Hypothetical burn question

[KarynAnne]

I love hypothetical questions. Mark hates them. I've tried to stab around at figuring this out, but no luck so here goes...

Take one Jotul Oslo wood stove with a mixed full load of misc. hardwoods. Nothing outrageous wood wise - no locust or anything. The hypothetical question assumes the exact same load for all scenarios.
1. Load runs full tilt without adjusting air flow for remainder of burn.
2. Load gets a good char for about 15 minutes, temp of thermometer on back right top of wood stove hits 600, then gets shut down to 1/2 for remainder of burn.
3. Same as 2 above except shut down to 1/4 for remainder of burn.

What would the approximate burn times of the three scenarios be based on #1 having a burn time of "x" amount of time?
What would the approximate temp differential be between full open and shut down as in 2 and 3?

Do you hate these questions as much as Mark? Want to take a stab at it anyway? Thanks.

 

[Roospike]

If you did #1 first your might not have a house to try #2 and #3

******************************************************************

#1 is just silly , over run the stove and a lot of the heat is going to go out the stack .

The same load of wood per all three setting is going to put off the same amount of BTU except #1 were a lot of it is going to the sky , the more damper you have open the hotter the fire and the more BTU per hour your going to give off ( per hour) Same BTU's in the 1/2 and 1/4 damper setting but changes the BUT per hour the wood is putting off.

#1 most BTU per hour, shortest burn time ( and wasted BTU up the pipe )
#2 average heat, average burn time
#3 less heat , longest burn time

 

[KarynAnne]

Yes, you're right of course - I'm just trying to frame the hypothetical. Feel free to suggest any other scenarios. Thanks.

 

[kevinmoelk]

Many questions for your question, lol.

First, I would use a manufacutred product to obtain the information. As Spike said, don't do #1, that's just insane... sorry, but it's true. #2 vs #3 I believe the results would be that #2 would be a hotter fire, but shorter burn time. #3 would burn longer at a lower temp. In all cases it would depend on your stove, how good your draw is, etc. (all of the little localized factors) to find out what temperatures you could achieve and burn times.

Also, are you starting from a dead cold stove? How you stack the logs will make a difference too. Hence the experiment again would be easier with a manufactured product. Give the experiment a shot (not #1) and find out.

-Kevin

 

[KarynAnne]

I guess I know the generalities. Let's forget #1.
How much longer would I expect #3 to last beyond #2? Those are the kinds of things that I'm trying to get a handle on. Thanks.

 

[KarynAnne]

Ok - #1 is out. I wouldn't do that anyway. It's just hypothetical.
Can you say more about the "manufactured product"? Is there some kind of meter that babysits the stove and records things like that for the average user? That would be cool.

 

[MountainStoveGuy]

Dont forget about number 1. Some stoves have such poor draft that they will only run wide open. The level of air you supply the secondary burn system is directly porportional to the amount of chimney you have to draw the air through the system....
As far as where you put the air, lets say you have strong draft, and you turn them down at the same stage of wood cumbustion as well. I would hypothise (is that a word??) that you would get 50% longer burn if your air was set at a 1/4" open vs 1/5" open.

 

[Roospike]

Thats just the thing about WOOD and wood stoves , no two loads are alike as it would be hard to have the exact same size load /cuts/splits and kind of wood ............ Thats where YOU come in. How well and how long the wood is seasoned is another factor.

 

[kevinmoelk]

Manufactured product would be the fire logs. BeGreen has some good write ups in the Wiki I believe concerning experiements he's conducted.

No automatic babysitter that I know of, but I'm relatively new to wood burning also. I'd just watch it and take a report say every 15-30 minutes.

-Kevin

 

[BrotherBart]

I love hypothetical questions. Mark hates them. I've tried to stab around at figuring this out, but no luck so here goes...

Take one Jotul Oslo wood stove with a mixed full load of misc. hardwoods. Nothing outrageous wood wise - no locust or anything. The hypothetical question assumes the exact same load for all scenarios.
1. Load runs full tilt without adjusting air flow for remainder of burn.
2. Load gets a good char for about 15 minutes, temp of thermometer on back right top of wood stove hits 600, then gets shut down to 1/2 for remainder of burn.
3. Same as 2 above except shut down to 1/4 for remainder of burn.

What would the approximate burn times of the three scenarios be based on #1 having a burn time of "x" amount of time?
What would the approximate temp differential be between full open and shut down as in 2 and 3?

Do you hate these questions as much as Mark? Want to take a stab at it anyway? Thanks.

#1 One hour and twenty-three minutes. Top temperature 1226 degrees

#2. Four hours and sixteen minutes. Top temperature 806 degrees.

#3 Seven hours and forty-six minutes. Top temperature 626 degrees.

 

[Rhone]

I believe all stoves and loads have a "sweet spot". That is, if you have the air too high the heat is leaving the unit before it can transfer and you have excess heat going out the flue. Too low, and your secondary burn isn't operating 100% of the time and you have unburned fuel out your flue. Somewhere, is the sweet spot. The place where, it's hot enough your secondary burn is staying constantly on and you're getting good combustion efficiency, and you've slowed the exhaust as much as possible maximizing the amount of time the heat takes to leave so it transfers the most amount of heat into the living space before heading out the flue. For me it's somewhere around 30-40% and differs for each load. The loads that are more compact, I need the air a little higher. Ones that are rather free I can move the air handle lower. Some people with multi story houses and really strong draft that setting may be the lowest they can set theirs, I have a ranch.

So, I say it's more like #2 or #3 and it takes a long time to learn. When starting from cold, I leave mine on max air until I see the flames are looping around the baffle up top which is usually about 5 minutes after the kindling gets moving. Then, I lower it to 75% for about 10 minutes to let it adjust. I then cut it back to about 50% for the next 20-30 minutes and will come back to check up on it to see if I should lower it to 30 or 40% depending on how things are going. If I'm heading out and lit a fire shortly before I I set it to 40% to be on the safe side, hope for the best, and head out. The 30-40% is an overnight burn for me (8+ hours) and I've learned is the most efficient and will raise my house 10-12F (I have a small house, big stove). If I burn it lower or higher, I extend or shorten my burn time but my house has consistenly been 2F or so colder after 12 hours so I think I found the sweet spot for my situation. You eventually learn when to adjust the air handle and how to load the wood, to give you the quickest start up time and most miles per gallon out of your load, usually takes a year or more though.

 

[KarynAnne]

I love hypothetical questions.
#1 One hour and twenty-three minutes. Top temperature 1226 degrees

#2. Four hours and sixteen minutes. Top temperature 806 degrees.

#3 Seven hours and forty-six minutes. Top temperature 626 degrees.

I'm holding you to it.

 

[KarynAnne]

Excellent - thank you. This was the closest to what I was looking for. I've suspected that there is a sweet spot - everything has one. I just haven't found the stove's yet.

I believe all stoves and loads have a "sweet spot". That is, if you have the air too high the heat is leaving the unit before it can transfer and you have excess heat going out the flue. Too low, and your secondary burn isn't operating 100% of the time and you have unburned fuel out your flue. Somewhere, is the sweet spot. The place where, it's just hot enough your secondary burn is staying constantly on, and you've slowed the exhaust as much as possible maximizing the amount of time the heat takes to leave so it transfers the maximum amount of heat into the living space before heading out the flue. For me it's somewhere around 30-40% and differs for each load. The loads that are more compact, I need the air a little higher. Ones that are rather free I can move the air handle lower.

So, I say it's more like #2 or #3 and it takes a long time to learn. When starting from cold, I leave mine on max air until I see the flames are looping around the baffle up top which is usually about 5 minutes after the kindling gets moving. Then, I lower it to 75% for about 10 minutes to let it adjust. I then cut it back to about 50% for the next 20-30 minutes and will come back to check up on it to see if I should lower it to 30 or 40% depending on how things are going. The 30-40% is an overnight burn for me, as coming back 8 hours later at that setting I will usually have gobs of hot embers.

 

[DiscoInferno]

My Ultima manual lists high, med, and low settings that are essentially your #'s 1,2,3. They give approx. respective burn times as 1-2 hours, 2-4 hours, and 6-8 hours. I guess those numbers for #2 and #3 are not too far off for full loads of good hardwood. #1 is useful for burning down a large coal bed with a couple of small splits, but would not seem to be a great idea with a firebox packed with dry wood. I'm not sure if this is typical, but all the "action" on my air control is at the mostly-closed end. Big differences from closed to say 15-20%. After that, it burns like it's close to wide open.

 

[Gunner]

Dont forget about number 1. Some stoves have such poor draft that they will only run wide open. The level of air you supply the secondary burn system is directly porportional to the amount of chimney you have to draw the air through the system....
As far as where you put the air, lets say you have strong draft, and you turn them down at the same stage of wood cumbustion as well. I would hypothise (is that a word??) that you would get 50% longer burn if your air was set at a 1/4" open vs 1/5" open.

Spent the weekend at a friends cabin that had a stove, draft sucked so bad the door had to be cracked the whole time, dampering it down was closing the door and leaving the air WIDE open. It was pathetic.

 

[KarynAnne]

[quote author="DiscoInferno" date="1169099240"] They give approx. respective burn times as 1-2 hours, 2-4 hours, and 6-8 hours.


THAT'S what I was after - thanks Disco.

 

[webbie]

That's one of the great things about wood - it does not respond to hypothetical scenarios!

MSG has it right - depending on the weather, the wind, the barometric pressure and the chimney a stove might work "normally" in one scenario, but not work as planned in another. Example: a strong warm chimney will tend to work best with the least relative air input, while a short cold one will seem "normal" only with the wider open situation.

 

[BrotherBart]

That's one of the great things about wood - it does not respond to hypothetical scenarios!

MSG has it right - depending on the weather, the wind, the barometric pressure and the chimney a stove might work "normally" in one scenario, but not work as planned in another. Example: a strong warm chimney will tend to work best with the least relative air input, while a short cold one will seem "normal" only with the wider open situation.

Kinda seems like Willie Nelson was talking about wood when he sang "I may not be normal, but nobody is."

 

[begreen]

I think this is a great question KarynAnne. Been wondering the same thing as well. With option #1 - we'll see about 2 hr burn time. (With our soft maple the results are not too heartenting.) We mostly follow step #2 in which we get about 1.5hrs a@ 550, 1 hr @ 400 and lower after that. With hardwood we see about 2 hrs @ 550-600, 2 hrs tapering down to 350. 2hrs more tapering down to 200. Option #3 extends the burn time but reduces the peak temps by about 50-100 degrees.

What are you seeing with the Oslo?

 

[KarynAnne]

I think this is a great question KarynAnne. Been wondering the same thing as well. With option #1 - we'll see about 2 hr burn time. (With our soft maple the results are not too heartenting.) We mostly follow step #2 in which we get about 1.5hrs a@ 550, 1 hr @ 400 and lower after that. With hardwood we see about 2 hrs @ 550-600, 2 hrs tapering down to 350. 2hrs more tapering down to 200. Option #3 extends the burn time but reduces the peak temps by about 50-100 degrees.

What are you seeing with the Oslo?

BG - You've got it...this is the information I was trying to get at. I'm not sure about the Oslo, but I'm trying to come up with something similar to what you've reported. I know that barometric pressure, etc will always make a difference. You can't really come up with answers in a laboratory. But the gist of what I'm trying to find out is exactly what you reported. Thanks.