aren't firing that the stove is not burning at max efficiency. Not necessarily. The secondaries can provide superheated air to the flames without it looking like the flames are coming from the secondaries. As long as the stove is in the sweet spot, it matters not where the flames appear.

I see your point and concur this is more-or-less true. It seems to me this 'secondaries without looking like the flames are coming from the secondaries' happens at higher air settings. I associate this with the same way you see 'flame standoff' at the head of a torch. With the wood stove, the air is rushing in so fast, it can create a pocket of fresh air around the secondary tube and may have to travel several inches to combine with wood gas and ignite. When I turn my secondary air down, I can draw the flame back to the pipe. I don't have scientific proof, but it 'seems' that the highest efficiency is had when the stove top temp is maximized while the air is minimized
 
I've learned that every time the stove is stuffed the air needs to be pinched off as soon as possible otherwise it can over fire (725º)

I also found that I have considerable more draft when it's alot colder outside. Therefore if I stuff the stove when it's real cold outside then stove top temps reach 700º-730º and I have no control over that happening. The stove just has to peak and return back to earth.
 
Backwoods Savage said:
HotCoals, we have a cat stove and still have not filled it this year. We are burning usually 3 or 4 splits simply because that is all the heat we require right now. We won't fill the stove until winter gets here, if it ever does this year.

Whatever works for you.
My stove room is fairly large(27x16) and I have good air flow throughout the two storys.
So even if the temps avg 40-45f for a 24 hour period I can load her up and not be heated out of the room.
That said there has been a few times I have opened the slider in that room.
Maybe I'm wrong but I have tried all kinds of things..but I keep coming back to the full long burns in low burn mode..I love it!
It seems I use less wood and less fuss!
When it was colder less then a full load on a 12 hour cycle works nice for me also. Cheers!
 
Benchwrench said:
I've learned that every time the stove is stuffed the air needs to be pinched off as soon as possible otherwise it can over fire (725º)

I also found that I have considerable more draft when it's alot colder outside. Therefore if I stuff the stove when it's real cold outside then stove top temps reach 700º-730º and I have no control over that happening. The stove just has to peak and return back to earth.

I don't think 725f is an over fire. MY PE runs at that and cruises.

I've had that puppy at 800F ( yeah, I was planning an escape route), but it backed off real quick.
 
LLigetfa said:
Some folk think if the secondaries aren't firing that the stove is not burning at max efficiency. Not necessarily. The secondaries can provide superheated air to the flames without it looking like the flames are coming from the secondaries. As long as the stove is in the sweet spot, it matters not where the flames appear.

Correct, with the indicator being no smoke out of the chimney.
 
HotCoals said:
Backwoods Savage said:
HotCoals, we have a cat stove and still have not filled it this year. We are burning usually 3 or 4 splits simply because that is all the heat we require right now. We won't fill the stove until winter gets here, if it ever does this year.

Whatever works for you.
My stove room is fairly large(27x16) and I have good air flow throughout the two storys.
So even if the temps avg 40-45f for a 24 hour period I can load her up and not be heated out of the room.
That said there has been a few times I have opened the slider in that room.
Maybe I'm wrong but I have tried all kinds of things..but I keep coming back to the full long burns in low burn mode..I love it!
It seems I use less wood and less fuss!
When it was colder less then a full load on a 12 hour cycle works nice for me also. Cheers!

HotCoals, I do think there is a difference in the way different stoves need to be operated. Glad you found what works for you.
 
another angle to ponder;

wiht a full load in the stove there is less available air space , but remember flue is pulling "X" air at a certain temp. filling up the firebox takes up a lot of space so more air is pulled through secondaries which do not get regulated in most cases the same way as the primaries, thus more air available for secondaries giving a more violent look due to the higher velocity and volume in the secondary air path.

another thing is that even without the secondaries, the primary air has a tighter path with the flue still pulling "X" through a smaller space so velocity (aka"turbulence") is higher
 
Another Angle is that fill it up to the top close to the secondaries makes for a small space to heat and main that heat so the hotter area burns better.

I do like the idea that it pulls more air thru the secondaries. Excellent thought.
 
Status
Not open for further replies.