How much heat energy is in firewood?

In the face of historically bitter winter weather, heating fuel prices are at record highs due to shortages in many traditionally plentiful fuel supplies. Wood for heat may be a practical alternative to fossil fuels for some homeowners.

Cutting green wood Photo Credit: Mike Schira, Houghton County, 2014

Cutting green wood Photo Credit: Mike Schira, Houghton County, 2014

Although deep snow and bitter cold winter temperatures may discourage many people from obtaining firewood this season, wood is an alternative heating fuel source worth consideration. Increased efficiencies of both indoor and outdoor wood burners coupled with rising fossil fuel prices are making wood a more attractive heating fuel source then it has been in many years.

It is difficult to put an exact heat energy value on wood. Complicating the problem of assessing firewood’s heat value is differing species, but mainly it is the variation in moisture content that creates challenges. Freshly-cut wood, with most of its growing moisture still within the cells, may contain as much as 60 percent or more moisture by weight. This fresh cut wood is generally referred to as “green wood”. Before wood can begin to generate heat this internal water must be evaporated or boiled off. Burning green wood uses up the wood’s energy available for heat as turning the water into vapor or steam uses energy.

Wood used as a fuel should be air dried or “seasoned” until its internal moisture level is about 20 percent or less. Usually, split green wood will dry to this more favorable moisture level if stacked in a way to allow outdoor air to circulate around the stacks for at least one full summer season. Some species of hardwood will require a longer time to dry then others. Red oak, for example, is slow to air dry due to a complicated interior cell structure.

The best way we have to compare relative heat values of various fuels it to break them down into heat units or BTU’s. A standard cord of well-seasoned hardwood (stack of wood 4’X 4’X 8’ or 128 cubic feet) contains the heat equivalent of about 20 million BTU’s. By way of comparison this is more or less equivalent to the heat value in 145 gallons of #2 fuel oil or 215 gallons of LP gas.

Michigan State University Extension suggests that individuals investigate the pros and cons of burning wood before getting too excited about changing over from your current heat energy fuel. Modern oil and gas burners are more efficient on average then wood burners, so savings switching to wood may not be as great as they would seem. Local zoning may restrict wood burners (restrictions generally apply to outdoor burners) as well as some insurance carriers; so homeowners are encouraged to look into this before beginning any changeover.

If a homeowner is fortunate enough to own or have access to a woodlot for harvesting firewood they have an edge on those who don’t. Remember to get permission before entering any property to remove wood. The Michigan DNR is issuing emergency fuel wood permits this year due to the extreme weather situation we are enduring. For more information on obtaining fuel wood permits for state owned lands or help in finding a local source of wood visit their “Personal Fuelwood Permit” web page.