Is high efficiency always tied to secondary burn?

[John Ackerly]

I've noticed that air/smoke from the air tubes ignites in a secondary burn only 10 - 20% of the time - when my stove is really hot and in overdrive. The rest of the burn cycle it appears that no air is coming out of the air tubes. My question is not whether I'm using my stove properly, but what the efficiency impact is of not having an obvious secondary burn from the air tubes. Without a good secondary burn, is a top-notch stove go back to 40 - 50% efficiency that older stoves had? To get 70% efficiency or more, do you need the secondary burn?

I suspect part of the issue may be that I'm using a stove made in 1994 and air flow through air tubes has improved since then, getting a good secondary burn for more of the burn cycle. Anyone have the same problem?

John

 

[pen]

Nope, don't have to have the crazy secondary burn going all the time. If there is enough primary air coming into the stove, the secondaries wouldn't have much to clean up anyway. The time that you'd really get creosote is when a large bed of hot coals gets a nice load of wood placed on it then the air is shut down too early. This and a general lack of air when closed down too far is how the old stoves get the title "smoke dragon." You are doing just fine so long as you don't have smoke pouring out of your chimney. If the exhaust is pretty much just heat waves, you are doing it right.

pen

 

[Backwoods Savage]

Secondaries or cat, you won't get the light show 100% of the time. Just enjoy it while it is there. Also, at the end of the burn there is nothing left there which would cause creosote. Enjoy the stove John!

 

[remkel]

Personally, I like when there is just a wavey dance of blue secondaries above the load of wood- it is almost hypnotic

 

[webbie]

Wood goes through various phases of burning. The easy way to think of it is in two phases.

The gases are driven of in the initial stages - that is when you usually see a nice flame and perhaps a lot of secondaries.

After a period of time, those gases are all consumed and the wood is mostly carbon - what we call charcoal. This is the phase with glowing logs and coals. At this point in the fire, wood is closer to coal or coke than it is to regular wood. Since there are few gases left at all, there are not as many flames or secondaries. The second phase might actually be more efficient, because not as much heat is going up the chimney.

 

[CTYank]

I've noticed that air/smoke from the air tubes ignites in a secondary burn only 10 - 20% of the time - when my stove is really hot and in overdrive. The rest of the burn cycle it appears that no air is coming out of the air tubes. My question is not whether I'm using my stove properly, but what the efficiency impact is of not having an obvious secondary burn from the air tubes. Without a good secondary burn, is a top-notch stove go back to 40 - 50% efficiency that older stoves had? To get 70% efficiency or more, do you need the secondary burn?

I suspect part of the issue may be that I'm using a stove made in 1994 and air flow through air tubes has improved since then, getting a good secondary burn for more of the burn cycle. Anyone have the same problem?

John

OTOH, in my Squirrel, whenever there's wood that's not fully burnt down to coals, if there's no secondary flame, there will be smoke.
Real simply put, the coals put in most of the energy required to expel distillates from the wood, and those distillates are largely burnt in the secondaries. Hot unburnt distillates are invisible in the firebox.
No counter-example seen in 10+ years. I expect that much has been learned since 1994. I hope so.

Overall efficiency can't be more than combustion efficiency. Unburnt volatiles will reduce efficiency- seems simple enough.

 

[John Ackerly]

Thanks all for insights and info. Helpful but I'm still confused. 1. I assume that secondaries in some stoves work better than others, and you are getting a robust secondary burn from air tubes for longer periods of time. Wouldn't that make that stove more efficient? 2. When my stove is smoking the most, secondaries are not firing off because stove is either not hot enough, and/or air is not getting sucked through the tubes. If a fan could kick in and circulate all that smoke back into my firebox, wouldn't I get more BTUs and higher efficiency out of it? 3. Unless its just high heat that triggers better secondary burns, what is it?

Surely someone out there knows this stuff! But no matter how long I stare at my air tubes, I can't figure it out.

 

[firefighterjake]

Thanks all for insights and info. Helpful but I'm still confused. 1. I assume that secondaries in some stoves work better than others, and you are getting a robust secondary burn from air tubes for longer periods of time. Wouldn't that make that stove more efficient? 2. When my stove is smoking the most, secondaries are not firing off because stove is either not hot enough, and/or air is not getting sucked through the tubes. If a fan could kick in and circulate all that smoke back into my firebox, wouldn't I get more BTUs and higher efficiency out of it? 3. Unless its just high heat that triggers better secondary burns, what is it?

Surely someone out there knows this stuff! But no matter how long I stare at my air tubes, I can't figure it out.

Keep staring at the tubes . . . if you stare long enough and hard enough eventually you will receive your answer . . . either that or see an image of the Virgin Mary.

 

[Rob From Wisconsin]

I've noticed that sometimes I get a "ping" sound just above my
baffle, and I think that can be also counted as a sort of "secondary burn"??

 

[firefighterjake]

I've noticed that sometimes I get a "ping" sound just above my
baffle, and I think that can be also counted as a sort of "secondary burn"??

I would think that just counts as metal warming up or cooling down . . .

 

[pyper]

Thanks all for insights and info. Helpful but I'm still confused. 1. I assume that secondaries in some stoves work better than others

That's for sure. I get secondaries most of the time with my new Jotul Castine. With the downdraft stove in our other house I rarely got them.

The first time I had a real fire in the Castine I went back and forth looking at the stack and adjusting the air control. I found that I could have smoke with secondaries, and no smoke without. So far I haven't been able to figure out how to get things just right without going outside and looking, but we've only had it in a couple weeks.

 

[WidowMaker]

How tall is your stack???

I have much the same problem, but I only have 12 ft of stack and don't think I'm getting a strong enought draft to fully engage the secondaries, therefore needing a small amount of primary air...JMHO, YMMV

 

[John Ackerly]

Interesting. I also have about a 12 foot stack. Never occurred to me that could impact secondary combustion. I also never shut air down completely because I noticed that made my glass get dirty when I did.

 

[oldspark]

Nope, don't have to have the crazy secondary burn going all the time. If there is enough primary air coming into the stove, the secondaries wouldn't have much to clean up anyway. The time that you'd really get creosote is when a large bed of hot coals gets a nice load of wood placed on it then the air is shut down too early. This and a general lack of air when closed down too far is how the old stoves get the title "smoke dragon." You are doing just fine so long as you don't have smoke pouring out of your chimney. If the exhaust is pretty much just heat waves, you are doing it right.

pen

I agree with that but for some reason with some conditions I have blazing secondaries with the primary air wide open, might be the wood is a little on the dry side because it sat in the wood box for several days.

 

[Rob From Wisconsin]

I've noticed that sometimes I get a "ping" sound just above my
baffle, and I think that can be also counted as a sort of "secondary burn"??

I would think that just counts as metal warming up or cooling down . . .

Actually, it occurs every couple of seconds on smaller fires that don't
display secondary combustion flames. It almost seems like a secondary
ignition just prior to entering the flue.

 

[Battenkiller]

If there is enough primary air coming into the stove, the secondaries wouldn't have much to clean up anyway. The time that you'd really get creosote is when a large bed of hot coals gets a nice load of wood placed on it then the air is shut down too early. This and a general lack of air when closed down too far is how the old stoves get the title "smoke dragon." You are doing just fine so long as you don't have smoke pouring out of your chimney. If the exhaust is pretty much just heat waves, you are doing it right.

I couldn't agree more with this post. In fact, you can do it right even when the wood is "not quite up to standards"... :roll:


I took this video against a backdrop of trees so the heat waves would be easily visible. Against a clear blue sky the stove looked like it wasn't even running, but it was cruising along at 750ºF with a roaring secondary burn going on behind the fireback.

I did this run last season just for giggles and to make a point (mostly to myself). It is not a fun way to operate a stove, however. Way too much babysitting. But a good example of the meaninglessness of comparing emissions, secondary combustion, and heater efficiency. Obviously, the stove is burning this sopping wet stuff (close to 60% MCdb) very cleanly, and producing great gobs of heat in the room, but we all know this can't be done efficiently. At 600º on the pipe, a lot of heat was being sent up the chimney. In fact, I could have gotten a lot more efficiency out of the wood by choking the air all the way down "old timer" style, but Lordy, what a mess that would have made in the flue - a case of great thermal efficiency but horrendous combustion efficiency.





BTW please refrain from the need to assume I am advocating burning wet wood by showing this video. I burned plenty of wet wood when I was young and stupid, but now that I am old and stupid I have come to realize that dry burns much better. I just wanted to show that you can get a clean, hot burn and stay very warm using bad wood, but you won't get very good fuel economy doing it.

 

[Danno77]

BK, did you post this before or after my other thread started? check it out if you haven't...

 

[Battenkiller]

BK, did you post this before or after my other thread started? check it out if you haven't...

Ha, ha! I posted this first. See the other thread for my comments there. :cheese:

 

[oldspark]

You two need to get a room. :lol:

 

[BrowningBAR]

If there is enough primary air coming into the stove, the secondaries wouldn't have much to clean up anyway. The time that you'd really get creosote is when a large bed of hot coals gets a nice load of wood placed on it then the air is shut down too early. This and a general lack of air when closed down too far is how the old stoves get the title "smoke dragon." You are doing just fine so long as you don't have smoke pouring out of your chimney. If the exhaust is pretty much just heat waves, you are doing it right.

I couldn't agree more with this post. In fact, you can do it right even when the wood is "not quite up to standards"... :roll:


I took this video against a backdrop of trees so the heat waves would be easily visible. Against a clear blue sky the stove looked like it wasn't even running, but it was cruising along at 750ºF with a roaring secondary burn going on behind the fireback.

I did this run last season just for giggles and to make a point (mostly to myself). It is not a fun way to operate a stove, however. Way too much babysitting. But a good example of the meaninglessness of comparing emissions, secondary combustion, and heater efficiency. Obviously, the stove is burning this sopping wet stuff (close to 60% MCdb) very cleanly, and producing great gobs of heat in the room, but we all know this can't be done efficiently. At 600º on the pipe, a lot of heat was being sent up the chimney. In fact, I could have gotten a lot more efficiency out of the wood by choking the air all the way down "old timer" style, but Lordy, what a mess that would have made in the flue - a case of great thermal efficiency but horrendous combustion efficiency.


BTW please refrain from the need to assume I am advocating burning wet wood by showing this video. I burned plenty of wet wood when I was young and stupid, but now that I am old and stupid I have come to realize that dry burns much better. I just wanted to show that you can get a clean, hot burn and stay very warm using bad wood, but you won't get very good fuel economy doing it.

My results match this as well. I have also done this with EPA stoves (Cat and Non-cat).