Max BTU/hr output from a BK

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SolarAndWood

SolarAndWood

Minister of Fire
Feb 3, 2008
6,788
Syracuse NY
I live on a windy ridge with a lot of glass and have to push the BKK to keep up when its cold and blowing. Last year, the best I could do was get a 12 hour burn cycle with good dry hardwood. Burn cycle to me is reload to reload in a manner that can be sustained for 7 months. With set it and forget it, max output over the 12 hour burn seemed to be setting the tstat @ 3 and the blowers on low. Then, 8 hours in, opening up the tsat all the way for the last 4 hours.

During the recent cold snap, I have reduced this to 8 hours by abandoning the set it and forget it convenience of the tstat and instead micro managing. My new scheme is something like tstat @ 3.25 blowers on high for the first hour or so. Then, gradually increase the tstat to wide open to maintain high stove temp. Then, start reducing the blowers to maintain stove temp. Adjustments every 1/2 hour seem to be adequate. By the time I turn the blowers off, what is left gets raked front center and gets a 1/2 hour or so to heat up for the next reload.

My highly scientific hand on the double wall stove pipe measurement technique says that the heat is not going up the pipe. Has anyone else tried this that has a flue probe? Yes Nate, I know that if I am running this hot, or for that matter above 1 on the tstat, I should be overheating the joint.
 
When I'm home, or the wife is home, the stove is getting new wood every 4-6 hours. Smaller loads, maybe 4-6-splits and T-stat no lower than 2.75. Then wide open for the last hour or two, fans on high. I can't load more than half full with this pine without having to keep the stat at 2.75 max. or it'll fry the cat. The smaller loads for the last few weeks have been working out quite well. Now the temps. are in the 50's for the next few days, so I can load it up and turn the t-stat down a bit, load twice a day at most.
 
I concur to what you are saying.
I more or less did the same thing on a few real cold days around 0 and windy and a little below around here.
But usually I'm on 2 or just a little more.
On real cold nights I'll leave the blower on low overnight..kicks the thermostat better.
 
But yeah..sometimes before reload I can bring the coals forward and get a few more hours of some really good heat full open on thermostat.
 
Beetle-Kill said:
I can't load more than half full with this pine without having to keep the stat at 2.75 max. or it'll fry the cat.
Click to expand...

Is that true after an hour? Or is it just the initial outgas you have to get over before you can open up the tstat? I can't run wide open for at least 2 hours after a reload either even with hardwood and the blowers wide open without smelling hot stove. Again, I'm not talking most efficient burn or least amount of work, just max heat per hour and have it end up in the house and not out the pipe.
 
HotCoals said:
But yeah..sometimes before reload I can bring the coals forward and get a few more hours of some really good heat full open on thermostat.
Click to expand...

Wait til April, that will heat the house for 24 hours.
 
Solar- what's been working good has been 2-3 split N/S over the coals, then 2-3 E/W on top. The bottom splits will be on fire before I finish placing the top ones. I'll have active flame for the first few hours, then it will ghost for awhile, then break up the chunks to let it burn down a bit and repeat. I'm not trying achieve anything but max. heat. It worked pretty good, even when it dropped to -31 one night. Inside of stove and flue/cap are all much cleaner than before, but that was my fault.
 
Not much to add, but two observations ...

SolarAndWood said:
My new scheme is something like tstat @ 3.25 ...
Click to expand...
I'm not sure how realistic it is to compare fractional thermostat settings with other owners on the forum. My stove came with the thermostat knob removed. I put it on somewhere that seemed reasonable. Although I have a very clear idea of how my thermostat settings work for me, I don't know how they compare with others.

My highly scientific hand on the double wall stove pipe measurement technique says that the heat is not going up the pipe. ...
Click to expand...
I'm not sure how you can tell that. One thing I'm pretty sure of, you're sending more heat up the pipe with double-wall. I went with single-wall. I have to be a little careful when re-loading when the fire isn't burnt down to coals; if I just fling the door open, it pulls some smoke into the house. But it's worth it to me. If I let the fire burn to coals like you're normally supposed to, it's a total non-issue; but occasionally you want to add a log or two before beddy-bye. BTW, BK told me that the double-wall rec is more or less CYA (from folks complaining of their stove leaking smoke into the house during reloads), and that if my stove works with single-wall, and they're not surprised it does, they got no problem with it.

It might be interesting, if we tried to get an idea of how much heat is lost with double-wall. I guess we'd need folks with IR thermometers to try to report the outer-surface pipe temps they see with both types of pipe under similar conditions, then use some physics to figure the difference in BTUs/hr radiated.

I made a new thread out of some calculation I did on this ...

https://www.hearth.com/econtent/index.php/forums/viewthread/70976/

The bottom line I got is that a typical 6" connector pipe is putting about 1K/3.5K/10K btu per hour of heat into the room (from radiation alone, never mind convection) if it is 50/100/200 degrees centigrade hotter than a typical room.
 
Beetle-Kill said:
I can't load more than half full with this pine without having to keep the stat at 2.75 max. or it'll fry the cat.
Click to expand...

Is that according to the BK probe, or the Condar?
 
RustyShackleford said:
I'm not sure how realistic it is to compare fractional thermostat settings with other owners on the forum.
Click to expand...

I agree. Especially when it comes to Ultras vs Non-Ultras I think there is a big difference. My guess is they use the same tstat which I assume means the Ultra puts out less heat for a given tstat setting?


My highly scientific hand on the double wall stove pipe measurement technique says that the heat is not going up the pipe. ...
I'm not sure how you can tell that. One thing I'm pretty sure of, you're sending more heat up the pipe with double-wall. I went with single-wall. I have to be a little careful when re-loading when the fire isn't burnt down to coals; if I just fling the door open, it pulls some smoke into the house. But it's worth it to me. If I let the fire burn to coals like you're normally supposed to, it's a total non-issue; but occasionally you want to add a log or two before beddy-bye. BTW, BK told me that the double-wall rec is more or less CYA (from folks complaining of their stove leaking smoke into the house during reloads), and that if my stove works with single-wall, and they're not surprised it does, they got no problem with it.

It might be interesting, if we tried to get an idea of how much heat is lost with double-wall. I guess we'd need folks with IR thermometers to try to report the outer-surface pipe temps they see with both types of pipe under similar conditions, then use some physics to figure the difference in BTUs/hr radiated.

I made a new thread out of some calculation I did on this ...

https://www.hearth.com/econtent/index.php/forums/viewthread/70976/

The bottom line I got is that a typical 6" connector pipe is putting about 1K/3.5K/10K btu per hour of heat into the room (from radiation alone, never mind convection) if it is 50/100/200 degrees centigrade hotter than a typical room.
Click to expand...

So, it would seem the double wall is worth it if the stove can transfer all the heat except what is required to maintain draft. My experiment over the past few weeks has been what is the mix that pulls the most heat off the stove without cooling the firebox. I think you are probably right that I may have left some heat on the table or up the pipe. That is why I am curious if someone has tried this with a flue probe instead of my highly scientific leather glove on the double wall measurement. I may pick up some single wall, I never get spillage and it probably would not matter in the middle of winter either way.
 
SolarAndWood,
Add a damper if you do the single wall if you really want to play around.lol.
 
Even with a straight up 8" cannon, I have never felt the need for a damper. These stoves have great burn control.
 
SolarAndWood said:
Even with a straight up 8" cannon, I have never felt the need for a damper. These stoves have great burn control.
Click to expand...
No doubt.
I have played with the by pass open and let her rip then adjusted the damper and air to see if i could get it to burn real hot like the old bk non cat..I could.
Only did it that one time.
I guess you're not supposed to let the flames lick the cat to much?
 
SolarAndWood said:
So, it would seem the double wall is worth it if the stove can transfer all the heat except what is required to maintain draft.
Click to expand...
Hmm, I'm confused. It seems to me that since double-wall will definitely be cooler on the outside, and my Stefan-Boltzmann calculation showed that it therefore radiates less significantly less heat into the house; so more heat is going to go up the chimney and therefore be wasted. Unless less heat goes into the chimney in the first place, but I don't quite see why that would be (?)

So I'd say that single-wall is better *IF* you can get away with it. Three reasons you might not be able to get away with it: 1. clearances to combustibles, 2. the stove leaking air out the door during reloading, or 3. the stove can't maintain a decent draft on low burns (I guess you can tell this because the cat goes inactive).
 
HotCoals said:
SolarAndWood,
Add a damper if you do the single wall if you really want to play around.lol.
Click to expand...

And if you really have time to burn, and want to see the effects of the fiddling you do with your stove...:

http://books.google.com/books?id=kQ...esnum=2&ved=0CBoQ6AEwAQ#v=onepage&q=measuring wood stove efficiency&f=false

Not sure if the above link is going to paste ok. You may have to copy and paste it into your browser to get it to work. It's a little dated, but would be a good way to see the effects of single vs double stove pipe, for instance.
 
sesmith said:
HotCoals said:
SolarAndWood,
Add a damper if you do the single wall if you really want to play around.lol.
Click to expand...

And if you really have time to burn, and want to see the effects of the fiddling you do with your stove...:

http://books.google.com/books?id=kQ...esnum=2&ved=0CBoQ6AEwAQ#v=onepage&q=measuring wood stove efficiency&f=false

Not sure if the above link is going to paste ok. You may have to copy and paste it into your browser to get it to work. It's a little dated, but would be a good way to see the effects of single vs double stove pipe, for instance.
Click to expand...

Man,PS really gets into way to deep for me..lol.
 
RustyShackleford said:
Unless less heat goes into the chimney in the first place, but I don't quite see why that would be (?)
Click to expand...

I agree. Unless you have draft issues, which I don't, the only time I see this being an issue is at very low burn in very moderate temperatures. I doubt that any stove can extract all the heat minus what is needed to maintain draft especially over the range of output the BK is capable of. It would seem it is time to give single wall a try.
 
SolarAndWood said:
RustyShackleford said:
Unless less heat goes into the chimney in the first place, but I don't quite see why that would be (?)
Click to expand...

I agree. Unless you have draft issues, which I don't, the only time I see this being an issue is at very low burn in very moderate temperatures. I doubt that any stove can extract all the heat minus what is needed to maintain draft especially over the range of output the BK is capable of. It would seem it is time to give single wall a try.
Click to expand...

Seems like a lot of trouble if you already have double-wall installed. I'd want to try to figure out what the temperature of the pipe exterior will be with single- and double-wall pipe, for similar burn conditions, and then go back to that calculation I did and see how many btu/hour it's gonna gain you. Could be a lot, maybe not so much; wish somebody would check my math. I think it's only be a real issue on pretty high burns, but that's important, that's when you're wanting to get as much heat as possible into your house.
 
I just know that on a long cruise my stack(single wall) 8 inches up is usually 200°..stove top just in front of cat gauge 350.
If I'm burning 525 stove top my stack at the same point is usually 350.
I use a ir gun.
 
HotCoals said:
If I'm burning 525 stove top my stack at the same point is usually 350.
I use a ir gun.
Click to expand...
If you're talking Fahrenheit, that 350 is 450 Kelvin. If we assume your room is 300 K (80 F), the formula (in the thread I linked) says your stove pipe is radiating about 6400 btu/hr into the room. *IF* your pipe has about 1 square-meter of surface area, and that temperature is pretty uniform along the height of the pipe (seems like it would be to me), and the emissivity really is about 1, and we ignore convective effects.

That's a pretty significant amount of heat, I'd say.

P.S. In that other thread, I put a clip from a spreadsheet showing heat loss of varying surface temperature and length of pipes.
 
HotCoals said:
I just know that on a long cruise my stack(single wall) 8 inches up is usually 200°..stove top just in front of cat gauge 350.
If I'm burning 525 stove top my stack at the same point is usually 350.
I use a ir gun.
Click to expand...

How much stove pipe do you have before you hit the chimney? What does the temperature gradient look like as you go up and how does a hot burn effect it? What happens to the stove top/pipe differential when you vary the blowers and the tstat independently? I might have to borrow an IR gun from work.
 
RustyShackleford said:
That's a pretty significant amount of heat, I'd say.
Click to expand...

The problem with this game is it is tough to isolate the variables especially over all the conditions the system is going to see through the course of the season. Operationally, my setup makes it very easy to burn 24/7/210. I'll have to get a gun and monitor it for a while. If my system is too conservative and I am leaving heat on the table, I would consider removing the side panels as well as the double wall.
 
Beetle-Kill said:
Solar- what's been working good has been 2-3 split N/S over the coals, then 2-3 E/W on top. The bottom splits will be on fire before I finish placing the top ones. I'll have active flame for the first few hours, then it will ghost for awhile, then break up the chunks to let it burn down a bit and repeat. I'm not trying achieve anything but max. heat. It worked pretty good, even when it dropped to -31 one night. Inside of stove and flue/cap are all much cleaner than before, but that was my fault.
Click to expand...

Are you going the small hot fire route or are you filling the box?
 
Small hot fire route. Not even big splits. I cleaned the stove a few days ago, now with 6 splits in it, they almost sit below door line. looks cool from 20' away, no wood visible, just flame.
 
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