A closer look at burning wood for heat
Published on March 14, 2023 at 2:55pm GMT+0000 | Author: Tucker Henderson0
The Prairie Spy
Alan “Lindy” Linda
Here’s a column for those of you–us–who heat with wood.
Wood is cellulose, a carbon based catch-all that includes a lot of different fuels. Paper, field corn, oats, beans, all kinds of grass. And so many more that it’s useless to keep listing them.
All of these substances share a common trait: When they are burned, if–and only if–they are pretty close to dry, they all give off about 9,000 Btus of heat per pound. It’s important that we are doing this by the pound. Straw gives off as much heat as oak? Well, apparently you haven’t quite grasped the size of the pile of straw that makes that pound. There’s quite a lot of it.
All the other fuels that we burn are priced according to how much heat they give off per some universal quantity. Oil and propane use the gallon measure; natural gas uses a cubic foot. In both these instances, those are carefully measured units. When you buy them, you know exactly what you’re buying, heat-wise. And you know exactly how many Btus you are going to get for your money from that quantification.
Wood on the other hand? Wood has some measured amounts, it’s true. Full cord, fireplace cord (½ a full cord). Some random-sized trailer full, loose piled. Some pickup box, slightly stacked. And who knows what.
And certainly, there are times when that full cord can (I say “can” here) give you some kind of your money’s worth, given that the measure of a cord, 4 X 4 X 8 feet, is correctly adhered to. And dry.
That’s the other problem we have not addressed. Remember, back up at the top, we talked about the “pound” measure. But we also mentioned the “dry” part of this. Yes, quantity is important, but the truth here regarding firewood is that moisture is more important. And here is where things go bonkers, because very little of what we wood-burners burn is truly dry. (It’s never totally dry. It’s quite rare for firewood to drop much below 10 percent.)
I recently sold several cords of firewood that had been stored inside a shed for a long time, some of it for over 40 years. It was very dry. If I had to guess–which is mostly what we all do with everything involving firewood–it was probably 10 percent or so. Because it was so dry, it was noticeably lighter in weight, when we were throwing it into a trailer. And because it was lighter–hence dryer– it burned quicker.
Luckily, the person who bought it burned it in an outside wood boiler. I say lucky, because had it been burned in a steel wood furnace with a fan taking the heat away, a great deal of this wood’s Btus would have gone up the chimney. Air isn’t all that good at wicking away super hot flames. It’s too light.
But water is. It’s heavy. Yes, this wood burned up quickly. No, it didn’t go up the chimney so wastefully because fortunately the water that surrounded the wood in that wood boiler can absorb a lot of heat, and store it quite nicely. So that was fortunate.
Back to the moisture thing. It takes about 1,000 Btus to turn a pound of water into vapor. Before you can burn any cellulose–wood, here–you have to vaporize out the moisture in that wood before it will burn.
Let’s say a cord of wood weighs about 3500 pounds. Let’s say furthermore, it’s not too dry. But pretty dry. Say, 25 percent. Now, wood will burn at 25 percent. It will burn at just about any percent, if you get it hot enough. Twenty-five percent of 3500 pounds is 875 pounds of water. (Which you just paid good money for.) To get rid of that water means changing 875 pounds of water to vapor. Take that times 1,000 Btus, and 875, 000 Btus of your heat just never appeared. And this is a best case scenario here, because a lot of people are burning wood that is damp enough to sizzle. (You might argue that you only have to reduce the percentage of moisture part way. Uh uh. The lower, the better.
I know several people who swear that the best heat is to burn green popple, or poplar. One of them burned 15 cords of it a winter, wood he cut just before it got cold. “Best way to heat ever,” he said to me.
I said nothing. Had that wood been reasonably dry, he could have heated with half as much.
Buying and burning firewood is somewhat like rolling dice at a casino. Buy a moisture tester, they’re inexpensive. That’s about the only way you can tell with firewood how dry it is.