BTU for a pound of Wood?

[Rockey]

Ok, so the BTU for a pound of wood at 10% MC is going to be XXXX BTU's. For arguments sake lets say it is 7000 BTU's. My real question is - does this valus take into consideration the BTU's from secondary combustion? How is the value derived?

 

[begreen]

The BTU value is not based on burning it in the woodstove. it's the basic energy content of the particular wood. Secondary combustion affects the efficiency of the stove. That is - if you put a pound of wood in the stove to burn, how much of this is returned to the room and how much up the stack as unburnt gases, heat, moisture.

 

[Rockey]

The BTU value is not based on burning it in the woodstove. it's the basic energy content of the particular wood. Secondary combustion affects the efficiency of the stove. That is - if you put a pound of wood in the stove to burn, how much of this is returned to the room and how much up the stack as unburnt gases, heat, moisture.

I understand. The BTU rating is the amount of energy in a given fuel. By definition a btu is the amount of energy needed to raise a pound of water 1 degree farenheit. Since a pound of wood (at a certain MC) has been given a BTU rating, does this include the amount of energy in the smoke it gives off also. I'm going to assume that the answer is yes.

 

[begreen]

Yes, I would think so too.

"One pound of very dry (zero moisture content) wood of any species has a calorific value of approximately 8,600 Btu (British thermal unit, which equals the amount of heat required to raise the temperature of one pound of water one degree F). Any moisture in the wood reduces the recoverable heat by carrying heat up the chimney during vaporization. Each pound of water vaporized uses about 1,200 Btu.

Additional Btu are lost through the formation of volatile liquids and gases during combustion, but these vary by the type of heating unit and should be considered part of the efficiency factor of the heating unit.

A pound of wood with a 20-percent moisture content contains 0.17 pound of water and 0.83 pound of completely dry wood and has a heat value of about 7,000 Btu. This is the base figure used in the heating comparisons made throughout this publication."
http://extension.missouri.edu/explore/agguides/forestry/g05450.htm

 

[thechimneysweep]

Hey Rocky,

Here's the way I've always understood it. If a pound of wood at a given moisture content contains 7,000 btu's of potential heat energy and you burn it in a woodstove rated at 70% overall efficiency, 4,900 btus will be delivered to the room (70% of 7,000 btu), and the remaining 30% (2,100 btus) will either not be extracted during the combustion process, or lost up the flue.

If a given stove develops good secondary combustion, more heat will be extracted from the wood. To the extent that this additional heat is transferred to the room, a higher overall efficiency rating will result. This is one reason today's EPA approved secondary burners achieve higher heating efficiency scores than pre-EPA models.

 

[Rockey]

Hey Rocky,

Here's the way I've always understood it. If a pound of wood at a given moisture content contains 7,000 btu's of potential heat energy and you burn it in a woodstove rated at 70% overall efficiency, 4,900 btus will be delivered to the room (70% of 7,000 btu), and the remaining 30% (2,100 btus) will either not be extracted during the combustion process, or lost up the flue.

If a given stove develops good secondary combustion, more heat will be extracted from the wood. To the extent that this additional heat is transferred to the room, a higher overall efficiency rating will result. This is one reason today's EPA approved secondary burners achieve higher heating efficiency scores than pre-EPA models.

That all makes sense to me. I think the efficiency of an insert is a bit misleading though. Although its effieciency rating is about the same as a stove, a good deal of that heat that is extracted will be lost out the back top and sides. I just find it hard to believe that 70% of the heat is going into my house and only 30% up the chimney.

 

[webbie]

Neither inserts nor stoves are actually tested for efficiency using standard tests (cordwood, etc.)....

Therefore, all numbers are either defaults (EPA defaults) or marketing-speak. It is very safe to assume that your "mileage" will be substantially less than most claims because manufacturers or dealers have no reason to understand things.

Inserts are often just a version of the same stove with panels around it. Therefore the same default or marketing numbers are usually used. Even if one manufacturer provided some sort of real numbers, it would not mean anything since the other makers cannot be accurately compared - the only real way to do this would be an unaffiliated independent lab that was not paid by the manufacturers.

 

[Rockey]

What are " EPA defaults"? Where do they get these numbers? I was under the impression that the stoves were actually tested just like automobiles are actually tested for mpg.

 

[webbie]

EPA defaults are just numbers that EPA allows them to put on there without any testing.
No, they are not tested in any way, shape or form for actually heat delivery efficiency. EPA is only concerned that they burn clean using 4x4's and 2x4's with spacer nailed around the ends.

You should be able to find a number of older threads about this here - reports from the EPA and the test labs pretty much say that manufacturers have no reason to test for efficiency nor to improve their stoves (other than lowering warranty claims).

Someday...when the market is big enough or when an independent lab or person figures out how to get paid for it, there will be some real world comparison tests with firewood. But don't hold your breath. As the test lab report says, there is really no reason (now) for manufacturers to either collect or share this data.

 

[BrotherBart]

The BTU value is not based on burning it in the woodstove. it's the basic energy content of the particular wood. Secondary combustion affects the efficiency of the stove. That is - if you put a pound of wood in the stove to burn, how much of this is returned to the room and how much up the stack as unburnt gases, heat, moisture.

I understand. The BTU rating is the amount of energy in a given fuel. By definition a btu is the amount of energy needed to raise a pound of water 1 degree farenheit. Since a pound of wood (at a certain MC) has been given a BTU rating, does this include the amount of energy in the smoke it gives off also. I'm going to assume that the answer is yes.

The answer is yes. The only thing that burns and gives off BTUs is smoke. The wood gets hot, gives off gasses/smoke and that is what burns. Or goes up the pipe if not burned.

 

[Andre B.]

Any energy content test results should have two values, the high heat value (HHV) and the low heat value (LHV). The HHV includes the latent heat of vaporization of the water in the wood and the LHV does not.
Almost all solid fuel burning systems are limited to the LHV. There are some natural gas, propane, and oil burners that operate in condensing mode which allows them to use the HHV of the fuel.

There have been some condensing wood stoves but they tend to be very high maintenance systems with corrosion and black water disposal problems.