Wood BTU

[Randy Acton]

I see a lot of charts that rate different woods and respective BTU content. What I don't see much of is how that number is derived.

If I look at a chart and it says (for conversation purposes only) that 1 lb of wood = 8,000 BTU. Does that assume 100% combustion or the combustion of the stove which the test was performed?

Since most stoves won't reach temps for 100% combustion, does that mean that the actual BTU output will vary depending combustion efficiency?

 

[johnpma]

Honestly it's all jibber jabber as far as I'm concerned....was just thinking about all this last night. I'm burning wood I cut split and stacked under blue tarps. I cut the wood in April, and it is burning great and cranking out the heat. I really wonder if 3 year seasoned wood would really give that much more????

I think when stove builders rate the burn BTU's it's done in a lab somewhere with the most ideal conditions which IMO is unrealistic

 

[gzecc]

Its all relative. Some trees come down with little moisture left in them. Some with a lot. Its best to bury your head in the sand and just burn it all as soon as you can.

 

[nyny]

the most reliable is the scientific weight per cubic foot. everything else is all over the map

 

[Paulywalnut]

Honestly it's all jibber jabber as far as I'm concerned....was just thinking about all this last night. I'm burning wood I cut split and stacked under blue tarps. I cut the wood in April, and it is burning great and cranking out the heat. I really wonder if 3 year seasoned wood would really give that much more????

I think when stove builders rate the burn BTU's it's done in a lab somewhere with the most ideal conditions which IMO is unrealistic

I have really noticed this year, I'm burning 3 year seasoned ash and the fire in the stove is so hot and clean. The secondaries are going full steam. All three rows in the Jotul stove. Crystal clear glass on the door too. I think it is a blurry line with BTU output though. I do like that dry seasoned wood though.

 

[BrotherBart]

Honestly it's all jibber jabber as far as I'm concerned....was just thinking about all this last night. I'm burning wood I cut split and stacked under blue tarps. I cut the wood in April, and it is burning great and cranking out the heat. I really wonder if 3 year seasoned wood would really give that much more????

I think when stove builders rate the burn BTU's it's done in a lab somewhere with the most ideal conditions which IMO is unrealistic

Yeah I did that for thirty years. And the folks here convinced me to let it dry for two or three. World of difference and cut my wood consumption by a third. Three years cut it in half.

Do what ya wanna do. I don't need to work that hard to stay warm.

 

[red oak]

My understanding is the charts show total BTUs. Some of those would go to burn off the moisture in your wood, so not all of those BTUs would be felt as heat in the house. I think the charts show 100% efficiency, which I for one do not have.

 

[Sprinter]

Since most stoves won't reach temps for 100% combustion, does that mean that the actual BTU output will vary depending combustion efficiency?

That's right. All wood has the potential of delivering about 8600 BTU per pound (more like 9000 BTU for some softwoods because of the extra resins) under complete combustion. Obviously no stove does this, so efficiency comes to play. All modern stoves with some form of secondary combustion are quite efficient and the differences are relatively minor. The greatest factors are under the control of the operator involving burning technique and moisture control.

 

[Sprinter]

I really wonder if 3 year seasoned wood would really give that much more????

It's not just a matter of time, but the moisture content of the fuel when burned, and that depends on several factors like species, relative humidity, air flow, etc. For some soft species and under good drying conditions, it is possible to burn efficiently after only 6 or 7 months but that experience seems to be pretty rare. Pine in Arizona comes to mind. But some species like oak are notoriously difficult to dry and can indeed take 3 years depending on the drying conditions.

The only way to know if your wood is really ready is by use of a simple and cheap moisture meter. All modern stoves are designed for maximum efficiency with wood at around 20% moisture content and that factor is a huge one. Stoves just lose so much heat trying to burn water...

 

[jebatty]

The "jabber" is based on a set of assumptions which often are not identified, those being moisture content (20%) and interior stack temperature (about 400F). From a wood chart, take the btu/cord of any species and divide it by the weight shown in the chart for a cord of that species. Do the same for several other species. You will end up with approximately the same btu/lb content for all species, and maybe slightly higher btu/lb for the pines (due to resin content). One chart I use ended up with about 6200 btu/lb regardless of species. The chart in Hearth.com works out to about 6400 btu/lb. For my analysis purposes, I use 6050 btu/lb which was derived from a careful wood burning study.

Drier wood = higher btu/lb and vice versa. Lower stack temp = higher btu/lb and vice versa. High efficiency wood gasification stoves/boilers can achieve stack temperatures down to about 250F without condensation, and stack temperature during a burn, including high burn, could go up to about 400-500F. My Tarm wood gasification boiler will have stack temperature in the 350-440F range pretty consistently, others achieve a lower stack temperature. My wood stove in the house has stack temperature in the 600-800F range during a good burn. Corresponding efficiencies would be roughly in the 80-85% range for the Tarm and about 70% for the wood stove.

The 20% moisture content assumption is based on the equilibrium moisture content of wood left to dry outside under "normal" drying conditions to the point where essentially no more water will evaporate from the wood, in other words, well seasoned wood and usually one to two years of outside drying. Of course, this varies by region and local conditions, by species, and often from split to split, so the 20% is an average. The 400F stack temperature is about the stack temperature that results form a reasonably efficient wood stove burning 20% moisture content wood, and of course this varies among stoves and local conditions, so it too is an average.

The key point is that all wood of the same moisture content has about the same btu/lb, and if you want to get fancy in analyzing your wood burning, do weighed wood burns, measure the moisture content of your wood, and get the interior stack temperature using a good probe thermometer. If your measurements come close to the assumptions and to the computed btu/lb from the chart, then you will have discovered your stove/boiler output over a period of time based on the weighed amount of wood you have burned.

 

[Woody Stover]

I'm burning wood I cut split and stacked under blue tarps. I cut the wood in April, and it is burning great and cranking out the heat.

What stove are you burning, an old smoke-bomber, or was the wood already dry when you cut it?

 

[Ram 1500 with an axe...]

Honestly it's all jibber jabber as far as I'm concerned....was just thinking about all this last night. I'm burning wood I cut split and stacked under blue tarps. I cut the wood in April, and it is burning great and cranking out the heat. I really wonder if 3 year seasoned wood would really give that much more????

I think when stove builders rate the burn BTU's it's done in a lab somewhere with the most ideal conditions which IMO is unrealistic

All I can say is now that I'm burning wood that has been split and single stacked in the sun and wind for over 2+ years! I am seeing a big difference in my insert output, it is easier to heat up the space and I'm using less wood....
The 3 year plan is the way to go......

 

[Maul4life]

I tend to burn by percentage of moisture. While I read a lot about getting under 12%, I regularly burn in the high teens. I do notice a big difference in heat output. The drier wood burns a lot warmer....and also less ash. At the lowest airflow setting on my stove, the 20% moisture content will not burn up all the way. The 12% wood burns completely away.
I don't think I am adding much to this thread, but just posting my observations.