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Energy Comparison Calculator (Wood Burners)

With the prospects of high energy prices for heating our homes some may consider the use of wood as an alternative heating source. The homeowner who is considring wood for home heating, either as a supplemental or primary basis, should give careful consideration to the economics involved and also to the amount of personal attention that is required to operate a wood burning unit.

Heating Your Home With Wood
by Ed Lee

To accurately compare costs for various types of heating fuels the homeowner must determine the usable heat - the amount of actual heat supplied to a home by the heating system. To determine the usable heat you multiply the heating value of the fuel (measured in BTU - British Thermal Units) by the operating efficiency of the heating system (measurement of how effective the system is at transferring the energy in the fuel into usable heat).

Non-wood systems

Natural gas produced in Oklahoma has a heating value of approximately 1 million BTU per MCF (1000 cubic feet). The average operating efficiency of a natural gas system is about 65%. The usable heat for the home is about 650,000 BTU for each MCF of fuel.

Propane has a heating value of 92,000 BTU per gallon and an operating efficiency of about 65%. The usable heat is about 59,800 BTU for each gallon of fuel.

Electricity has a heating value of 3413 BTU per kilowatt-hour (Kwh). An operating efficiency of 100% provides 3413 BTU of heat for each Kwh.

Wood Based Systems

Wood varies in heating value depending on species and moisture content. For cost comparisons, a cord of air dried mixed hardwoods will be used with a heating value of 24.5 million BTU.

The operating efficiencies of wood heating units vary with the type of unit and operating environment.

Conventional fireplaces are found in over half the single family residences in the United States. It has an average operating efficiency of about 10% and provides a usable heat value of 2.45 million BTU per cord of wood (stack of wood 4 ft. x 4 ft. x 8 ft., two ricks equal one cord if the logs are 24 inches long). The addition of glass doors and simple heat exchangers may increase its operating efficiency up to 20%.

A Convective fireplace is a metal fireplace with a double wall around the firebox. With an operating efficiency of 40% it will provide about 9.8 million BTU of usable heat per cord of wood.

A fireplace insert is designed to insert into a conventional fireplace. With an operating efficiency of 40% it will provide about 9.8 million BTU of usable heat per cord of wood.

In airtight stoves the only air allowed into the firebox must come through the air inlet openings. The average operating efficiency of an airtight stove is about 35%. Usable heat is about 13.475 million BTU per cord of air-dried mixed hardwoods.

A non-airtight stove has loose fitting doors and side panels. Franklin stoves, parlor stoves, and old fashioned potbelly stoves are examples of non-airtight stoves. The units have an operating efficiency of about 25%. Usable heat is about 6.125 million BTU per cord of air-dried mixed hardwoods.

Central wood burning furnaces have an airtight firebox, can be attached to duct work, and can heat an entire house. Some units can burn both wood and a secondary fuel such as natural gas, propane, or electricity. They have an average efficiency of about 55%. Usable heat is about 13.475 million BTU per cord of air-dried mixed hardwoods.

Other Considerations

Safety is the most important consideration when locating and installing a wood burning unit. Proper clearances to combustible materials, such as walls and furniture, must be provided even if a portion of efficiency is sactificed.

If possible consider how the heated air is going to move through the living space. It is easier to heat one large room than three smaller rooms even though the total floor space is the same. Minimize air flow restrictions by keeping doors open and by locating the stove to minimize the number of coners around which the heat must flow.

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Non-wood Cost
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If the economics are positive for wood as an alternative, we suggest that you review the publications in the Heating With Wood Series from the Oklahoma State Extension Service, they will discuss in more detail the factors you need to successfully install and utilize a wood burning heating system.

This article condensed from information in NREM-9440 Firewood: How to Obtain, Measure, Season, and Burn by Joseph F. Gerling and Max R. Craighead, Oklahoma Cooperative Extension Service-Division of Agricultural Sciences and Natural Resources.