Wood Heating

by Michael Kuhns, Extension Forestry Specialist, and Tom Schmidt, Forester*

Introduction

Wood is a source of heat used by many Utahns. More firewood will be burned as the cost of other energy sources like gas and electricity rises. In order to use firewood effectively, an understanding of species characteristics and firewood volumes is needed.

Species Characteristics

Firewood from different species or types of trees varies widely in heat content, burning characteristics, and overall quality. Table 1 presents several important burning characteristics for most species used in Utah.

TABLE 1. Firewood Facts (Sorry for any gaps in the data)

Species Weight (lbs./Cord) Green Weight (lbs./Cord) Dry Heat per Cord (Million BTUs) % of Green Ash Ease of Splitting Smoke Sparks Coals Fragrance Overall Quality
Alder   2540 17.5   Easy   Moderate Good Slight  
Apple 4850 3888 27.0 135 Medium Low Few Good Excellent Excellent
Ash, Green 4184 2880 20.0 100 Easy Low Few Good Slight Excellent
Ash, White 3952 3472 24.2 121 Medium Low Few Good Slight Excellent
Aspen, Quaking   2160 18.2   Easy   Few Good Slight  
Basswood (Linden) 4404 1984 13.8 69 Easy Medium Few Poor Good Fair
Beech   3760 27.5   Difficult   Few Excellent Good  
Birch 4312 2992 20.8 104 Medium Medium Few Good Slight Fair
Boxelder 3589 2632 18.3 92 Difficult Medium Few Poor Slight Fair
Buckeye, Horsechestnut 4210 1984 13.8 69 Medium Low Few Poor Slight Fair
Catalpa 4560 2360 16.4 82 Difficult Medium Few Good Bad Fair
Cherry 3696 2928 20.4 102 Easy Low Few Excellent Excellent Good
Chestnut     18.0           Good Good
Coffeetree, Kentucky 3872 3112 21.6 108 Medium Low Few Good Good Good
Cottonwood 4640 2272 15.8 79 Easy Medium Few Good Slight Fair
Dogwood   4230 High   Difficult   Few Fair    
Douglas-fir 3319 2970 20.7 103 Easy High Few Fair Slight Good
Elm, American 4456 2872 20.0 100 Difficult Medium Few Excellent Good Fair
Elm, Siberian 3800 3020 20.9 105 Difficult Medium Few Good Fair Fair
Fir, White 3585 2104 14.6 73 Easy Medium Few Poor Slight Fair
Hackberry 3984 3048 21.2 106 Easy Low Few Good Slight Good
Hemlock   2700 19.3   Easy   Many Poor Good  
Honeylocust 4640 3832 26.7 133 Easy Low Few Excellent Slight Excellent
Juniper, Rocky Mountain 3535 3150 21.8 109 Medium Medium Many Poor Excellent Fair
Larch (Tamarack)   3330 21.8   Easy-med   Many fair Slight Fair
Locust, Black 4616 4016 27.9 140 Difficult Low Few Excellent Slight Excellent
Maple, Other 4685 3680 25.5 128 Easy Low Few Excellent Good Excellent
Maple, Silver 3904 2752 19.0 95 Medium Low Few Excellent Good Fair
Mulberry 4712 3712 25.8 129 Easy Medium Many Excellent Good Excellent
Oak, Bur 4960 3768 26.2 131 Easy Low Few Excellent Good Excellent
Oak, Gambel     30.7              
Oak, Red 4888 3528 24.6 123 Medium Low Few Excellent Good Excellent
Oak, White 5573 4200 29.1 146 Medium Low Few Excellent Good Excellent
Osage-orange 5120 4728 32.9 165 Easy Low Many Excellent Excellent Excellent
Pine, Lodgepole   2610 21.1   Easy   Many Fair Good Fair
Pine, Ponderosa 3600 2336 16.2 81 Easy Medium Many Fair Good Fair
Pine, White   2250 15.9   Easy   Moderate poor Good  
Pinyon   3000 27.1   Easy   Many      
Poplar   2080 Low   Easy   Many Fair Bitter  
Redcedar, Eastern   2060 13.0   Easy Low Many Poor Slight Fair
Redcedar, Western 2950 2632 18.2 91 Medium Medium Many Poor Excellent Fair
Spruce 2800 2240 15.5 78 Easy Medium Many Poor Slight Fair
Spruce, Engelmann   2070 15.0 78 Easy   Few Poor Slight  
Sycamore 5096 2808 19.5 98 Difficult Medium Few Good Slight Good
Walnut, Black 4584 3192 22.2 111 Easy Low Few Good Good Excellent
Willow 4320 2540 17.6 88 Easy Low Few Poor Slight Poor

Green weight is the weight of a cord of freshly cut wood before drying. Dry weight is the weight of a cord after air drying. Green firewood may contain 50% or more water by weight. Green wood produces less heat because heat must be used to boil off water before combustion can occur. Green wood also produces more smoke and creosote (material that deposits on inside walls of chimneys and may cause chimney fires) than dry wood. Firewood should therefore always be purchased dry or allowed to dry before burning. Dry wood may cost more than green wood because it produces more heat and is easier to handle.

A wood's dry weight per volume, or density, is important because denser or heavier wood contains more heat per volume. Osage-orange is a very dense firewood with limited availability in Utah. It's included here to show what a very dense wood is like. It contains almost twice the heat by volume of cottonwood, one of our lightest woods. In general it is best to buy or gather dense woods such as oak, hard maple, or ash. Hardwoods, or woods from broadleaved trees, tend to be denser than softwoods or woods from conifers. Some firewood dealers sell "mixed hardwood" firewood. This may or may not be desirable, depending on the proportion of low- density hardwoods such as cottonwood that are included.

The amount of heat per cord of dry wood is presented in Table 1. Heat content is shown as a percent of dry green ash, a fairly common, dense firewood. Values above 100 signify a higher heat content than green ash and below 100 a lower heat content.

Table 1 also contains information on other characteristics that determine firewood quality. Ease of splitting is important because larger pieces of wood must usually be split for good drying and burning. Fragrance and tendency to smoke and spark are most important when wood is burned in a fireplace. Woods that spark or pop can throw embers out of an open fireplace and cause a fire danger. Conifers tend to do this more because of their high resin content. Woods that form coals are good to use in wood stoves because they allow a fire to be carried overnight effectively.

Firewood Volume

Though firewood dry weight is important for determining heat content, firewood is normally bought and sold by volume. The most common unit of firewood volume is the cord, also known as a standard or full cord. A cord is an evenly-stacked pile containing 128 cubic feet of wood and air space. Though a cord can be piled in any shape, a standard cord is generally thought of as a stack of wood 4 feet tall, 8 feet long, and 4 feet deep (Figure 1). To figure the number of cords in another size or shape pile, determine the pile's cubic foot volume and divide by 128. A randomly-piled stack of wood will generally contain more air and less wood than one neatly piled.

Some dealers sell wood by the face cord or short cord (Figure 2). A face cord is a stack of wood 4 feet high, 8 feet long, and as deep as the pieces are long. Pieces are commonly 12 to 18 inches long, so a face cord may contain 32 to 48 cubic feet of wood and air.

Another common firewood measure is the pickup load (Figure 3). This is a very imprecise but common measure. A full-size pickup with a standard bed can hold about 1/2 of a full cord or 64 cubic feet when loaded even with the top of the bed. Small pickups hold much less. Random loading will decrease this amount further.

A randomly-piled stack or pickup load of wood will contain more air and less wood than one neatly stacked. Crooked, small diameter, and knotty or branchy pieces also reduce the amount of wood in a pile.

Firewood with standard cord
Figure 1. Standard Cord - Total Volume = 128 cubic feet

Firewood with a face cord
Figure 2. Face Cord - Total Volume = 32 to 48 cubic feet (depending on piece length)

Firewood in pick-up truck
Figure 3. Pickup Load - Approximate Total Volume = 64 cu. ft.

Buying Firewood

Species, volume, dryness, and need for splitting should be considered when buying firewood. The information here and in other publications should give you the basic information you will need to be an informed buyer. However, knowing your dealer is the best way to ensure that you are getting what you are paying for.

For More Information

A number of good publications are available to help you learn more about using firewood for heating.

Burning Wood and Coal by Susan Mackay, L. Dale Baker, John W. Bartok, Jr., and James P. Lassoie. 1985. Northeast Regional Agricultural Engineering Service, Riley Robb Hall, Cornell University, Ithaca, NY 14853. (607) 256-7654. 90 pp.

The Wood Burner's Encyclopedia by Jay Shelton and Andrew B. Shapiro. 1976. Vermont Crossroads Press, Box 333, Waitsfield, VT 05673. 155 pp.

Wood Heat Safety by Jay Shelton. 1979. Garden Way Publishing Co., Charlotte, VT 05445. 165 pp.

* Tom Schmidt is a former Forester for the Nebraska Forest Service. This web page is based partly on a University of Nebraska fact sheet titled "Heating With Wood: Species Characteristics and Volumes".