secondary burn

[fishinAK]

Ok so I thought that I understood the idea of secondary burning. However, after reading one of the recent posts I think I had the idea wrong. Could someone please explain the idea of a secondary burn chamber, how it works, what it does, etc. Thanks so much!
oh ya I have a Jotul F3CB.

 

[wahoowad]

Same stove, same question.

https://www.hearth.com/econtent/index.php/forums/viewthread/1048/

 

[biggins08]

I guess my stove does it a little different....
No other non-catalytic wood burning insert gives you extra long burn times and steady even heat like the Harman Exception Insert. The Exception’s non-catalytic design incorporates an afterburner which mixes air and wood gases at very high temperatures so that more complete combustion can occur. This design is so clean-burning that it meets tough EPA standards without the need for a catalyst, but more importantly it delivers very long even burning fires. Other non-catalytic inserts have air tubes above the fire which shortens the burn time and causes heat spikes early in the burn cycle. The Harman Exception Insert is more than an outstanding source of heat, it is also an attractive addition to your home. Various decorative options including choice of color, decorative tiles and hood let you choose a look that is right for your home

 

[BrotherBart]

The Exception is different. I was out looking at one last week. It has an "afterburner" chamber behind the fire box where the secondary burn occurs. Same principle but just not burning at the top of the fire box but burns in the chamber.

Kind of a slick setup.

 

[wg_bent]

Very slick. But the basic principle of what's happening is the same as stoves with burn tubes. The smoke that is being released from the burning wood (primary combustion) is mixed with very hot air which is ignited by the primary burn. This way the wood is burned completely. A CAT works a bit different.

My point here is that I don't see why burn tubes would result in a shorter burn time when compared to a CAT or an after burner technology like Harman and VC use.

 

[MountainStoveGuy]

Very slick. But the basic principle of what's happening is the same as stoves with burn tubes. The smoke that is being released from the burning wood (primary combustion) is mixed with very hot air which is ignited by the primary burn. This way the wood is burned completely. A CAT works a bit different.

My point here is that I don't see why burn tubes would result in a shorter burn time when compared to a CAT or an after burner technology like Harman and VC use.

Because you cant control the air to the secondary burn system, If you have a strong drafting chimney then you will pull more air through the seconary burn chamber. you can contol the air flow much more on a cat.

 

[Roospike]

Ya"ll must be talking secondary "burn tubes" Because the P.E. Summit is right up there in Long burn times ( 8-12 hours ) So if your talking secondary burn chambers or non-cats in general then that whole way of thinking is out the window.

 

[Roospike]

Very slick. But the basic principle of what's happening is the same as stoves with burn tubes. The smoke that is being released from the burning wood (primary combustion) is mixed with very hot air which is ignited by the primary burn. This way the wood is burned completely. A CAT works a bit different.

My point here is that I don't see why burn tubes would result in a shorter burn time when compared to a CAT or an after burner technology like Harman and VC use.

Because you cant control the air to the secondary burn system, If you have a strong drafting chimney then you will pull more air through the secondary burn chamber. you can contol the air flow much more on a cat.

The design would go like this for the non-cat stoves. You have so much inlet air on an inlet of a stove per the size of the stove. Now when you add the secondary burn chamber air inlet the main air in let is sized down. They dont just drill a bunch of big air holes in a stove and try to sell it .......... its all design and the main air inlet and the secondary air inlet are sized to each other and the size is per the firebox sf. I'm sure theres a lot more to it per the mathematics of how a stove burns and how they come up with XX air inlet size. The ole big holes in the front of a stove and a screw air inlet are a design of the past. There not just a box with adjustable holes per how ever you feel to set it at.

 

[Dunadan]

Sorry I came to this thread late. I was doing a search on "secondary burn" and came across this. I'll respond here since the link referenced above to the thread wahoowad started is from the beginning of the year.

I had my insert installed on Friday, and just stopped in the stove shop to talk to them about my experiences so far. Wanted to see if they thought I was doing things right. It turns out, from what they said, I didn't fully understand the secondary burn, and might have been doing things wrong.

My insert has a bypass (to the flue) on it and an air control. I had thought that to get secondary burn I needed to 1) have the air control wide open so air was blowing into the firebox, and 2) the bypass closed.

What I was told today (which jives with something said in the thread linked to above) is that actually the air control doesn't control how much air is entering the system, but where the air goes. To get a secondary burn, the control has to part or all the way closed so that the air entering the stove is re-directed into the burn tubes (I think) thus initiating the secondary burn. By keeping the air control all the way open, I was just blowing the air into the stove and thus not getting secondary burn.

I was told the bypass does not affect secondary burn at all. I thought if it was open, all the smoke and gasses would exit right up the flue, and therefore there would be nothing to burn in the secondary burn. Apparently this is not the case.

Also, the secondary burn does not ocurr just up in the baffles near the burn tube holes, but will occur everywhere in the firebox.

I'm going to give this a try as soon as the weather cools down again and see how it works.

 

[fishinAK]

So if it occours everywhere in the fire box. Then what difference does it make where the air enters. Oh man I feel more confused now than I did when I started this thread weeks ago.
So maybe I dont need to be so obsessed with trying to get the flame high enought to reach the burn tubes.?

 

[Dunadan]

From what the dealer said, secondary combustion (SC) does not occur just at the burn tubes. At least for my Lopi, the burn tubes shoot oxygen into the firebox which create the proper "ingredients" for combustion. Possible the reason it matters where the air enters is if it enters from the the bottom front of the firebox (which I believe is where mine enters) it is immediately consumed by the fire? No chance to mix with the gases?

I thought you needed these roaring fires with flames billowing through the baffles for SC to occur, but they said that's not the case.

I do agree though, it is confusing.

 

[Roospike]

Sorry I came to this thread late. I was doing a search on "secondary burn" and came across this. I'll respond here since the link referenced above to the thread wahoowad started is from the beginning of the year.

I had my insert installed on Friday, and just stopped in the stove shop to talk to them about my experiences so far. Wanted to see if they thought I was doing things right. It turns out, from what they said, I didn't fully understand the secondary burn, and might have been doing things wrong.

My insert has a bypass (to the flue) on it and an air control. I had thought that to get secondary burn I needed to 1) have the air control wide open so air was blowing into the firebox, and 2) the bypass closed.

What I was told today (which jives with something said in the thread linked to above) is that actually the air control doesn't control how much air is entering the system, but where the air goes. To get a secondary burn, the control has to part or all the way closed so that the air entering the stove is re-directed into the burn tubes (I think) thus initiating the secondary burn. By keeping the air control all the way open, I was just blowing the air into the stove and thus not getting secondary burn.

I was told the bypass does not affect secondary burn at all. I thought if it was open, all the smoke and gasses would exit right up the flue, and therefore there would be nothing to burn in the secondary burn. Apparently this is not the case.

Also, the secondary burn does not ocurr just up in the baffles near the burn tube holes, but will occur everywhere in the firebox.

I'm going to give this a try as soon as the weather cools down again and see how it works.

How to explain ..........

#1 The front air inlet / window air is warm heated air and is used for air combustion .

#2 The secondary air is separate and is added per burn tubes and burn baffle and is super heated air by the fire and flames / hot coal .... an when added to unburned gasses it makes combustion with the smoke.

#3 secondary combustion is at the top of the fire box (a) because thats where the unburned gasses are where the super heated air is introduced . Secondary combustion can happen anywhere from above the logs, to the top of the fire box , around the tubes and baffles and to the pipe.

 

[DonCT]

I will see if I can take a short high quality video of the secondary burn on my Heritage. Sometimes it's only visible from the burn tubes, other times it fills the entire fire box. It's quite mesmerizing :coolsmile:

 

[Roospike]

I need to take a picture one night after i have set the stove to an all night burn ........
At an all night burn ( or long burn time ) after the stove has been reloaded and all the big massive flames have died down ........
The logs are burning at a 80% blue flame with some yellow and a touch of orange and every so often ( 3-8 seconds ) what i call a "ghost of flames" are set off in the middle of the fire box and float for a few seconds and then go away and again 3-8 seconds later it happens again. The ghost of flames are the secondary burn gasses .

 

[Roospike]

I will see if I can take a short high quality video of the secondary burn on my Heritage. Sometimes it's only visible from the burn tubes, other times it fills the entire fire box. It's quite mesmerizing :coolsmile:

Ha, you posted as i was posting . Yep ....... Thats the "ghost of flames" i was talking about. Its just awesome.

 

[fishinAK]

Ok I think it is begining to make sense. thanks. But can SC be invisible. What about when I have a really good hot bed of coals going but very little flame. I know its hotter than heck even though there is very little flame and no smoke. is there still SC?
thanks guys

 

[laynes69]

Im my wood furnace I get secondary combustion at the baffle mostly. I have air that comes under the fire, then I have air that comes in behind the fire near the top. The flames heat the baffle, the rear air goes along the baffle, gets heated, and the flames from below ignite the remaining gasses. I have learned when this occurs, where if I dont have it hot enough, it will just smoke. So at nighttime I burn it hot, then shut it down a little. If I peek into the damper in the door, I have orange and blue flames dancing right under the flame baffle. Keep up the temp and this will occur all night, and produce a clean burn. Also I only use seasoned hardwoods, which is the key. Wet wood makes horrible fires. Im sure in an EPA burner, its alot easier for a cleaner burn, but when I burn mine I can get no smoke from the chimney at all. Just a little more thinking and the right air settings to get it to happen.

 

[fishinAK]

oops posted at same time fellas. thanks. ya I get the "ghost flames"!

 

[wahoowad]

fishinAK,

I have your stove. You have 2 air control levers on the front: The PRIMARY air control lever at the top and the START UP air control lever below the glass. The Jotul documentation says the next part best:

When first starting a fire: The primary control lever should
be at the far right position, and the start-up air lever
should be in the far right position. This will allow for the
maximum amount of air to enter the stove and assist in
starting the fire quickly and most effectively.
Once the fire is established the start-up air lever should
be closed and the primary air lever should be adjusted
according to the amount of heat desired.

You also have a third air entry port (that you can not control) hidden behind the rear heatshield. This port is where the air comes from that will flow through your reburn baffle and out those little holes. The more you close the PRIMARY air control lever the more air the stove will suck in through this rear port and this is when you will start to see the reburn.

You might wonder why not just let the PRIMARY air flow in and cause this reburn? I think the PRIMARY air might be too cool? Anyway, reducing combustion air via the PRIMARY lever will allow more reburn via air coming in this rear port. This rear port is part of the EPA air control stuff and ensures you don't let your stove smolder and emit more pollution.

On my stove I get a good burn going and pretty much shut down the PRIMARY all the way. I get enough air for combustion coming from this rear port and gives me my longest burns (which is still only 3 to 4 hours).

 

[bobm]

Really great thread, on my Oslo F500, what is the material just above the SS pipes? Thanks Bob

 

[babalu87]

"ghost of flames"

That is a perfect analogy to what my Morso does (best stove ever mind you)

It is quite mesmerizing.
Especially after a day of cutting next seasons wood while sipping a 2nd, 3rd glass of McClellands

 

[Dunadan]

So I gave the new instructions a try. At first I was thinking to my self this is never going to work. My firebox was going out with air on low. There was no way I was going to get secondary burn this way. Then I decided that my stove might not be hot enough. Surface temp was only reading slightly over 300. So I open my bypass all the way, open the air all the way, play with my wood (chuckle) and move it around a bit ('cmon now) and it takes off with flames everywhere. After about 20 minutes of this I close the bypass, turn the air control until it's about 1/3 open (so more air is being sent through the burn tubes) and sit back and watch.

Within moments I started getting secondary combustion all over the place. Not a little wisp here or there, and not what I thought was secondary burn (the air blowing out of the burn tubes, through the flames in the baffes, leaving a little dimple in the flame). I'm talking full blown secondary combustion.

All I can say is I never would have thought low air, with almost nothing left of my original splits, would get me that type of burn.

My stove person told me that's the key to getting the long burns out of these stoves. With the air low, and the secondary burn going, it's the gasses that are burning, not so much the wood. That's why she said she can get 2-3 hours or more of burn out of a hot bed of coals, and only a split or two.

Sorry if this is obvious information to some of you vets, but since I'm a newb, it's all new to me

Here are a few pics...

Burn 1
Burn 2
Burn 3
Burn 4
Burn 5
Burn 6

 

[begreen]

By jove I think you've got it!

Nice pictures. What is the bright spot in the upper left of each one? It looks like a pilot light burning.

 

[Roospike]

Thats it !

Here is a film loop of my stove from the start of a fresh fire to over 3 hours later with a reload in between.

I didnt take pictures long enough to show the "Ghost flames" but does show the secondary burn .

http://invite.filmloop.com/x?TiGQ7F6rkWljtamOMeI1Ev2k1CLKdfik

 

[Roospike]

By jove I think you've got it!

Nice pictures. What is the bright spot in the upper left of each one? It looks like a pilot light burning.

Its a porch light ........... . :wow: