Temps required for a clean burn

[mnowaczyk]

I know there must be a thread on this, and maybe I'm abusing Hearth.com... but it's just so easy to get quick help on here. I can't resist. THANK YOU everyone.

Is 1200 degrees the temperature required for a secondary burn... that should result in nearly no creosote buildup?

The following makes it look like 1000 degree combustion air temp will assure a clean burn. Does that sound right?
http://www.arb.ca.gov/cap/handbooks/wood_burning_handbook.pdf

Of course we'll never get our stove face temps this high. It seems like it's common to have around 450 stove face temp and still get a clean burn. (I just got a cheap IR today and I'm learning.)

 

[Backwoods Savage]

450 degree stove top temperature should get you a clean burn but you should be able to go a couple hundred degrees hotter with no problem. We regularly run our stove top to 650 degrees up to 700, but usually try to stay closer to 650. Above that and we start to get nervous. When the weather is not so cold then we run 450 most of the day and sometimes even lower.

 

[mnowaczyk]

450 degree stove top temperature should get you a clean burn but you should be able to go a couple hundred degrees hotter with no problem. We regularly run our stove top to 650 degrees up to 700, but usually try to stay closer to 650. Above that and we start to get nervous. When the weather is not so cold then we run 450 most of the day and sometimes even lower.

Stove top temp seems to be the thing to measure (maybe because it's easy). It seems like everyone says temps over 400 degrees make a clean burn. That seems like a rule of thumb, but to get academic about it, exhaust temp would be the thing to measure, right? (But oh... cat stoves are going to be cooler.) I guess if you have an exposed stove pipe you could have a probe-style poked into the exhaust, right? (This wouldn't be a possibility on my insert... at least as far as I know.)

But back to your stove-top temp... If stove temp IS a good way to measure, are we assuming that a hot stove is going to force splits to fire up quick enough to not create creosote?

You could have a 500 degree stove, shovel out all the coals, and then toss in a wet log with some newspaper and get tons of creaosote... right? (Or mroe likely, toss a wet log on top of coals for a slow ignition.)

After a 600+ night last night that cleaned all my glass, I was educated to see that the glass was still clean in the morning with stove temps as low as 200 degrees (and not much but coals left). I guess what I'm learning is that the creosote burns off new logs (especially in cold fires), but cool downs of dried coals is going to be much less likely to create creosote. Is that a fact?

This morning I got my stove face (doors and glass) up to 450 (from 200) in a couple minutes, but saw the glass clouding up (and the thermometer in the top corner of the face was almost at 400). I think I was making creosote. I wonder if an exhaust thermometer would have told me something helpful. ... Maybe that's a good reason to go with a hearth stove as opposed to a fireplace.

I sure like the looks of that Hearthstone Fireview! Ironic to see that's what you own. Like it?

I'm learning, and really appreciate the education!

 

[Backwoods Savage]

First and foremost, what causes creosote is bad fuel. Also, bad fuel makes it difficult to get higher temperatures. If you take bad fuel and the flue is too cool, you are definitely going to have creosote. But that does not have to be. What can one expect with good fuel? As an example, we put up a new SS chimney when we installed our last new stove and after 2 years we got a cup or so of soot out of the chimney; no glazing of creosote on that chimney at all. The wood we burned during this time had seasoned from 4 to 6 years, outdoors. We have a thermometer on the pipe (we have horizontal out through the wall) about 16" from the stove and it will vary from 200 degrees to 350. Of course it gets higher when we load the stove and put the draft open full but once the fire is going good and we turn the draft down the temperature usually runs 300-350; lower in spring and fall.

Yes, cat stoves can run cooler and ours is a cat stove but it is not made by Hearthstone, it was made by Woodstock.

Where you stated, "...are we assuming that a hot stove is going to force splits to fire up quick enough to not create creosote?" It is not when getting those splits burning when you get creosote because you will have the draft full open and the flue gets pretty hot. You will get the creosote after you cut back on the draft and if the wood is not dry, that smoke will cool as it goes up the flue and as it cools, you get creosote.

Your glass getting dirty says that your fuel is not dry or at least not as dry as it should be. If you have good wood your glass will stay clean. Because your wood is not as dry as it should be, you have to burn the fires hotter to keep from getting too much creosote. I do hope you are checking your chimney often....like monthly. Clean as needed.


Oh yes, you asked if we like our Woodstock Fireview and yes, we love it. One of the biggest reasons we like it is because we now burn only half the amount of wood we used to and we stay a whole lot warmer too. That is hard to beat.

 

[Adios Pantalones]

Backwoods nailed it. If you want to burn wet wood without getting a lot of creosote- you need to run it full open for a good long time, until the water is driven out. This can take a surprising amount of time and consume a lot of wood. Listen for hissing or look for water coming out of the end. There's a clip in my firing video that shows an extreme case- very wet wood in a VERY hot (2300F) kiln. (I do that on purpose though)




Shameless promotion

 

[Adios Pantalones]

BTW- I'm not advocating burning wet wood, or running full open all day... Gotta play CYA on that

 

[DanCorcoran]

I think you're mixing up soapstone (a woodstove material) with Hearthstone (a stove manufacturer).

 

[mnowaczyk]

Backwoods Savage:
I was just reading your Gloves post. I think I might have to submit a new topic for HOT gloves (as opposed to cold).

Apparenlty I can't read. Your signature says Woodstock right in it. Maybe I was thinking about stone in soapstone. That is right about the exhaust being cool a lot of the time.

So I will assume that with a cool chimney I better let in all the air I can until that stove is pretty darn hot. It took over an hour to get from 200 to 600+ this afternoon. I'm happy with that startup performance, but wish it took less labor. Must be the wood. (Threw my kindling and dry log on around 9 AM before the 14 month old son had to go to the ER to get his forehead glued back together. I was up to 400 from 200 in less than a half hour, but back at it again around 3 PM.)

Messing with getting the fire really hot before letting it run itself leads to my search for gloves that might resist heat a little better than leather. It's just so much easier to stick those hands in the fire than try to move junk all around with the tongs, poker and shovel. Maybe I should just get some welding gloves. Maybe we'll chat on that thread.

THANK YOU FOR ALL THE HELP TO EVERYONE ON HEARTH.COM

 

[Martin Strand III]

I know there must be a thread on this, and maybe I'm abusing Hearth.com... but it's just so easy to get quick help on here. I can't resist. THANK YOU everyone.

Is 1200 degrees the temperature required for a secondary burn... that should result in nearly no creosote buildup?

The following makes it look like 1000 degree combustion air temp will assure a clean burn. Does that sound right?
http://www.arb.ca.gov/cap/handbooks/wood_burning_handbook.pdf

Of course we'll never get our stove face temps this high. It seems like it's common to have around 450 stove face temp and still get a clean burn. (I just got a cheap IR today and I'm learning.)

Delaware:

Yup, you're in the ball park. Sorry to say your previous responders have led you astray. Unless they're talking 'relatively' clean burn...

Unless you have a catalytic converter (which ignites and reaches the temps you've mentioned) in your metal wood burning stove or a stove and flue that can withstand constant temps > 1000* F or so, you are not approaching the temperature required for complete combustion of wood.

Creosote forms from the cooling and condensation of unburned wood GASES (creosote is not found in the wood per se) when combustion of wood is incomplete. This happens usually in your chimney quite some distance from the stove where the chimney is cooler than about 180* F or so. Measuring 'stove top' or flue temps just above the stove are meaningless to determine this since the critical stack temperature for creosote formation is many feet away. Of course, if one is way off base and burns wood only by 'smoldering', creosote will form in the chimney closer to the stove and maybe there too.

Yup, now days information can be had quickly in the wild, wild west. Being careful what you retain as accurate is the challenge. Good hunting.

Aye,
Marty

 

[Backwoods Savage]

Del, you are right about getting the stove and flue hot after a reload but I doubt you have to get the stove top to 600 before cutting back on the draft. Waiting to reach that temperature you very well may be losing a lot of heat up the chimney.

 

[Adios Pantalones]

You know it's the temp INSIDE that's really important. The temp outside is used a guide to what's happening in the stove, but it does take a while for the mass to soak up the heat and show a good temp outside. I start cutting my stove back long before it's hot outside. I also don't use a thermometer because I'm a bad boy rebel livin on the edge.

Bwahaha! ya. tell my wife that one.

 

[savageactor7]

You know it’s the temp INSIDE that’s really important.

Exactly. While 400-450 stove top may be the magic #...it's still very much hotter in the stove. And more so if you have the blower on...so I go by how things look inside too. Good dancing flames with a clean glass imo means you have a hot enough fire to minimize creosote.

 

[nojo]

Adios,
you got a chimney fire going there. Call the FD.

 

[Adios Pantalones]

LOL- someone did call them once. They though it was about the coolest thing ever.

 

[Battenkiller]

LOL- someone did call them once. They though it was about the coolest thing ever.

40 cu ft, eh? Nice! You add the green wood to help control the rate of temp increase, I'm assuming?

I have a small collection of pit-fired pottery, but I think it's cool how the high temps inside the kiln melt the ash right onto the work.

Sorry for the hijack, but that's just a wicked cool thing to see.

 

[Battenkiller]

Measuring 'stove top' or flue temps just above the stove are meaningless to determine this since the critical stack temperature for creosote formation is many feet away. Of course, if one is way off base and burns wood only by 'smoldering', creosote will form in the chimney closer to the stove and maybe there too.

I'm in 100% agreement about the meaninglessness of stove top temps to monitor the burn inside the firebox. But while I understand what you are saying about flue temps, I disagree that they are meaningless. We need some indicator to give us feed back about the quality of the burn, and the only way I can think of doing that is to monitor flue gas temps. IMO, the hotter the better (up to about 1000ºF). Hot gases rise faster in the flue, and velocity is the name of the game. Cool flue gases linger in the flue long enough to collect on the walls which is the problem. Fast rising gases improve the draft as well, providing hotter temps and more complete combustion inside the box, particularly in the burn zone. When the draft in the flue is improved, the air can be cut down which will increase the velocity of the intake air, giving you a livelier and more energetic burn. This is the principle behind a forge blower, where as more air is delivered faster to the coals, temperatures inside the fire itself rise dramatically.

Here's a composite photo I took last year of my old stove during a starting burn using three progressively smaller intakes air openings, each one 1/2 of the one before. No flue pipe temperature data because I din't use a flue thermo back then, but all I can say is that the stove was warm to the touch when I began and the flue pipe was scathingly hot before I started to reduce the air. Each time I halved the air, you could hear the stove accelerate and see the temperature inside the burn zone increase by the change in color. At 1/8 air, the coals inside were yellow-white... well over 2000ºF. Unfortunately, the fire was so bright that I had to make adjustments in the exposure to prevent it from blowing out the photo, so you really can't see what it looked like in person.

If I were to use stove top temps at the time I took the photos (taken about five minutes apart) at that particular time to monitor the burn, I would have predicted it would have been making massive amounts of creosote, but that was how I ran that stove with thin splits and dry wood, and there was never a trace of creosote in the flue anywhere near the stove. I'll guess that stove top temps eventually rose to about 700-800ºF within about 1/2 an hour from then.

This is the first time I've posted a photo on this site, so I hope it shows up. And don't beat me up about the condition of my poor little Jotul clone. It was a great little heater that cost me nothing but some stove cement, a couple of feet of gasket rope and a few hours work on a buddy's guitar. It saved me about $50,000 in electric heat over 18 years of 24/7 burning from Nov-Apr each season, and ate about 80-90 full cords of wood without complaing too much until the very end. And while it looks like it is leaking air badly around the joints, it was still quite air-tight, and shutting the air down completely in that stove would snuff the fire right out.

 

[nojo]

LOL- someone did call them once. They though it was about the coolest thing ever.

40 cu ft, eh? Nice! You add the green wood to help control the rate of temp increase, I'm assuming?

I have a small collection of pit-fired pottery, but I think it's cool how the high temps inside the kiln melt the ash right onto the work.

Sorry for the hijack, but that's just a wicked cool thing to see.

Where are you located? I want to see!

 

[nojo]

It was a great little heater that cost me nothing but some stove cement, a couple of feet of gasket rope and a few hours work on a buddy's guitar.

Find someone to trade me a modern stove for my 1994 Strat Plus Delux with Lace sensors and hot rail in the bridge, roller nut locking tuners, the works!

 

[Adios Pantalones]

nojo- I'm in Londonderry, NH

Batten- I don't think that the residence time is the whole story. At high temp, things are just plain less likely to condense. faster exhaust will exit without cooling as much in the pipe, and cooler gas will have more residence time, so they aren't independent variables to be sure.

 

[Pagey]

Some very good points in this thread, and a very interesting read. Flue gas temps will, at the end of the day, be the major factor in determining the amount of condensation allowed in the flue - cooler gases, more condensation (however, if you burn up 90% of that material BEFORE it even reaches the flue, you're going to have even less condensation to begin with). But I think with today's EPA non-cat stoves that use secondary air, there is in fact at least SOME amount of correlation in stove top temps and what is considered "clean burning". Of course those temps will vary by manufacturer, stove top thickness, baffle material, etc. But in a non-cat stove, that smoke needs to hit around 1,100F up there by the baffle to combust and thus burn off MOST of the organic/volatile materials before they can even get to the flue. And this 1,100F seems to translate to a stove top temp of around 500F on up in a non-cat secondary air stove. Not sure about the downdraft types. And since a cat lets the smoke burn at 500F (half the temp required in a non-cat), one could arguably achieve a cleaner burn with a lower stove top temp.

Does that make sense, or am I totally off base? Please correct me if I am wrong in my assumptions, which I freely admit is what I just posted - assumptions. I don't want to be guilty of assuming bad information!

 

[Martin Strand III]

As an armchair wood burning engineer, like the rest of you, I predict before long and before being allowed to burn in your home the Green Police will test each chimney with a combustion gas analyzer which measures stack O2, stack CO, stack temperature and stack pressure (draft) just like the Bezirgsschornsteinfegermeister in Germany.

In the mean time, burn hot and clean.

Aye,
Marty

 

[Battenkiller]

Batten- I don't think that the residence time is the whole story. At high temp, things are just plain less likely to condense. faster exhaust will exit without cooling as much in the pipe, and cooler gas will have more residence time, so they aren't independent variables to be sure.

Oh, I agree, but I thought we had already hashed out the temperature aspect of creosote condensation. I was just trying to bring another factor into the equation. Truly, they are interdependent variables, in that faster exhaust will lead to faster intake which will lead to higher burn temps which will lead to hotter flue gases which will lead to even faster exhaust gases... which is a positive feedback loop that will lead to runaway combustion without a limiting factor. In most stoves, the design allows enough fuel in a full load to create a nightmare and a half in that event, so the limiting factor built into the design is usually air intake and not fuel. Velocity will only increase to a certain point as you close the intake, and beyond that no more air will be able to enter the box, and then less and less as the intake opening is closed further and further. Open the intake all the way and the velocity drops. More air can certainly enter in as the burn increases, but the design of the stove should be such that a flue of the recommended size and height can't draw hard enough to increase it beyond a point, which means that the intake air will actually cool the fire after a point. In every case, hot fires lead to more complete combustion which leads to lower creosote production and hotter exit gases that hopefully are too hot to condense on the flue walls. In no case is merely using perfectly dry wood a guarantee of low creosote production. I could soot up a chimney real fast with kiln-dried wood if I wanted to.


On the stove I showed the photos of, I actually set the intake air for a long burn with a 5/16" metal rod that always sat directly under the door. If I opened draft all the way, the burn rate would increase to a point, but the velocity would drop to the point where the burn was not vigorous and internal temps would actually drop, slowing the whole thing down. If I went below the 5/16" draft opening, the stove would gradually suffocate no matter how hot it was beforehand. Like every other stove, there was a sweet spot for every desired output/burn time combination.

Anyway, I've wasted so much hot air on this forum lately that I'd have been better off blowing it into my stove to keep the temps up.

 

[VTHC]

But in a non-cat stove, that smoke needs to hit around 1,100F up there by the baffle to combust and thus burn off MOST of the organic/volatile materials before they can even get to the flue.

SO wouldn't that translate to a potentially lower flue gas temperature? Since all the VOCs and such are burned, there is theoretically less creosote forming materials to condense on the chimney walls. I just installed a probe thermometer on my double wall stove pipe, and the temps were reading around 500*F 18" up from the collar while my stove top was ~650*F. If i turned down the primary air to a point where all i saw was ghost flames coming from the re-burn tubes the flue temps decreased to ~400*F... is this considered a "clean" burn if i have good secondary burn? The stove top temp dropped slightly to ~600*F...

 

[RonB]

With my stove, I would consider that a clean burn (600 stove, 400 flue). Actually, I am able to reduce the air somewhat to drop the flue temp to about 300. If your glass stays clean and there is no visible smoke out the stack then it's clean.

 

[Martin Strand III]

OK. We need a word or so about combustion air.

The minimum amount of air required to completely burn wood can be calculated from wood's chemical formula, the weight of wood being burned and the % of carbon dioxide you end up with (max at about 20% for wood) in the flue gasses. This is called theoretical combustion air or stoichiometric air. It is somewhere between 3 - 4 cubic meters of air per Kg of wood. In the real world, more air travels through the firebox than theoretical combustion air. The extra air which does not participate in the exothermic chemical equation converting wood to carbon dioxide (carbon monoxide is created in incomplete combustion) and water is called excess air.

Too much excess air going through the firebox does not help in the exothermic chemical equation, passes through into the flue and goes up the chimney wasting heat and cooling the flue gasses.

This was taken into account in the design of your stove and explains why the fire, after being established, can be felt (and measured) to cool down by opening the firebox door allowing in too much air for an efficient fire. It can also happen with the door closed with the air intake control open too wide if the fuel supply is scant.

That's the story and I'm sticking to it.

Aye,
Marty
Grandma used to say,"It's possible to have too much of a good thing."