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Wood Heat as a Comparison

Wood heat often is not the cheapest heat source and it requires more work. It is less convenient than heating with fuels fed automatically. However, if the cost of wood is low enough, you will save money. For those who enjoy work, the cutting, splitting and stacking of wood can be a pleasant form of exercise.

To compare heating costs you must convert costs of different fuels to cost per unit of heat. Cords of wood, gallons of oil, kilowatt hours of electricity, therms of natural gas and cubic feet of LP gas must be converted to a common heat content unit for comparison. The kilowatt hour and the therm are precise quantities of energy. But the volume of wood in a cord can vary considerably and the potential heat in a cubic foot of wood varies for different species. In addition, the heating value of wood, the stoves used, and the frequently required stove adjustments make wood heating calculations less than precise when compared to other sources of fuel. 

TABLE 1. HEATING VALUES OF LOCAL WOOD

SPECIES OF WOOD

BTU/Cord (million)

Mountain Mahogany

35

Black Oak

26

Douglas-fir

25

Juniper

24

Lodgepole Pine

21

Ponderosa Pine

20

White Fir

19

Incense-Cedar

18

CAUTION: Mountain Mahogany and pitchy pine limbs will burn very HOT!!  Do not regularly use these woods in wood stoves as excessive high heat will damage the fire box and the chimney.

TABLE 2. COMPARISON OF OTHER FUELS TO A CORD OF WOOD

To compare with other heating fuels use the factors below to obtain equivalent cords of wood:

Oil

divide no. of gallons by 175

Coal

divide no. of pounds (not tons) by 1600

Gas

divide no. of thousand cubic feet (MCF) by 28

Propane Gas

divide no. of gallons by 220

Electricity

divide no. of kilowatts by 6500

NOTE: A cord of wood equals a tightly stacked 4 foot by 4 foot by 8 Wt rectangle of wood or 128 cubic feet.

TABLE 3. HEAT CONTENT OF COMMON FUELS

FUEL             UNITS PER MILLION BTU EFFICIENT HEAT
Natural gas                            15.4 therms
Fuel oil                                   11.1 gallons
Coal                                            .067 ton
LP gas                                    16.5 gallons
Electricity                             293.1 kwh (kilowatt hour)

TABLE 4. COMBUSTION EFFICIENCY OF TYPICAL HEATING UNITS

TYPE                                                EFFICIENCY
Standard fireplace                            up to 10%
Fireplace inserts/tube grates           up to 20%
Simple updraft stove (Franklin)        up to 30%
Airtight stoves                                    up to 60%
Pellet fuel stoves                                up to 90%

Using the above tables a person can determine whether wood heat is the most efficient way to heat the home. Or another question could be "How much can I pay for wood before it exceeds the price of my alternative heat source?"

The following example shows the step by step method of determining the most efficient way to heat the home. Obtain your own local fuel costs for the example. Remember, wood burns at a lesser efficiency than other fuels and the efficiency of the stove is also important. To do a proper comparison we must change to the BTU measurement (British Thermal Unit). See table #3.

EXAMPLE: "If I used 600 gallons of #2 fuel oil last year at a average price of $0.70 per gallon, would I be better off burning seasoned juniper purchased at $65.00 per cord?"

A simple calculation would show that you spent $420.00 for heat last year ( 600 gallons x $0.70/ gallon = $420.00 ).

From table #2 we calculate the fuel oil equivalent to a cord of wood equals 3.4 cords for the winter ( 600 gallons divided by 175 = 3.4 cords ).

Therefore, if we had to purchase wood for heat it would cost us $221.00, or a whole lot cheaper. ( 3.4 cords x $65.00/cord = $221.00 ) Actually, we must go a step further and compare the efficiency of the wood and the stove. To do this we must change to a BTU measurement (British Thermal Unit).

From table #4 we see that our wood stove has an efficiency rating of 60%.

At a value of $65 per cord our wood could be compared by using the following equation:

$65/cord divided by 24/mBTU divided by 0.60 = $4.51 per mBTU

Thus to heat our home with wood would be cheaper. But because the wood stove is only 60% effective, we would have to purchase more wood than the original calculation would suggest:

3.4 cords divided by 0.60 efficiency = 5.7 cords

Thus 5.7 cords at $65-00 per cord = $370.50 for our winter heating bill.

Note: Your needs could be lower, depending on the size of the house and how well insulated it is.

Now, to determine how much we can pay for wood before its price exceeds the price of fuel oil:

Divide $420.00 paid for fuel oil by 5.7 cords of wood needed and we can pay up to $73.68 per cord before heating with fuel oil would be cheaper.

You can do the same calculations for other sources of heat or compare the BTU's of other firewood available. Remember, these figures are not that precise for wood due to its nature. Other factors such as moisture content and stove operation can greatly reduce the efficiency of burning wood. Wet wood alone can reduce the efficiency of a wood stove by an additional 50%. Much of the heat produced must be used to boil out the "free" moisture and the heat output of the stove will not increase much above 212 degrees, the boiling point of water, until the "free" moisture is gone.

From table #I we see that juniper has a value of 24 per million BTU of efficient heat per cord.

From table #3 we see that fuel oil has a value of $7.77 per million BTU of efficient heat

( $0.70 x 11. 1 = 7.77 ).

Next: Wood Storage and Proper Drying