Wood as Fuel

A good, well-seasoned hardwood will provide approximately half as much heat value per pound as does good coal (Table 5-4). Wood is easy to ignite, burns with little smoke, and leaves com­paratively little ash. On the other hand, it requires a larger storage space (commonly outdoors), and more labor is involved in its preparation.

Wood is commonly sold by the cord for heating purposes. A standard cord measures 4 ft high by 8 ft wide and contains wood cut to 4-ft lengths for a total of 128 ft3. Only about 70 percent of a standard cord actually represents the wood content, however, because of the existence of air spaces between the wood.

The heat value of wood depends upon the type of wood being burned. Heat values per cord for a number of different types of wood are given in Tables 5-5 and 5-6. The data used to com­pile the tables were obtained from Use of Wood for Fuel (U. S. Department of Agriculture Bulletin No. 753). The wood heat

Table 5-4

Typical Firewoods

Name of Wood

Type of Firewood

Combustion Characteristics

Ash, white

Hardwood

Good firewood

Beech

Hardwood

Good firewood

Birch, yellow

Hardwood

Good firewood

Chestnut

Hardwood

Excessive sparking (can be dangerous)

Cottonwood

Hardwood

Good firewood

Elm, white

Hardwood

Difficult to split, but burns well

Hickory

Hardwood

Slow, steady fire; best firewood

Maple, sugar

Hardwood

Good firewood

Maple, red

Hardwood

Good firewood

Oak, red

Hardwood

Slow, steady fire

Oak, white

Hardwood

Slow, steady fire

Pine, yellow

Softwood

Quick, hot fire; smokier than hardwood

Pine, white

Softwood

Quick, hot fire; smokier than hardwood

Walnut, black

Hardwood

Good firewood, but difficult to find

Table 5-5 Heat Value (Million Btu per Cord) of Wood with 12% Moisture Content*

Name of Wood

Heat Value

Equivalent Coal Heat Value

Ash, white

28.3

1.09

Beech

31.1

1.20

Birch, yellow

30.4

1.17

Chestnut

20.7

0.80

Cottonwood

19.4

0.75

Elm, white

24.2

0.93

Hickory

35.3

1.36

Maple, sugar

30.4

1.17

Maple, red

26.3

1.01

Oak, red

30.4

1.17

Oak, white

32.5

1.25

Pine, yellow

26.0

1.00

Pine, white

18.1

0.70

*The equivalent coal heat values are based on 1 ton (2000 lb) of anthracite coal with a heat value of 13,000 Btu/lb.

Table 5-6

Heat Value per Cord (million Btu) of Green Woods*

Name of Wood

Heat Value

Equivalent Coal Heat Value

Ash, white

26.0

1.00

Beech

27.1

1.04

Birch, yellow

27.2

1.05

Chestnut

19.2

0.75

Cottonwood

18.0

0.69

Elm, white

22.2

0.85

Hickory

29.0

1.12

Maple, sugar

27.4

1.05

Maple, red

23.7

0.91

Oak, red

27.5

1.06

Oak, white

28.7

1.10

Pine, yellow

23.7

0.91

Pine, white

17.3

0.67

*The equivalent coal heat values are based on 1 ton (2000 lb) of anthracite coal with a heat value of 13,000 Btu/lb.

Values are determined for cords containing approximately 90 cu ft of solid wood (i. e., about 70 percent of a standard cord). Note that the heat values for both green and dry wood (12 percent moisture content) are given in Tables 5-5 and 5-6. It is important to remem­ber that the amount of moisture in a wood will affect its burning characteristics. A green wood will burn much more slowly than a drier one, and its fire will be more difficult to start.

All wood can be divided into either hardwood or softwood. Contrary to popular belief, the basic difference between the two groups is not the hardness of the wood. In other words, a wood that can be classified as a softwood is not necessarily softer than a hardwood. These terms do not refer to the physical properties of the wood but to its classification as a coniferous (needle or cone-bearing) or deciduous (broad-leafed) tree. A softwood comes from a coniferous tree, whereas a hardwood is obtained from a deciduous tree. You will find that some softwood trees produce wood that is harder than the wood obtained from some hardwood trees.

Chimney or flue fires are always a hazard when wood is used as a heating fuel. Resins and soot collect in these areas over time and can be ignited if there is a flare up of the fire. The possibility of this happening can be eliminated or greatly reduced by cleaning the chimney or flue from time to time.

Softwoods (e. g., white or yellow pine) contain greater amounts of resin than do hardwoods. This tends to give softwoods flamma — bility, but it also results in more smoke.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers