January tale of the tape + ComfortMax 75 Stove Efficiency

1,452 lbs. coal burned
602 lbs. compressed sawdust blocks burned

Theoretical coal burned if zero wood was burned = 1,785 lbs. (based on January's HDD's and past history)

Coal saved by burning wood blocks = 333 lbs.

333 lbs coal / 602 lbs. wood = 0.5532 lbs. coal saved for every lb. of wood burned

Coal BTU's not expended = 333 lbs. x 12,250 BTU's/Lb. = 4,079,250 coal BTU's saved

BTU's/Lb. for ~8% moisture wood = 7,850

Wood BTU's Expended = 602 lbs. x 7,850 BTU's/Lb. = 4,725,700 wood BTU's expended

Coal Boilers overall efficiency (from comparison of 7 years coal boiler data to 10 years all electric data) = ~71%

Ratio of Coal BTU's saved to Wood BTU's expended = 4,079,250 / 4,725,700 = 0.8632

ComfortMax 75's Real World Efficiency when burning wood = 0.8632 x 71% Coal Boiler Efficiency

ComfortMax 75's Real World Efficiency = 61.3%

How much did the 602 pounds of wood blocks cost vs the 333 pounds of coal you could have used cost? Feel free to use cheap costs on those blocks cuz i know they go on sale sometimes in the spring, or bulk prices. They do burn nice.

warminmn wrote: ↑

Thu. Jan. 31, 2019 8:42 pm

How much did the 602 pounds of wood blocks cost vs the 333 pounds of coal you could have used cost? Feel free to use cheap costs on those blocks cuz i know they go on sale sometimes in the spring, or bulk prices. They do burn nice.

Last May the wood blocks were on sale at Coalway for $150/ton. I believe I paid $160 a ton at their once a year May sale a few years ago. I'll call it $155 per ton. This is a you pick it up yourself price. And pre-tax.

Delivered bulk pea coal has generally ran me $240 to $250 per ton here, so I'll call it $245. Also pre-tax.

602 lbs. wood blocks x 155/2000 = $46.66

333 lbs coal x 245/2000 = $40.79

Since (per someone on this forum quite recently) the EPA rated the ComfortMax 75 at 94% efficiency on wood, and DS Machine rates it a tad conservatively thereby by listing it at 92% on their website, and I have it at closer to 61% efficient overall in the real world, I suspect that rather similar general conclusions might logically be drawn for the highest efficiency rated (or presumed) hand fired (fed) coal stoves in general.

Rob R. wrote: ↑

Thu. Jan. 31, 2019 9:15 pm

Larry, most appliances have an efficiency curve. If you are not firing this stove very hard, it is likely giving up some combustion efficiency.

There are other inefficiency causes as well. One is that my living room, dining room, and kitchen zone (all of these being on one zone) runs about 5 degrees hotter than the boiler keeps it on average when running the ComfortMax. If one permits about 3% fuel wasted per added degree (which is a figure often seen for set-back T-Stats), that alone could put it at closer to 75% efficient. But then it is speculatively hypothetical, and no longer "real world".

I've always been one to divide the manufacturers BTU rating by 2. Since DS Machine rates the ComfortMax at 75,000 BTUH (input I presume), I'd call it an honest 37,500 BTUH input stove.

37,500 BTUH x 4.75 hours between feedings = 178,125 BTU's

I burn between 3 and 4 blocks of compressed sawdust, so lets call it 3.5 blocks, at 3.5 lbs per block. Or 12.25 lbs. of blocks per typical feeding.

12.25 Lbs. x 7,850 BTU's/Lb. = 96,163 BTU's fed to the stove per every 4.75 hours

96,163/178,125 x 100 = ~54%

So I'm running it at 54% of full capacity on average.

To reach full capacity I would need to load it with roughly 7 compressed sawdust blocks per feeding.

lsayre wrote: ↑

Thu. Jan. 31, 2019 8:54 pm

Last May the wood blocks were on sale at Coalway for $150/ton. I believe I paid $160 a ton at their once a year May sale a few years ago. I'll call it $155 per ton. This is a you pick it up yourself price. And pre-tax.

Delivered bulk pea coal has generally ran me $240 to $250 per ton here, so I'll call it $245. Also pre-tax.

602 lbs. wood blocks x 155/2000 = $46.66

333 lbs coal x 245/2000 = $40.79

Thats mighty close to the same cost even if your off a couple dollars. For occasional use the blocks would be hard to beat if you cant cut wood, or dont want too. I know my Dad likes them but its been a couple years since he used any. The main disadvantage would be them getting wet, like pellets, I imagine.

Yep they puff up and crumble...

Thought I’d bump this thread since I am waiting on a review of wood burning from Ultra and his new Comfortmax 75 he bought last year.

Some really good information in this thread.