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Friday, January 15, 2016

Wood Science 101 (22) - Which Log or Pellet is Right for Your Stove?

Last post I alluded to my work talking to folks about firewood and pellets at the Pennsylvania Farm show. I thought today I would share those ideas with the rest of you.

1. Firewood

People love to talk firewood, especially if they burn it for heat. This year, our display included about a dozen different sticks of firewood. But these sticks were special. All were of different species, and sizes...but they all had the same mass, or what we call in wood circles, dry weight.

Testing folks knowledge of the wood they burn.
I kept the labels turned down and let folks see if they could guess a few of the species. Most couldn't, although a few picked out oak and maple.

My favorite demonstration was to pick up a piece of black locust (Robinia pseudoacacia, fifth from the bottom in the picture) and hold it up against the piece of Douglas-fir (Pseuodtsuga menziesii, third from bottom). The two pieces are almost exactly the same length, but the locust is only about half the thickness of the Douglas-fir. When asked which piece of wood provided the most heat, many folks would guess the Douglas-fir, simply because it was twice the size.

But of course, you know the rest of the story...since the two pieces weigh the same amount, and energy value is roughly correlated to mass of the wood, then the two pieces contained roughly the same heating value, even though the stick of Doug-fir was much larger. Which then led us on the the importance of density of different species, knowing which species is which, and relating the price per cord (which is a volume, not a weight measure) to whether or not a certain cord of firewood is worth the price being asked. Sometimes we even got into the moisture content of the wood.

If you care to dig more deeply into the topics, there are many good online references, including

Firewood BTU Comparison Charts at chimneysweeponline.com, and

Heating with Wood: Species Characteristics and Volumes by Utah State Extension authors Michael Kuhns and Tom Schmidt, which has some great explanatory text, charts with firewood properties, and diagrams on different measures of firewood.

2. Pellets

My conversations with folks on wood pellet stoves were even more interesting, because pellet stove owners, on average, seem to know much less about wood and what makes their stove work well and what doesn't. But they all want to know the bottom line...what is the best brand of pellet?

Most of the conversations were held around the pellet pit while we watched their kids and grandkids dig around and organize the pellets.

Highlight of the farm show for two-year-olds.
As for the golden question, I explained to them that pellet stove effectiveness is a function of many variables...size of space being heated, brand of stove, quality and species of the pellets, outside temperature variation, home construction and insulation, and on and on. Where it got interesting was in a few cases, the folks countered with stories of trying Brand A versus Brand B and deciding that Brand A pellets went further. My usual response to comments of this type is..."And the outdoor temperature was the same when you tried the two different brands?"

[Deer in headlights look.]

So on the way home Sunday night, I got to thinking...there has to be a way for folks to make a somewhat reliable comparison of pellets. And this is what I came up with...

1) Buy two or more different brands of pellets, preferably by the bag.
2) Set your pellet stove on your preferred temperature setting...AND DON'T ADJUST IT DURING THE TRIAL. You can turn it down to a night-time setting each evening, if you make sure to turn it down and back up at the exact same times each day. Keep temperature fiddlers away from the thermostat.
3) Begin a week on one brand of pellets, and stay on that brand for a whole week.
4) Each day, write down the high and low outdoor temperatures for the day, and from these, calculate the average temperature for the day. (High + low)/2
5) At the end of the week, record the average temperature for the week (sum of daily averages / 7) and record the total amount of pellets used during the week.

Keep a simple chart that looks something like this:


In this simple example, we can see that even though the average temperature was about 2 degrees (F) lower in Week 1 using Pellet X, about three fewer bags were used to keep the house warm. Here we have quantitative proof that Pellet X was more effective for this home and this stove.

This trial could be extended for as many brands as you care to try, and the longer the trial is extended, the better. I would recommend running the trial for at least a month, and preferably for an entire heating season. Over the course of several weeks. you can average the weekly results for each brand, and eventually the temperature averages during which you trialed each brand will converge to nearly the same average temperature...but the total usage of each brand will differ by the effectiveness of each pellet in your home, under your heating requirements with your stove.

If you care to take your analysis to the next step, you could compare the pellet effectiveness achieved during your trial to the cost of each pellet product. For instance, say you find that at an average outdoor temperature of 30 degrees F over several months, you used 10% more of Pellet Y than Pellet X. So Pellet X is more effective in your stove, right? But, if Pellet Y is 20% less expensive than Pellet X, they Pellet Y makes more sense from a pellet cost standpoint.

Many folks worry more about ash content than they need to...the major brands are certified to PFI standards to contain less than 1% ash content, and most contain even less than that. If you find a certain brand to produce too much ash consistently over time, then you'll know it, and the efficiency and cost effectiveness testing described above will help you decide if that extra amount of ash is worth the trouble.

So, have some fun the rest of this winter, and take control of your wood heating solution. Crank the numbers and increase your satisfaction in warm, bright wood heat.

1 comment:

Clancy T Labrador said...

Hey Doc, pound for pound the black locust will produce less ash. So it is not as efficient as the Douglas fir.

See us westerners all know that D-fir produces significantly more heat in the Fire Square. Here's how it goes, we get sweat more on the cutting side, sweat more while splitting and stacking, same amount of British what ever units (darned metric, what ever happened chains and links) and shovel out the stove twice as much.

So there you go D-fir is better for all around heat.

We'll miss you at the Western Pallet Association in Palm Springs.