I'm Completely Confused About Airflow And Its Effect In Stove!

[turbocruiser]

Well, I have read and read and read myself into complete confusion! The "understanding" that I have always had is that more air moving into the stove should mean having a hotter fire and that less air moving into the stove should mean having a cooler fire. I thought this "understanding" of mine was validated with what my owner's manual states about airflow in several different spots such as...

"Caution: Unit may overheat if door is left open for too long."

and a little later...

"Caution: If the body of your unit, flue baffle or any part of the chimney connector starts to glow, you are overfiring. Stop loading fuel immediately and close the draft control until the glow has completely subsided."

But then in thread after thread recently regarding overfiring, I'm reading about advice that seems to suggest the opposite where one would purposely open the door to cool the fire.

Please understand that I am not arguing with anyone but myself here! In other words I simply cannot reconcile what's what anymore and I'm asking for clarity!

My own observations of my stove suggest that with the door closed and the air control fully open I get a good roaring fire in no time that within twenty minutes or so needs to be damped down to 1/2 open. I can then run the stove load after load by just waiting for good glowing coals, opening the air to full open for about 30 seconds, opening the door to add several more splits, closing the door as quickly as possible, leaving the air at full open for another 30 to 60 seconds at the most, then going right back to 1/2 open. The stove seems to cruise (as you all call it) at about 650 to 750 with this method. If I see anything above 750 on the chimney connector or on the actual top of the firebox I damp it down a small fraction of an inch and it settles into 650-750 again. Further, if I damp it down all the way the temps drop pretty dramatically. If the stove is damped down all the way with a good thick glowing bed of coals on then it will last a long time that way with temps in the 350 to 450 range (that's what we shoot for for our overnight burns so far) but if the stove is damped down before the fire takes the first batch to coals, then it will literally snuff out the whole thing.

So basically everything I'm seeing is that more air equals more heat and I'm trying to really reconcile that both for regular situations as well as overfire situations. I cannot imaging climbing above 750 even slightly and in response immediately opening the door though and that's where i'm obviously not understanding something. I'd appreciate any clarity y'all can give because right now the whole thing is about clear as mud mixed with ash and coal and then again with mud! Thanks.

 

[learnin to burn]

I'm sure some others will jump in here but I'll give it a shot. Yes, More air means larger/hotter fire, less air smaller fire. There are differences when it comes to EPA stoves though. With the stoves, the more air you put into it, the more heat is going up the chimney which provides less heat from the stove itself. By closing down the draft you provide less air and heat to go up the chimney so the stove retains more heat to radiate. When you reload on a real hot bed of coals (lets say the stove is still over 400 deg.) it's real easy to close down the draft and have the secondaries take off creating an over fire situation. If I'm understanding some of the other posts about opening the door to cool the stove down, what's happening is the secondaries are being taken out of the loop and the cooler air from the room is rushing in and either cooling thing off just enough to get it out of over fire range even though the extra air will increase the size of the fire or the extra heat is being pulled/sucked out through the chimney or a combination of both.

I hope that makes some sense and clears things up at least a little for you.

 

[Wallace]

I'll explain it from a firefighters perspective. On a house fire when visibility is terrible and the house is extremely hot, one of the things we need is ventilation. We put a crew on the roof and have them cut a 4x4 hole. The interior crew knows that the fire is going to intensify. But along with the fire getting bigger, we get much better visibility and cooler temps! Most of the smoke and heat go out the vent hole in the roof effectively cooling the inside! Hope this helps!

 

[branchburner]

With many stoves an open door basically turns the stove into a fireplace. Allowing the huge amount of air exposure kills the efficiency of the fire. Instead of being focused and directed through the fire to intensify it, the over-supply of air flow (with the door open) in effect bypasses the fire, pulling excess heat up the flue and stopping any secondary combustion. It does seem counterintuitive, but just try picturing an over-fire in a fireplace or a campfire - without being in a contained box, there is no way to get high-temperature combustion without something like a bellows to direct and amplify the airflow to the fire.

 

[ScotO]

I'll explain it from a firefighters perspective. On a house fire when visibility is terrible and the house is extremely hot, one of the things we need is ventilation. We put a crew on the roof and have them cut a 4x4 hole. The interior crew knows that the fire is going to intensify. But along with the fire getting bigger, we get much better visibility and cooler temps! Most of the smoke and heat go out the vent hole in the roof effectively cooling the inside! Hope this helps!

EXACTLY Wallace......here's a little test.....next time yer outback cooking marshmallos on the campfire with the kids, observe the fire and the heat/intensity of the flames.....then take, say, a two foot section of 1/2" pipe and put it near the base of the fire....start blowing into the pipe and observe how it stokes and intensifies.....what I am getting at is if you open your door the whole way up, all the heat rushes up the chimney, but if you crack the door and open the draft the whole way, you are effectively stoking the fire, intensifying the heat in the firebox and all of the heat is NOT going up the flue.......it can be confusing but if you look at it "outside the box" it can make sense.......

 

[Battenkiller]

With many stoves an open door basically turns the stove into a fireplace. Allowing the huge amount of air exposure kills the efficiency of the fire. Instead of being focused and directed through the fire to intensify it, the over-supply of air flow (with the door open) in effect bypasses the fire, pulling excess heat up the flue and stopping any secondary combustion. It does seem counterintuitive, but just try picturing an over-fire in a fireplace or a campfire - without being in a contained box, there is no way to get high-temperature combustion without something like a bellows to direct and amplify the airflow to the fire.

This hits the nail right on the head IMHO. And it's not just true for EPA stoves, it's true for any stove. All stoves, furnaces and forges work by containing the heat to increase the the temperature up into the range where complete combustion can occur. Combustion air is restricted to a high-velocity flow directly into the hottest part of the fire. Open the air up and you get a more intense burn... to a point. But open the doors all the way and all that incoming air will have a strong quenching effect on the fire, and both temps and efficiency plummet.

This is why cracking open the door or the ash pan can overfire a full stove, but giving it too much air by opening the doors all the way will have the opposite effect. My stove was designed to be used either as a stove or as a fireplace. I've played around a lot with both methods and have lost interest in the fireplace mode. No real heat, even with that massive cast iron box surrounding it. Low flue temps as well, leading to a lazier draft in spite of the wide open front. But start to close those doors a bit at a time and you soon have a raging inferno with the same load of wood as the air gets more and more restricted and focused into a higher velocity stream.

 

[turbocruiser]

I'm sure some others will jump in here but I'll give it a shot. Yes, More air means larger/hotter fire, less air smaller fire. There are differences when it comes to EPA stoves though. With the stoves, the more air you put into it, the more heat is going up the chimney which provides less heat from the stove itself. By closing down the draft you provide less air and heat to go up the chimney so the stove retains more heat to radiate. When you reload on a real hot bed of coals (lets say the stove is still over 400 deg.) it's real easy to close down the draft and have the secondaries take off creating an over fire situation. If I'm understanding some of the other posts about opening the door to cool the stove down, what's happening is the secondaries are being taken out of the loop and the cooler air from the room is rushing in and either cooling thing off just enough to get it out of over fire range even though the extra air will increase the size of the fire or the extra heat is being pulled/sucked out through the chimney or a combination of both.

I hope that makes some sense and clears things up at least a little for you.

Awesome, that makes sense and again I'm not arguing any but by that logic there isn't the act of opening the door just moving the problem from the firebox into the chimney? In other words, say I'm cruising along at 750 actual stove top temp and I load a load early and it totally takes off. If my stove top temp is 750 then the chimney was probably already way above that temp, then I add additional fuel and additional air and get panicked as I allow the stove top temp to get to 850 let's say, then I open the door to cool the firebox but now the full force of that overfire is in my chimney and at least as far as I can imagine I intentionally make that problem much worse by intentionally leaving the door totally open until I think the firebox area is alright. Again I'm not arguing any at all but I'm basically just trying to work through all these contractions in my own mind so the sounding board here is helping immensely Thanks so sincerely and please keep it coming; sooner or later my lightbulb will lightup here! Thanks.

Edit: I meant to quote what Wallace wrote as well and say that also totally makes sense but in that analogy, if the attic is the firebox then the "chimney" is the air and the air in that instance can handle whatever temperatures were released into it. I'm still wondering what happens to the chimney if the response to an overfire is intentionally opening the door to the stove. Again, no argument, just confusion. Thanks.

 

[Battenkiller]

I'm still wondering what happens to the chimney if the response to an overfire is intentionally opening the door to the stove. Again, no argument, just confusion.

Well, the flue gas temp should drop rapidly because the gases are being diluted by cool, fresh air. However, one other possible response is a chimney fire if it's dirty. Chimney fires are usually hard to start because the exhaust gases are usually oxygen-deprived from the combustion process. If you have an extremely hot stove and you suddenly add a lot of excess air, the conditions are much better for creosote ignition. Best reason to maintain a squeaky clean flue.

 

[begreen]

One thing that confuses a lot of people is the misconception that when the air control is closed down all the way there is no air getting to the fire. In most modern stoves that is not so because they have an open secondary air port and a stop that prevents the air control from closing fully. The purpose of this is to keep the glass clean with the air wash and to avoid smoldering (and polluting) fires. Because of this partial air, a fire can burn quite hot if it is in the strong outgassing phase. (This is when a fresh charge of wood is loaded into the stove on a hot coal bed.) Opening the stove door at that point will flood the stove with cold air that will dilute the raging fire, the fire temperature will drop and so will the flue temperature. That is, if as BK pointed out, there is not a serious accumulation of creosote.

So yes, it can be confusing, because for every condition, there are variables. Variables can be the type of wood stove, whether it has a bypass or not, the stage of the fire, the state of the chimney, etc.

 

[BrotherBart]

Opening the door also breaks the negative pressure vacuum from the chimney draft that is focusing the secondary air and the unregulated primary air into the burning wood. As well as has been mentioned flooding the firebox and chimney with cooler air.

 

[BrotherBart]

BTW: Don't wait for an over-fire to try it out. Get the stove up to five hundred degrees and swing the door open check out the results.

 

[turbocruiser]

One thing that confuses a lot of people is the misconception that when the air control is closed down all the way there is no air getting to the fire. In most modern stoves that is not so because they have an open secondary air port and a stop that prevents the air control from closing fully. The purpose of this is to keep the glass clean with the air wash and to avoid smoldering (and polluting) fires. Because of this partial air, a fire can burn quite hot if it is in the strong outgassing phase. (This is when a fresh charge of wood is loaded into the stove on a hot coal bed.) Opening the stove door at that point will flood the stove with cold air that will dilute the raging fire, the fire temperature will drop and so will the flue temperature. That is, if as BK pointed out, there is not a serious accumulation of creosote.

So yes, it can be confusing, because for every condition, there are variables. Variables can be the type of wood stove, whether it has a bypass or not, the stage of the fire, the state of the chimney, etc.

Actually one thing that I was aware about was that with my stove no matter how much I close the air there's still a small hole about 1/4 inch in diameter which stays open but again when it is closed that completely the fire almost always goes out; if there is already a good thick glowing bed of coals they will slowly work themselves to ash that way but if there is any wood in there it almost always remains relatively unburnt so that was why I thought my owner's manual operating instructions instruct to completely close the air in the event there ever was an overfire condition. Can a hole only 1/4 inch in diameter still feed the fuel with enough air to maintain the overfire or even to make it much worse? Perhaps the thing here is some stoves don't close the air that much and for those stoves the method of opening the door works when the method of completely closing the air control doesn't work??? Thanks, I really appreciate everyone's advice and also patience.

 

[BrotherBart]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

 

[Danno77]

BTW: Don't wait for an over-fire to try it out. Get the stove up to five hundred degrees and swing the door open check out the results.

That's naughty. My manual tells me to always open my primary air fully and then slowly open the door! ;-)

 

[tfdchief]

I'm still wondering what happens to the chimney if the response to an overfire is intentionally opening the door to the stove. Again, no argument, just confusion.

Well, the flue gas temp should drop rapidly because the gases are being diluted by cool, fresh air. However, one other possible response is a chimney fire if it's dirty. Chimney fires are usually hard to start because the exhaust gases are usually oxygen-deprived from the combustion process. If you have an extremely hot stove and you suddenly add a lot of excess air, the conditions are much better for creosote ignition. Best reason to maintain a squeaky clean flue.

BK, Really good point, not often thought of or mentioned.

 

[tfdchief]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

BK, that is an interesting comment. It really has me thinking. It would seem to me that the size of the fire box would be irrelevant, because the amount of wood would be relative to the size of the fire box.......smaller fire box, less wood needed, less btus needed to overfire. Bigger fire box, more wood needed, more btus needed to overfire. Am I missing something?

 

[BrotherBart]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

BK, that is an interesting comment. It really has me thinking. It would seem to me that the size of the fire box would be irrelevant, because the amount of wood would be relative to the size of the fire box.......smaller fire box, less wood needed, less btus needed to overfire. Bigger fire box, more wood needed, more btus needed to overfire. Am I missing something?

Less of a blast of gasses from a smaller load out-gassing. I don't know how in the world I could every get my F3 CB or F100 to over fire. The big problem with them is getting them hot and keeping them hot. A smaller steel stove might be a different story though.

 

[Danno77]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

BK, that is an interesting comment. It really has me thinking. It would seem to me that the size of the fire box would be irrelevant, because the amount of wood would be relative to the size of the fire box.......smaller fire box, less wood needed, less btus needed to overfire. Bigger fire box, more wood needed, more btus needed to overfire. Am I missing something?

Less of a blast of gasses from a smaller load out-gassing. I don't know how in the world I could every get my F3 CB or F100 to over fire. The big problem with them is getting them hot and keeping them hot. A smaller steel stove might be a different story though.

Well, I was over firing my little propane tank converted into a woodstove before I devised a way to crank the door closed a little tighter. Think it said 800 at one point on the iR gun! Not sure how many cubic feet it was, but not very big!

 

[tfdchief]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

BK, that is an interesting comment. It really has me thinking. It would seem to me that the size of the fire box would be irrelevant, because the amount of wood would be relative to the size of the fire box.......smaller fire box, less wood needed, less btus needed to overfire. Bigger fire box, more wood needed, more btus needed to overfire. Am I missing something?

Less of a blast of gasses from a smaller load out-gassing. I don't know how in the world I could every get my F3 CB or F100 to over fire. The big problem with them is getting them hot and keeping them hot. A smaller steel stove might be a different story though.

Well I am not doubting your real life experience, but so far, my little Hampton, 1.34 Cu. In. fire box gets hot real easily. And my Old Little Buck, which has a very small fire box, will get just as hot as I will let it, Yikes. So, again, you don't think that the "Less of a blast of gasses from a smaller load out-gassing" really makes a difference, because it has less of a fire box to heat up and to over fire? I expect BK to jump in here anytime and set me straight LOL ;-)

 

[BrotherBart]

Like I used to say at the first of any presentation: "Given what I know at this moment, and I feel sure I don't have all of the facts that are out there, here is what I think.". ;-)

 

[tfdchief]

Like I used to say at the first of any presentation: "Given what I know at this moment, and I feel sure I don't have all of the facts that are out there, here is what I think.". ;-)

:cheese: Hehe, Ok, It's late.

 

[Battenkiller]

Like I used to say at the first of any presentation: "Given what I know at this moment, and I feel sure I don't have all of the facts that are out there, here is what I think.". ;-)

Maybe I should preface all my posts with that comment. Save me a lot of grief.

 

[Jackfre]

I've not installed my stove yet, but did read the install/operation manual today. Essentially there is a basic description of how the unit should operate and that is followed by one caveat after the other saying that "every system has a personality". I have tons of pre-cat experience, but this should be interesting coming to grips with the "system". Great post and terrific responses. Much obliged!

 

[tfdchief]

Like I used to say at the first of any presentation: "Given what I know at this moment, and I feel sure I don't have all of the facts that are out there, here is what I think.". ;-)

Maybe I should preface all my posts with that comment. Save me a lot of grief.

Auh, come on BK, that's no fun......I like it better when you stick your neck out...... ;-P Your admirer, Chief

 

[begreen]

Frankly, I don't know how you would overfire a 1.4 cf firebox stove. It is a more frequent concern with larger fireboxes with a full fuel charge in them.

With negligence it can be done with the Castine. I came close when learning about the compressed wood products and have seen some poor stoves that this has happened to. If you want to push the stove to 900+ it is possible, especially if you stoke it like a fireman and leave the air control half open.