How good is your wood?

[Nofossil]

I've been trying to get a load of consistent wood so that I can do another calibrated efficiency run. Unfortunately, the wood that I have right now is highly variable and not of the best quality. Got me wondering how important the quality of your wood is to overall performance.

As you can see in the picture, my wood is mostly small. While that makes it easier to handle, especially for the women, it means there's a lot more bark and crud per unit of good stuff.

I also have a mix of hard wood and soft wood, and varying degrees of punkiness. There are a few pieces of apple, hickory, locust, and black birch that are solid. Lots of the pieces have some degree of dry rot - the large poplar chunk is all punky, and two of the buckthorn sticks have a surface layer that's starting to go.

Moisture content is all over the place. The smaller buckthorn and read cedar are around 20%, while the hickory and larger buckthorn are around 30%.

Does anyone have a sense of how much improvement I might expect if my wood were better? I have to think that I'd be happier if it were consistently solid, straight, dry, and perhaps larger diameter. Unfortunately, it looks like it will be next year before i have a load that's consistent.

 

[Eric Johnson]

The big problem with wood-fueled appliances is exactly the inconsistency of the fuel--not only the fuel in an individual pile, but trying to engineer a machine that will perform as expected with a wide variety of fuels of varying quality and condition across a large installed base. Tough row to hoe, vs. say, oil or natural gas. So by that measure, the more consistent your fuel supply, the better off you're likely to be and the easier it will be to dial in your boiler.

I cut mostly beech in the 10-14" range, DBH. Since I use the whole tree, I have a nice mix of split chunkwood and limbs. I usually use the limbs to start my fires (or rejuvinate the bed of coals) and toss the bigger, split stuff on top for the long burn. My wood is all at least 2 years old, but I have found big differences in moisture content, mostly relating to how long the wood has been sitting in my barn. Typically, I'll fill the barn up with about 10 full cords by Memorial Day, and it's pretty darn dry by the time the heating season rolls around. I have some stuff that hasn't been sitting in the barn all that long, however, and I tend to get blue smoke on startup, which to me is a sign of excessive moisture. I suspect that if I fooled around with the air supply, I could work around that.

I suspect that species, size of wood, moisture content and the general condition of the wood all have an impact on performance.

 

[leaddog]

I think that it would be better if you are trying to get good comparisons that it would be best to have compareable lots of wood. If you take your wood and brake it up into like lots I think you will get fairly comparable results. If you are trying to get the most btu's per hr then that is another story. It all depends what you are trying to get. As long as you are compareing apples to apples that gives a baseline. I think to get the most efficient burn you should have 20% hardwood, no bark, equal lengths, 4in splits, firebox water temp at 180f, and go from there but that isn't the real world. The real world is what you 've got so make as equal lots as you can and go with that.
leaddog

 

[Rick Stanley]

Eric,

Do you split your wood and put it directly in the barn as you cut the trees, or does it stay outside for awhile?

 

[Eric Johnson]

I cut and block the trees up in the summer, then let them lay on the ground until the following summer. Then I haul the rounds home and split and stack them over the course of the summer. The following spring, they go into the barn. So It's actually 3-year-old wood (or older) by the time it gets burned. I don't like to haul green wood. The stuff that sits around for a season in blocked form is a lot lighter to haul and easier to split. The only downside is that the ends stain.

 

[jebatty]

I'm going to go out a "limb" with this one (pun fully intended), and suggest that except for MC, if you weigh the wood, in a mixed load, presence of bark, punk, etc. really doesn't make too much difference. It's BTU's per pound, not per volume, that count.

As to MC, again with a mixed load, likely mixed MC, and perhaps that too may be ignored.

If you run several measured weight burns, with different mixed loads, then take a average, you're probably going to come pretty close.

So, I think just weighing the wood and ignoring the rest might end up giving pretty accurate results when averaged over several burns. At least they should be pretty accurate for many of us, who often burn mixed loads because that's what we have.

 

[Nofossil]

I think that you're right about BTU/lb being more or less constant regardless as long as the MC is the same.

However, I think there's no question that if your wood is small, it's finished much more quickly.

The extra surface area drives much faster combustion, and messes up the secondary air ratio.

 

[jebatty]

Agree fully -- air control will play a role. But in a mixed load, small and large, and averaged, do you think this too will average out to provide a reasonably accurate result?

 

[Willman]

How about trying to find KD hardwood offcuts from a local woodworker for your testing purposes. Also pallet skids could be used for uniformity. usually they are 2 x 4 in size. You could mix in the slats.Same sizes, weights and MC would remove some variables from the equation. When it warms up you could try out KD softwood.
Will