Secondary Burn and efficiency

[Bill]

While reading another thread about secondary burn and the high temperatures needed, when you damper the stove at night, how efficient is your burn? Also are you letting a lot of volatile gases escape without being converted to heat?

 

[Vic99]

My unscientific observation: When I damp down and secondary burn engages it is a super efficient burn emissions-wise. Very little smoke escapes the chimney. I think when the inside the firebox temp falls enough, efficiency decreases. This first happens at <1,110 F until ~600 F I think. If I go out with the dog very early morning of the middle of the night, sometimes I can smell more smoke. Obviously that could also be neighbors, atmospheric conditions, etc. Less efficient.

 

[BurningIsLove]

Even when you damper down the primary air control all the way on a modern, EPA-approved stove, there is a secondary air supply that keeps the secondary combustion going which efficiently burns all the gases & smoke. The EPA regs. require this so you cant damper down so much that the stove smolders & burns uncleanly.

Its great because your firebox fuel lasts a long time w/o reloading, and the secondary combustion of the waste gases/smoke keeps the temp hot enough to sustain itself.

 

[BurningIsLove]

Well, yes. When the fuel burns down to coals are no more waste gases/smoke to reburn, of course the stove will cool down. But since the bypass is closed, the draft up the chimney will also drop significantly, and the heat from the coals will remain in the firebox vs. going straight up the chimney as if the bypass was open. So even then in all-coals mode, there is still plenty of useful (efficient) heat being generated. You shouldnt smell any smoke at this point since there really isnt anything left to produce smoke.

 

[oak194]

ok well let me ask this then, yesterday i brought my quad 7100 up to temp, and after the house was as warm as I could take, i closed the draft down, and within an hour my front glass blackened to the point you could not see thru it. Which as far as i can figure was from the fire just smoldering. correct?
Am i doing something wrong? this is my first epa wood burner, i'm used the the old simple, no burn tube type.

 

[daveswoodhauler]

If the glass blackened up with the air shut down, I would say that your wood is not completely seasoned. I did a test with my new stove and used some wood seasonsed for about 1 year, and other for 5-6 months....same type of wood...so just a little experiment. The one for 12 months burned long and true with no black smoke/frost...wood only 5-6 months seasoned produced mush more soot/smoke on the glass. Just an observation, as yours might be due to different stove/different draft, etc...
Also, before you shut her down all the way you really need to get a good bed of coals and have the fire run hot for 30 minutes or so
Good luck.

 

[Jags]

ok well let me ask this then, yesterday i brought my quad 7100 up to temp, and after the house was as warm as I could take, i closed the draft down, and within an hour my front glass blackened to the point you could not see thru it. Which as far as i can figure was from the fire just smoldering. correct?
Am i doing something wrong? this is my first epa wood burner, i'm used the the old simple, no burn tube type.

A whole bunch of that depends on the draft that your system produces. It IS possible to have a system that when the stove is turned to its lowest setting, will "snuff" out the fire. Meaning that it does not let in enough air to continue combustion. It happens. Doesn't really mean that you are doing anything wrong, its just a characteristic of YOUR setup.

Some people drop their air control to its lowest setting. If I do that, I will have charcoal chunks the size of your fist that are unburnt. I have to stay 5-10% open to get a complete burn.

Edit: and as was said above, unseasoned wood is probably the BIGGEST culprit of clouding up the glass.

 

[BurningIsLove]

I cant claim to be familiar w/ the Quad stove and how the air is supposed to circulate in them for secondary (or primary) combustion. Many new stoves have the primary air come into the firebox just above the glass to create an 'airwash' current to keep the glass (mostly) clean. Not sure if the Quad has the same design type, but usually those burn tubes also distribute air to keep the glass clean (someone Im sure will comment who knows the stove)

On my stove, when its first coming up to temp, I occasionally get some black soot on the front glass, but once it's up to temp, that burns off and doesn't reform. If you are getting soot on the glass, chances are it's not hot enough, but again w/ the caveat that I am not an expert on your stove. A

A couple of quick things you can check:

Are the emissions coming out of your stack dark or clean? (they should be clean. If they are sooty, then its not burning hot enough most likely)
What does your stove temperature read when the soot starts to form?
Are you dampering down on primary air immediately after you close the bypass? (alot of stoves need some time at full blast to initiate secondary b4 dampering down)

 

[Adios Pantalones]

too little air/draft or wet wood will cause that, Oak194

 

[mranum]

ok well let me ask this then, yesterday i brought my quad 7100 up to temp, and after the house was as warm as I could take, i closed the draft down, and within an hour my front glass blackened to the point you could not see thru it. Which as far as i can figure was from the fire just smoldering. correct?
Am i doing something wrong? this is my first epa wood burner, i'm used the the old simple, no burn tube type.

A whole bunch of that depends on the draft that your system produces. It IS possible to have a system that when the stove is turned to its lowest setting, will "snuff" out the fire. Meaning that it does not let in enough air to continue combustion. It happens. Doesn't really mean that you are doing anything wrong, its just a characteristic of YOUR setup.

Some people drop their air control to its lowest setting. If I do that, I will have charcoal chunks the size of your fist that are unburnt. I have to stay 5-10% open to get a complete burn.

Concerning the draft, Isn't it true that the draft will be significantly weaker this time of year vs in the dead of winter when temps are a lot colder?

 

[Jags]

Yep, outside to inside temp. differential can greatly affect the draft.

 

[BurningIsLove]

Concerning the draft, Isn't it true that the draft will be significantly weaker this time of year vs in the dead of winter when temps are a lot colder?

Almost certainly (location dependent of course!). My stove doesn't usually draft optimally/efficiently unless the outdoor temps are below 35. But atmospheric pressure can affect that range significantly for proper draft.

 

[Bill]

Even when you damper down the primary air control all the way on a modern, EPA-approved stove, there is a secondary air supply that keeps the secondary combustion going which efficiently burns all the gases & smoke. The EPA regs. require this so you cant damper down so much that the stove smolders & burns uncleanly.

Its great because your firebox fuel lasts a long time w/o reloading, and the secondary combustion of the waste gases/smoke keeps the temp hot enough to sustain itself.

Let me clarify my question, first my wood was split and stacked two years ago, tested with moisture meter, and the wood is dry. I have a straight chimney, double stainless inside, class A outside. What I am referring to is as you damper the stove down, the inside of the box drops, to light off the secondary combustion you need a minimum amount of heat. Even though you cannot close the secondary combustion air, the stove is not hot enough for secondary combustion. My feeling is it then lets combustible smoke and gases up the chimney.

When I start a rip roaring fire with lots of dry kindling, there is no secondary combustion, until the entire stove gets hot, and it occurs best when I damper down about half way, after the fire is nice and hot.

Just a thought I would kick around with my fellow pyros.

 

[BurningIsLove]

Let me clarify my question, first my wood was split and stacked two years ago, tested with moisture meter, and the wood is dry. I have a straight chimney, double stainless inside, class A outside. What I am referring to is as you damper the stove down, the inside of the box drops, to light off the secondary combustion you need a minimum amount of heat. Even though you cannot close the secondary combustion air, the stove is not hot enough for secondary combustion. My feeling is it then lets combustible smoke and gases up the chimney.

When I start a rip roaring fire with lots of dry kindling, there is no secondary combustion, until the entire stove gets hot, and it occurs best when I damper down about half way, after the fire is nice and hot.

Just a thought I would kick around with my fellow pyros.

Gotcha. In the optimum environment (good draft, dry wood, all of which it sounds like you have)....even when the internal temp of the firebox drops off, as long as secondary combustion was already occuring, then I find that secondary burn to be self sustaining since it creates so much heat itself. So even as the firebox cools and produces additional unburnt gases/smoke, secondary will continue and sometimes even accelerate as it has more raw material to burn, making it even hotter. This is what sometimes happens in Dutchwest non-cat stoves that we call 'thermonuclear' events. Even with the primary air completely closed, it just makes the stove angry and the flu gas temp rise uncontrollably.

Below a certain amount of primary air, the primary combustion temp will drop to the point where the temperature differential is too great which will stall secondary and 'choke out' the stove. This would produce a lot of unburnt gas/smoke coming out the top of the stack. As you've already surmised, finding that optimum setting on the primary air that allows for the slowest, clean burn without stalling out the stove is the holy grail. On a given stove, that setting isnt fixed and changes with every reload depending on lots of factors like split size, type, moisture content, draft factors, etc.

 

[oak194]

Wow, thanks for the info, and ideas. This new zc fireplace is light years ahead of my old one.
It's going to take me a little while to figure it out, that and some cold weather would be great!!

Thanks once again!

This is worth its weight in gold!