Clean burn with no secondaries?

[partybob99]

First I have to say I am not burning ideally seasoned wood.. Im burning exclusively white ash. What I've found is many times I've started a burn with slightly darkened glass on my napoleon 1402 and after a few hours I have completely clean class and with light-tan fire bricks. During this time I often see NO secondary burning. Am I burning cleanly or am I building up cresote because I dont have a strong secondary burn?

When I burn the "wetter" logs (about 60% of the time), I've found I have to keep the damper nearly fully open to keep active flames going, but I still have clean glass with the light-tan fire bricks.

What do you think? Is clean glass/light firebrick always a good indicator of an efficient burn?

 

[NH_Wood]

Yes, clean glass usually indicates a fairly clean burn. How about the chimney? Do you see smoke exiting the chimney for the first 2-3 hours of the burn (before the load coals)? This is one of the best signs of efficient burning - no smoke = secondary combustion efficiency. You are at least lucky that your poor wood is ash - one of the best woods to burn if no well seasoned wood is available. Be sure to burn hot, if you need to keep your primary air relatively open to do so, do it. Keep a close eye on your flue - check for creosote and clean if necessary. Burning less that optimal wood is no fun. If you can, try getting your wood ordering for next year now. Stay away from oak for next year. The trick to burning well seasoned wood is to get well ahead on your wood supply. Shoot for 2-3 years ahead if you can. Cheers!

 

[Backwoods Savage]

Keep in mind that if you have to keep the draft nearly fully open that most of your heat is just going up the chimney rather than staying in the stove to warm your house. White ash, even if not fully seasoned should burn without a full open draft. The key to burning it is to put the wood in on reloads a bit sooner than you normally would. You want a BIG coal bed for burning not properly seasoned wood. Of course that brings on its own problem with buildup of coals. There are no 2 ways about it, poor fuel = poor results.

 

[Danno77]

I dunno. I find that with bad wood my glass gets dirty ONLY when the air is closed down. If your stove has an air wash system, then the hottest and cleanest burning place on your stove is right where that air is coming in. When you are allowing a LOT of air to come in it (the glass) stays pretty clean.

I guess the real question is how HOT are you getting the thing? stove top temps might help those more familiar with secondary combustion figure this out.

 

[precaud]

Many folks think that, unless they see distinct "tongues of flame" streaming from the secondary holes, the secondary air must not be working. This also leads many to believe that it is recycled gasses coming out of the burn tubes. Neither of these are true.

First, there is nothing but preheated air coming out of the burn tubes. When the stove is filled with unburned gasses released from the wood, the air shoots out of the tubes and into this pool of gasses. If it is hot enough, they ignite on contact.

Sometime in the second half of burning a load, the wood is still outgassing but not nearly to the extent it was before. The firebox temperature is higher than it was in the first half - high enough to, as you note, burn off the stuff that collected on the surface of the bricks and glass. There is still the same amount of secondary air coming out of the tubes. But the stove is not filled with unburned gasses. So most of the secondary air passes unused out the chimney.

It's one of the inefficiencies we have to live with to get clean burning in the earlier phase of the burn.

 

[savageactor7]

What do you think? Is clean glass/light firebrick always a good indicator of an efficient burn?

Yup imo it is.

We burn seasoned wood WOT 95% of the time and I can't ever recall any secondary burn flames jumping like I see posted here. But whenever I go outside I eyeball the chimney and am comforted by the lack of smoke so I know that the secondaries are doing the job of re-burning smoke.

imo almost clean glass is pretty much the visual benchmark of a efficient burn. Still a good idea to mirror the chimney when the situation permits to confirm...esp so if you know you burning unseasoned wood.

Pretty much all wood burners have been in a likewise position...hey if we're doing risky things or not quite right things a wise man like yourself can still take precautions or implement control measures to make those risky moves at least incrementally safer.

In your case clean glass reps the hotter or efficient burn that will work best...sorry to ramble.

 

[LLigetfa]

We burn seasoned wood WOT 95% of the time...

Of course this would vary by stove, draft, wood, etc. My stove definately has a sweet spot WRT to how much to open it up. Anything beyond about half throttle just wastes fuel and gets me no more heat.

The colour of the firebrick and the cleanliness of the glass are good indicators of more complete combustion. As for keeping the flue clean, if in doubt when burning wood before its time, sending more heat up the flue is better to keep the extra moisture from condensing.

 

[joshlaugh]

I watch my stack for smoke and always ensure a clean viewing glass to make sure I am getting a good clean burn. I burn the same species of wood in both my Heritage and Jotul but only see the secondary burn tubes working on the bigger Heritage. I am confident that both are clean burning though and I know that some folks here get great viewing secondary burn but I am not always one of them. I have not had any real creosote problem due to burning good seasoned wood and keeping the fires hot. Good luck

 

[Danno77]

I have a cheap nonEPA stove, so maybe that's the difference. I can't imagine running that wide open and not having problems. at least that's when I have good seasoned wood.

 

[HotCoals]

Heck..you could make a old smoke dragon burn fairly clean at times with lots of flame going.
I did it for years with the old non-cat BK.
Burning long without smoke is where the cat comes in.
I'm sorry..but I don't see secondary tube burners doing that as well...maybe that's just me.

 

[Danno77]

BTW, it's my understanding that the stove won't "pull" air out of the secondaries (fully, at least) unless the primary is closed down a little. it's not like the secondary tubes have forced air.

 

[precaud]

BTW, it's my understanding that the stove won't "pull" air out of the secondaries (fully, at least) unless the primary is closed down a little. it's not like the secondary tubes have forced air.

Actually, every EPA stove I've examined (except perhaps the Jotul F602) has more 2ndary than primary air under all conditions. There are many reasons for this, it's not worth going into them all here. Oh, and they do have forced air, to an extent... that's one of the benefits of preheating.

 

[boatboy63]

BTW, it's my understanding that the stove won't "pull" air out of the secondaries (fully, at least) unless the primary is closed down a little. it's not like the secondary tubes have forced air.

Bingo. Think of it this way...most stoves have a 6" chimney, exhaust heat from the fire is lighter than room air so it wants to go up the chimney. As it starts moving up the chimney, it places a vacuum on the stove to allow more air to go. Since air will take the path of least resistance, if the primary air supply is open all the way, it will pull from there. When primary air supply is reduced, the chimney will "draw" or "draft" from the next area of least resistance. Since the secondary holes are open, air will be pulled from these holes and into the stove where they mix with any smoke that is hot enough to ignite. It takes around 1100* for this smoke to ignite and burn.

 

[precaud]

Bingo. Think of it this way...most stoves have a 6" chimney, exhaust heat from the fire is lighter than room air so it wants to go up the chimney. As it starts moving up the chimney, it places a vacuum on the stove to allow more air to go. Since air will take the path of least resistance, if the primary air supply is open all the way, it will pull from there. When primary air supply is reduced, the chimney will "draw" or "draft" from the next area of least resistance. Since the secondary holes are open, air will be pulled from these holes and into the stove where they mix with any smoke that is hot enough to ignite. It takes around 1100* for this smoke to ignite and burn.

Actually, the primary air source isn't as dominant as you say it is. Yes, flow resistance is a factor. But other things being equal, the draft will pull more from sources (openings) closest to it. Since most secondary air systems have holes closer to the chimney (the source of the draft) than the airwash, there will be plenty of air pulled through the secondary at all times.

In most EPA stoves, the primary air opening is smaller than the secondary, in some cases by quite a bit.

 

[Danno77]

Bingo. Think of it this way...most stoves have a 6" chimney, exhaust heat from the fire is lighter than room air so it wants to go up the chimney. As it starts moving up the chimney, it places a vacuum on the stove to allow more air to go. Since air will take the path of least resistance, if the primary air supply is open all the way, it will pull from there. When primary air supply is reduced, the chimney will "draw" or "draft" from the next area of least resistance. Since the secondary holes are open, air will be pulled from these holes and into the stove where they mix with any smoke that is hot enough to ignite. It takes around 1100* for this smoke to ignite and burn.

Actually, the primary air source isn't as dominant as you say it is. Yes, flow resistance is a factor. But other things being equal, the draft will pull more from sources (openings) closest to it. Since most secondary air systems have holes closer to the chimney (the source of the draft) than the airwash, there will be plenty of air pulled through the secondary at all times.

In most EPA stoves, the primary air opening is smaller than the secondary, in some cases by quite a bit.

So, you are saying that if I did an experiment and closed the primary completely, that the stove would get more air through the secondary than if I plugged the secondary completely and let it draw from the primary?

 

[precaud]

So, you are saying that if I did an experiment and closed the primary completely, that the stove would get more air through the secondary than if I plugged the secondary completely and let it draw from the primary?

Absolutely. And the primary air control lets you do exactly that. The secondary air is large and always on; the primary air is smaller and controllable.

For example, the secondary air inlets on my Quad are twice the area of the primary inlet.

 

[LLigetfa]

BTW, it's my understanding that the stove won't "pull" air out of the secondaries (fully, at least) unless the primary is closed down a little. it's not like the secondary tubes have forced air.

Actually, every EPA stove I've examined (except perhaps the Jotul F602) has more 2ndary than primary air under all conditions. There are many reasons for this, it's not worth going into them all here. Oh, and they do have forced air, to an extent... that's one of the benefits of preheating.

The amount of air "pulled" from the secondaries will vary by how much the flue "draws". On my stove the secondaries and the primary are controlled as one but the secondary is limited by the size and number of holes in the burn tubes. The primary is whatever amount of down wash air overcomes upward convection and the air wash system is a larger opening than all the secondary burn tube holes combined.

If air takes the path of least resistance which I'm sure it does, there is more potential primary air than secondary air but of course the primary air must overcome upward forces. Closing down the air would result in less downward force on the air wash and the air would turn around sooner to become secondary air rather than primary air. The zipper air helps to direct the convection forces in a circular motion to draw more of the air wash in as primary air. It is easy to see these circular convection forces when a Birch log with bark is placed on a hot bed of coals creating smoke.

 

[precaud]

LLigetfa, if that's true, your stove is not representative of the norm. Most (99.8%) EPA stoves have totally separate secondaries and primary air systems. The secondaries are larger area and fixed open, and the primaries are variable. So the primary dances around the secondary, not the other way around. And you can't ignore the proximity factor. A constant pressure will favor a nearer source. Doesn't matter whether it's blowing or sucking.

I see this almost daily at the health club jacuzzi. The air compressor that makes the bubbles feeds into a 5-foot tube from one end, with holes along the tube. Guess which end has the most and strongest bubbles, by far? The one closest to the end where the pressure source is.

You can model this electrically by a power source feeding a series string of resistors, grounded at the opposite end. The power available at each resistor node gets smaller as you move away from the source.

 

[Danno77]

LLigetfa, if that's true, your stove is not representative of the norm. Most (99.8%) EPA stoves have totally separate secondaries and primary air systems. The secondaries are larger area and fixed open, and the primaries are variable. So the primary dances around the secondary, not the other way around. And you can't ignore the proximity factor. A constant pressure will favor a nearer source. Doesn't matter whether it's blowing or sucking.

I see this almost daily at the health club jacuzzi. The air compressor that makes the bubbles feeds into a 5-foot tube from one end, with holes along the tube. Guess which end has the most and strongest bubbles, by far? The one closest to the end where the pressure source is.

You can model this electrically by a power source feeding a series string of resistors, grounded at the opposite end. The power available at each resistor node gets smaller as you move away from the source.

that doesn't make sense, because the secondaries may empty out into the stream, but the source of the air isn't any closer in proximity than the primary air inlet.

 

[HotCoals]

Just get a cat stove..problems solved...lol.

 

[precaud]

that doesn't make sense, because the secondaries may empty out into the stream, but the source of the air isn't any closer in proximity than the primary air inlet.

It is possible to find an exception for every norm. True, most primary air paths are shorter (hence less resistance) than the secondary air paths. (Known exception: Waterford 100B "Leprechaun")

It's not that secondaries MAY enter into the stream; they WILL. They are always the same size, and always on, and a good percentage of their outlet opening is closer to the pressure source than the primary outlet is. As I've demonstrated in threads about burn time, you can close your primary air at the end of a burn and yet the coals will still burn down. Why? Because the secondary air is still open.

 

[LLigetfa]

that doesn't make sense, because the secondaries may empty out into the stream, but the source of the air isn't any closer in proximity than the primary air inlet.

It could well be that my stove is not typical. There is but one air control that dumps into the doublewall cavity.

The 3 secondary tubes penetrate into this doublewall space and the lesser of the diameter of the tubes or the size and number of holes govern how much of that air they take. The air wash system has a larger opening than the 6 holes the tubes add up to.

What you need to realize is that on a low setting, less of the air wash reaches the base of the coals and so instead of being as much primary air, more of it turns upward and becomes secondary air. I have watched how the secondary burn tubes behave under a variety of conditions and they don't vary nearly as much as does the primary air from the air wash system.

I've attached a line drawing from the manual. 15, 16, & 18 are the secondary burn tubes and 13 is a perforated metal diffuser for the air wash. The flue exit is top left.

 

[precaud]

I've attached a line drawing from the manual. 15, 16, & 18 are the secondary burn tubes and 13 is a perforated metal diffuser for the air wash. The flue exit is top left.

And where is the common plenum that feeds the pri and sec? It's not shown in the drawing.

 

[BrotherBart]

...but the source of the air isn't any closer in proximity than the primary air inlet.

Not true on most EPA stoves. The primary air enters near the bottom rear of the stove, travels to the bottom front and then up to the top to feed the airwash from where it travels down the glass and then back into the fire. The secondary air only has to travel up the manifold in the rear of the stove and the around the sides to the air tubes. Less than a third of the distance the primary air has to travel in my stove.

On your exempt stove primary just comes in at the front and down the glass into the fire.

 

[precaud]

On your exempt stove primary just comes in at the front and down the glass into the fire.

Many compliant stoves are that way, too. Your F100, for one...