Firebox temp to BTUs

[Wolves1]

Does anybody know if there is a chart that gives an idea about temperature inside stove using cat thermometer converted to BTUs?

 

[DougA]

I don't think they relate. A 4 cu ft firebox will be generating a ton more BTU than a 2 cu ft. at the same temperature.
There are charts that will give you BTUs for each species of wood. Then, find out the efficiency of your stove and do the math. You will be in the ballpark but most stoves will give you optimum or maximum BTUs on their specs, regardless of wood.

 

[brad wilton]

I wouldn't think fire box size would change btu's two pieces of wood will give of same btu regardless of stove size

 

[DougA]

Maybe I missed the point of his question. I am assuming you would fill a stove full.

 

[brad wilton]

true never mind

 

[iamlucky13]

The short answer is no.

The slightly longer answer is it varies by stove and fan use and determining the correlation is a fairly advanced heat transfer calculation worthy of an engineering grad school project.

DougA's method can get you a halfway decent estimate with much less effort. Weigh your load, calculate the BTU content based on the wood (usually about 8000 BTU/pound for properly seasoned wood), and measure your burn time. Combined with the stove's rated efficiency you can calculate a rough estimate of the average output over that time. With a little more work, you can take temperatures at different points in the burn for multiple loads to get some averages and estimate the kind of correlation you're asking about.

 

[Jan Pijpelink]

Does anybody know if there is a chart that gives an idea about temperature inside stove using cat thermometer converted to BTUs?

http://forestry.usu.edu/htm/forest-products/wood-heating

 

[rideau]

I believe the theoretical BTU for 100 % dry hardwood is about 8000 BTU per pound. Thus even very dry hardwood (say 12 % moisture content) is only 88 % wood. .88*8000 = 7040 BTU theoretical output at 100 % efficiency. But you have to take into account the BTU's lost to evaporation of the water. I believe the figure generally used is a potential of about 6500 BTU's from a "dry" pound of wood, and that is the figure you have to multiply by the efficiency of your heating device. Even the few most efficient wood stoves heat at only about 90% efficiency, so .9 * 6500 = 5850 BTU output per pound of dry hardwood burned. My figures may be off a bit, but you get the idea. Best to figure a maximum BTU production of about 6000 BTU's per pound of dry hardwood.

A 63% efficient stove , maybe 4100 BTU' s output per pound (EPA default efficiency rating on secondary burn stoves)
A 72 % efficient stove, maybe 4680 BTU's output per pound (EPA default efficiency rate on catalytic stoves).
An 80% efficient stove, maybe 5200 BTU's output per pound.
I think this is fundamentally right.

Am I wrong?

 

[rideau]

And, if I am correct, you can clearly see the difference in potential heat output between the new hybrid stoves and the BK's rated at 90%, and a 63% efficient stove: 5850 BTU's vs 4100 per pound: 40% more heat into the home. Thus the impressive heat output these stoves claim, and owners observe. Firebox size isn't by any means everything.

 

[Wolves1]

Love all the info thank you. I have a digital thermometer that reads the cat temp and wanted to know if the stove temp is ex 800 how many BTUs it was delivering.

 

[Oldman47]

Love the all the info thank you. I have a digital thermometer that reads the cat temp and wanted to know if the stove temp is ex 800 how many BTUs it was delivering.

I think the methods outlined above were intended to give you a way to generate your own correlation for your stove. Since every cat is not the same, cat temperature will not be the same BTU/hr from stove to stove. There might be a way to relate cat temperatures to BTU/hr if you know lots of other factors such as cat volume, cat thickness front to back, and cat temperature profile.

To me it sounds like a good master's thesis for a college kid. Maybe they could show a simple relationship we do not know about that would allow folks to estimate the expected performance of any cat based on its face temperature or the probe temperature.

As far as moisture impacts, I call them penalties, that rideau refers to, don't forget the huge penalty associated with actually boiling that water at around 970 BTU/lb of water. A couple of ounces of water in a pound of wood is stealing about 120 BTU just to turn to steam. That is 12% moisture wood, not wet wood. It also averages about 1 BTU/lb to get a 1 degree rise in temperature and you pay for that all the way from room temperature to 212ºF so figure at least another 140 degrees from 72ºF to 212ºF. That is another 17 BTU/lb for that 12% moisture wood. I would subtract at least 140 BTU from the numbers that rideau came up with.

 

[Wolves1]

I think the methods outlined above were intended to give you a way to generate your own correlation for your stove. Since every cat is not the same, cat temperature will not be the same BTU/hr from stove to stove. There might be a way to relate cat temperatures to BTU/hr if you know lots of other factors such as cat volume, cat thickness front to back, and cat temperature profile.

To me it sounds like a good master's thesis for a college kid. Maybe they could show a simple relationship we do not know about that would allow folks to estimate the expected performance of any cat based on its face temperature or the probe temperature.

As far as moisture impacts, I call them penalties, that rideau refers to, don't forget the huge penalty associated with actually boiling that water at around 970 BTU/lb of water. A couple of ounces of water in a pound of wood is stealing about 120 BTU just to turn to steam. That is 12% moisture wood, not wet wood. It also averages about 1 BTU/lb to get a 1 degree rise in temperature and you pay for that all the way from room temperature to 212ºF so figure at least another 140 degrees from 72ºF to 212ºF. That is another 17 BTU/lb for that 12% moisture wood. I would subtract at least 140 BTU from the numbers that rideau came up with.

Thank you, may be a project one day to figure it out I was hoping somebody did the work already.

 

[Oldman47]

Do you know any engineering students that need a post grad project for a masters or doctorate? Why not suggest it to them? It certainly fits the typical required profile of "adding to existing knowledge".

 

[Wolves1]

Unfortunately I don't but my 12-year-old son says he wants to be an engineer so I can wait to goes to college .