The heat value of wood per unit of weight is about the same for all species: 8,600 Btu per pound, ovendry weight. Exceptions to this rule are very resious species, which have slightly higher values. The heat value of hardwood bark is slightly lower than that of wood, while the energy content of pine bark is slightly higher.

The heat value of wood per unit volume, at any given moisture content (MC), depends on its specific gravity or relative density. The higher the specific gravity (SG) of a wood, the denser the wood per unit of volume, and the higher its heat value.

The table below groups common tree species by relative density classes. Each class represents a different range of specific gravities of wood, and each class has been assigned an average specific gravity factor. This factor is used to calculate the approximate amount of heat energy available in a given volume of dry wood.

To calculate the GHV of a cord of dry wood, two assumptions are first made:
1. A cord(CD) of dry wood contains 80ft³ of solid wood.
2. Each pound of ovendry wood, regardless of species can produce 8,600 Btu of heat energy.

Relative density classes, based on specific gravity, for common tree species, with average specific gravity factors.

Specific gravity is based on ovendry weight and volume at 12% moisture content. The average specific gravity factors shown here were arbitrarily chosen to represent all species within a certain density class.

The formula for gross heat volume is:

GHV = SG factor X [6.23 lb./ft³(wt. of water)X 80 ft³/CD. X 8600Btu/ovendry lb.]

[6.23 lb./ft³(wt. of water)X 80 ft³/CD. X 8600Btu/ovendry lb.] = 42,862,400 Btu/cord.

This formula is used to derive approximate GHV's‡ for each of the three density classes:

   Low Density Woods: 
      GHV = 0.45 X 42,862,400 Btu/cord = 19.3 million Btu per cord. (MMBtu/CD.)
      
   Medium Density Woods:
      GHV = 0.55 X 42,862,400 Btu/cord = 23.6 MMBtu/CD.
   
   High Density Woods: 
      GHV = 0.65 X 42,862,400 Btu/cord = 27.9 MMBtu/CD.
      

Use the following table to provide a more precise GHV, if a species' average specific gravity is known.

Gross heat value of ovendry wood by specific gravity

‡These GHV's a re only approximations of Btu's available from a cord of dry wood of a species group. If a more precise heat value is needed for a particular species, the average SG of that species must be used in place of the density class SG factor. SG values for most woods can be found in The Wood Handbook (USDA FS, For. Prod. Lab. Ag Hdbk No. 72)

For example, if we knew the average SG of balsam fir were 0.36, we could use the preceding table to determine a more exact GHV for a dry cord of balsam fir: 15.4 million Btu. This figure is certainly a better approximation than the 19.3 milion Btu derived through the use of the first and the generalized GHV formulas for broad density classes.

Note that the GHV's calculated by the density class method are only approximations; they are meant to provide relative values for large groups of woods. More realistic GHV's can be derived, however, by using the same general formula and by substituting a more precise SG value for the standard SG value shown in the first table.

Also not, at this point, that gross heat values represent the maximum Btu values for ovendry wood. To find the actual, or net, heat value of a cord of green or partly dried wood, other factors must be considered, such as the amount of water in the wood and the combustion efficiency of the device in which the wood is to be burned.

Heat Value of Green Wood All wood except that artificially dried in the laboratory contains some amount of water, commonly expressed as moisture content (MC). Though there are two accepted methods of calculating MC, most wood technologists calculate MC ont he basis of ovendry weight (ODWT), as follows:

MC% = (Green weight-ODWT)/(ODWT) X 100 [Equation 1]

In the above equation, the green weight (GWT) and ODWT of a given volume of wood must be known. If the MC and ODWT of a cord of wood are known, GWT can be calculated as follows:

GWT = ODWT(1+(MC%/100)) [Equation 2]

Using the same relationships, and knowing both GWT and MC, ODWT can be found this way:

ODWT = (GWT/(1+(MC%/100)) [Equation 3]

if MC, ODWT, and GWT of a cord are not known, but the SG of that wood is, we can calculate ODWT another way:

ODWT/CD = SG X 6.23lb/ft³ X 80ft³/CD. [Equation 4]

All of these equations are helpful in calculating the low heat value (LHV) of wood. This value is defined as the amount of net energy available in a given volume of wood after accounting for the energy needed to evaporate the water in the wood, but before accounting for the the combustion efficiency of the device in which the wood is burned. After determining LHV, combustion efficiency can be factored in to arrive at the bottom line, net heat value (NHV.)

To estimate LHV it is necessary to know the weight of water contained in a given volume of wood. That water must be evaporated before wood can burn. This water evaporation requires about 1,210 Btu per pound of water, and the total energy needed to evaporate all the water in the wood must be deducted from GHV to arrive and LHV.

Take as an example a cord of “green” red maple. Assume approximate heat values are all that is required. The table above suggest the GHV for red maple to be 23.6 million Btu per cord. To simplify the process, assume green woods have an average MC of 75% ODWT basis. [The reported average MC for red maple is 70%]. At 75% MC, how many pounds of water are in a cord of red maple? This value would be the difference in weight between a green cord and an ovendry cord.

Recall that equation 4 can be used to calculate ODWT/CD, if we know the SG. Because an approximation of LHV is all that is required for this example, use .55 as average SG (see table above).

ODWT/CD = 0.55 X 6.23 X 80 = 2,741 pounds.

With values for MC and ODWT, equation 2 can be used to calculate GWT:

GWT/CD = 2,741(1+ (75%/100)) = 2,741(1.75) = 4,797 pounds

Weight of water in the cord would be:

Wt. of water/CD = GWT/CD - ODWT/CD = 4,797lb - 2,741pounds = 2,056 pounds

The amount of energy needed to evaporate this weight of water is 2.5 million Btu per cord, i.e.,

1,210 Btu/lb of water X 2,056lb of water/CD

Now deduct this value from GHV to arrive at LHV:

LHV = 23.6 MMBtu/CD -2.5 MMBtu/CD. = 21.1MMBtu/CD

Calculation of net heat value requires reducing LHV to account for energy losses which occur during the combustion process. These Btu losses may be due to excessive air entering the combustion chamber, excessively high temperatures of flue gases, ro simple radiation losses. Btu losses are generally related stove or furnace efficiency.

Efficiency of stoves, fireplaces, and other heating devices are affected by the design, quality of installation, location, indoor and outdoor temperatures, and use patterns. Fireplaces range in efficiency from -10 to 15%, box stoves 20-40%, and airtight stoves 25-70%.

To calculate NHV it is necessary to know which type of heating device will be used to burn the wood, and then to apply a combustion efficiency factor to the LHV.

Assume an airtight stove is to be used to burn a cord of red maple, and that it's a new, carefully installed and properly located stove. Its combustion efficiency factor will be at least 60%.

Net Heat Volume (NHV) = 0.60 X 21.1 million Btu per cord. = 12.7 million Btu per cord.

For this example, under assumptions made, a green cord of red maple burned in a 60% efficient airtight stove would produce approximately 12.7 million Btu's net heat energy.

If the average reported SG for red maple (0.54) and that species' average reported green MC (70%) had been used in the calculation of NHV (in an effort to derive a more precise value), the NHV would have been approximately 12.5 million Btu per cord, only 0.2 millin Btu off the approximation [1.6% error].

 The process of calculation NHV can be summarized this way: 
 
 1. GHV (MMBtu/CD) = SG X 42.862 MMBtu/CD.
 2. LHV (MMBtu/CD) = GHV - [1,120 Btu/lb X lb. water/CD]
 3. NHV (MMBtu/CD) = Combustion efficiency factor X LHV

When asserting the energy content of a cord of firewood, the most important parameter is the ovendry density for that species because a pound of dry wood of any species has about the same energy content. If all species were sold at the same price, the best buy would, of course, be the denser wood, assuming equal moisture contents. When buying wood by the pound, the most important parameter is moisture content, and the driest wood would be the best buy.