Questions on BTU/Efficiency Ratings on Solid Fuel Furnaces

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Post Mon. Oct. 14, 2013 12:39 am

Awhile back it was stated that in practical home usage a wood stove would probably only produce about 50 to 60% of its rated BTU output. This is because when the stoves are rated they are stuffed full with the driest wood possible and burned as hot as possible. Does the same hold true for solid fuel furnaces?

How accurate are the efficiency ratings claimed by the manufacturers? One claims that their 150,000 BTU input furnace is 78% efficient. Therefore it's output would be 150,000 x 78% = 117,000 BTUs of output. However, if, like the wood stove, you'd only get 50 to 60% in practical home usage, would the output be 50 to 60% of the 117,000 BTU output, or 50 to 60% of the 150,000 BTU input?

The same manufacturer claims that their 180,000 BTU input furnace is 82% efficient because it has a larger heat exchanger than their 150,000 BTU unit. Does this seem reasonable? I read somewhere that no solid fuel furnace produces these kinds of efficiencies. I look forward to your responses...Gary

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Richard S.
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Post Mon. Oct. 14, 2013 4:35 am

Gary the major reason for the higher efficiencies is becsue you can create a convoluted path for the flue gases with a coal unit. Flue temperatures are much lower on a coal unit, unless it just got done stoking I can place my hand on my flue pipe, you aren't going to do that with a wood stove. For example take this cutaway diagram for a Keystoker boiler, note how low the exit for the flue gases is. The bed where the coal is burning is above where the gases exit and pulled through that internal water jacket before exiting.


Let's take my Van Wert as another example, note the door for accessing the fire pot is slightly lower than the height af the flue pipe:
When you ope that access door you don't see any exit for the flue pipe do you. ;)
If look at side view you can get a better idea why, you can't see it in this picture but the box extending on the back is just for the flue gases. Internally a plate extends below the fire pot that the gases have to go below before they can then go up and exit into the flue pipe.

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Location: Harrison, Tenn

Post Mon. Oct. 14, 2013 8:55 am

Richard has it covered but I will add that people find that their coal appliances heat more then they are rated for when compared to wood appliances they have experienced in their pasts. My VF 3000 is doing a job it technically would not do if it was a wood boiler rated as it is. In fact, it replaced a wood boiler of much higher input/output.


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Rob R.
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Post Mon. Oct. 14, 2013 9:19 am

Traditional stoker boilers are rated in Net BTU's and also include a rating for connected load...these units meet or exceed those figures. Remember, these units served as heating plants for large homes, often with no backup system...not the like a wood boiler that is "helping out" the oil boiler sitting next to it.

Axeman Anderson, EFM, Keystoker, and Van Wert all published detailed ratings and recommendations for their units. Others may have done the same, but that list is all I am familiar with.

My EFM 520 has no problem hitting the published output for 10 lbs per hr of feed. In fact, it seems like EFM left a buffer to allow for poor quality coal.

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Post Mon. Oct. 14, 2013 8:36 pm

BTU values and efficiency values are not connected really. The BTU output is the BTU output.

I would think that the 78% efficiency is likely close to what the unit can do.

Used to be that wood burning stoves (not pellets) were poor in efficiency 'cause people did not really care ... they are getting better.

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Post Tue. Oct. 15, 2013 6:35 pm

Thank you all for your input. Greatly appreciated...Gary

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