Good Fires Start With Good Wood

By Mike McClintock (reprinted courtesy of the Washington Post)

When you buy firewood, the size of the pile is only one factor to consider. Getting your money's worth of wood and having fires that burn efficiently also depends on whether logs are hardwood or softwood, seasoned or unseasoned, and how much potential heat they contain.


Last week, we sorted out the differences among face cords, fireplace cords and other variations of the basic 4-by-4-by-8-foot stack. This week, let's look at the wood itself, including the kinds of firewood that provide the best fires and the most heat.


What burns best?

You might care most about the crackling charm of a fire without much concern for heat, or you might want to burn wood purely for the BTUs -- the heat it produces. But the same factors that make a quick-starting, long-lasting blaze also tend to make fires that produce the most cost-effective heat.


If you're buying firewood in bulk as supplemental fuel, you probably should take the trouble to compare the cost and heat output of fires to the cost and heat output of your furnace. After all, why split, stack and carry piles of wood if it's less expensive (and a lot cleaner) simply to turn up the thermostat?


The potential heat in logs varies greatly, particularly between hardwoods and softwoods. Generally, however, 150 gallons of fuel oil will provide about the same heat as a cord of seasoned hardwood. This means that it's not cost effective to pay more than 150 times the price per gallon of fuel oil for a cord of wood. For example, when fuel oil costs $1 per gallon, a cord of seasoned hardwood ready for burning should cost no more than $150.


To make practical choices, you need to know approximately how much heat is in the cord of wood you plan to buy, and use the common denominator of BTUs (British Thermal Units, a standard measure of heat) to compare it to the heat in other fuels.


There are too many variables to make precise comparisons. Among the most significant: the operating efficiency of a fireplace, which might be 50 percent, vs. the efficiency of a modern furnace, which might be 85 percent or 90 percent. But it's easy enough to convert cubic feet of gas or kilowatt hours of electricity to the common denominator of BTUs. To find the BTUs per cubic foot of natural gas multiply by 1,031. To find the BTUs per kilowatt hour, multiply by 3,412. You don't have to convert for fuel oil, which is rated at about 150,000 BTUs per gallon.


Once you price a cord of wood, for example, $200 for a cord of ash, which is rated at about 20 million BTUs, you can calculate that a dollar's worth of wood provides about 100,000 BTUs. If fuel oil costs $1 per gallon, which supplies about 150,000 BTUs, the oil will be more cost-effective than firewood, though hardly as romantic.


If you're prepared to do the multiplication, figure that one kilowatt-hour of electricity yields about 3,400 BTUs. One cubic foot of natural gas (approximately one Therm) yields about 1,000 BTUs. One gallon of liquefied petroleum gas (LPG) yields about 95,000 BTUs. One gallon of residual fuel oil yields about 150,000 BTUs.


Seasoned and unseasoned wood

Most firewood suppliers advertise hardwood, although loads of the high-priced wood sometimes are padded out with less expensive softwood. Hardwood is more dense than softwood -- a cord of oak can weight twice as much as a cord of pine -- because there is more wood fiber packed into each log. More wood fiber to burn means more BTUs.


Hardwoods such as oak, maple and birch weigh about 4,000 pounds per cord and contain more than 20 million BTUs of potential heat. Softwoods such as fir, spruce and hemlock weigh 2,500 pounds or less per cord and contain 10 to 15 million BTUs of potential heat. Piles of hardwood and softwood might be the same size, but there is more fuel in the stack of hardwood.


In any case, you'll realize the potential heat only if the wood is seasoned (stacked to dry) for at least a year, and longer for some hardwoods. After four or five years, however, most cut logs start to deteriorate -- sooner if you pile them unprotected in the weather.


Seasoning is so important that you would be better off buying a cord of softwood that's dry than a cord of hardwood that's green. One reason: Recently cut wood, which might contain more than one-third moisture by weight, uses up a lot of its BTUs converting moisture to steam instead of radiating heat.


You can try to beat this waste of energy by splitting unseasoned logs into small pieces, or even into kindling. But the fire will still be difficult to ignite, hiss and pop (sometimes violently), yield only marginal heat and produce a lot of creosote -- a gummy residue that can reignite to cause hidden fires in chimneys and flues.


In contrast, seasoned firewood lights more easily, delivers more heat and produces less creosote because the flue gases (and the flue itself) are hotter.


Spot green wood by its tight-fitting bark, which has to be peeled off, and wet, sometimes spongy end grain. Seasoned wood generally has looser, more brittle bark, and is darker on the ends of the logs, which have a spider-web pattern of small cracks.


© 2001 The Washington Post Company
This article is the second of a three part series. For the other two go to the community web site at www.lakebarcroft.org/docs/reprints/firewood1.html and www.lakebarcroft.org/docs/reprints/firewood3.html.