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Calculate BTU’s Produced By a Pellet Stove

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by whitsett2014, Jan 24, 2011.

  1. whitsett2014

    whitsett2014 New Member

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  2. whitsett2014

    whitsett2014 New Member

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    Everything you need has been discussed at length. If you are too dumb/lazy to put it together yourself, don't blame me.
  3. turbotech

    turbotech Feeling the Heat

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    I looked at the EPA website table and a Regency wood stove is listed at 12K to 44K BTU/hr, default efficiency is 63. Looking at the Regency website it states 75K BTU/hr. What is the difference between EPA measurement and the Regency measurement?

    Is the 78% an overall (combustion, electric, heat transfer) efficiency or is it just the heat exchanger? If it is overall then the heat transfer / exchanger could be down to 40% efficient. Kind of sad.
    If it was 40% and something like Maine's Choice pellets (7700 btu/lb) were burned then it comes out to 123K BTU/bag. Only slightly better than a gallon of oil in a typical furnace.
  4. kofkorn

    kofkorn Feeling the Heat

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    The EPA website is giving the output BTU/ hr. The Regency website is using the Input BTU value.

    If you take EPA's output BTU and divide it by the efficiency you should get the input BTU value

    44,0000 / .63 = 69,814

    That's pretty close to Regency's number.

    IMO, the 78% overall efficiency number is pretty much useless for calculations. It's a simple average of combustion, electrical and exchanger efficiencies. This average gives as much value to the electrical efficiency as compared to the heat exchanger. Now I don't feel that the electrical energy used by the stove is even close to the combustion energy (pellets) used by the stove. The overall efficiency shouldn't be a simple calculation. Without additional information it is very difficult to make any inferences off of this overall efficiency number.
  5. turbotech

    turbotech Feeling the Heat

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    That is what I was guessing but wasn't sure. Since the Regency stove in question has an "optional" blower then how does that play into the EPA output value?

    The pellet stove "overall" efficiency is what confuses me. Suppose I had a wood stove with 70K BTU/hr input and 45K BTU/hr output. If I ran it at roughly 50% then that is 35K BTU/hr input so that means a 35K to 40K BTU/hr pellet stove could replace it? Not sure because of the "overall" efficiency thing.

    I don't understand why a wood stove is rate at 63% on the heat exchanger, but the pellet stove is rated (98+99+40)/3 ~= 78%. That means the pellet stove has a 40% efficient heat exchanger. Doesn't make sense interpreting things that way.
  6. kofkorn

    kofkorn Feeling the Heat

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    I would imagine that the optional blower wasn't considered during the EPA value. They would likely only test the stove as received.

    Regarding efficiency, your thoughts sound right. On the other hand, I've found that a direct comparison between a pellet stove and a wood stove is a difficult thing to do. They generate their heat with very different methods. A wood stove creates much more radiant heat that affects everything in near proximity, while a pellet stove focuses much more on heat convection, using a blower to move the heat around. Simply using the efficiencies to decide on which stove to use will likely leave you disappointed when you have a cold snap or some other unusual situation. Especially if you are typically running your wood stove at 50%, and then buy a pellet stove that is based on that 50% value. If you get a cold spell, your pellet stove would already be maxxed out.

    I don't know how the heat efficiency on a wood stove is evaluated. They could be averaging the combustion and exchanger efficiencies in the same way that the pellet stoves are evaluated. So by that method you could see an average like: (95 + 31)/2 = .63 This is a pure example though. I have no idea how they generate the number.
  7. jtakeman

    jtakeman Minister of Fire

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    Dumb and lazy? Ha! I wasn't the one who came asking the questions and got most of his answers for free even. :sick:
  8. tjnamtiw

    tjnamtiw Minister of Fire

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    Oh man, I would have expected nothing less out of Whitshitt than that!!!! He comes crawling here to get us all to help him and then craps on us like that! Heck, he had it set in his mind what he wanted to do and wouldn't take advice or criticism. He'll make it far in the Corporate world but not in the REAL world. I wouldn't want to work next to him. He is a typical college kid with no sense of how to work with people. I've seen his kind many, many times in my factory as local school interns. Good riddance.
  9. turbotech

    turbotech Feeling the Heat

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    I see this quite a bit where some newbie comes to forum with a "new, great idea no one ever thought of" and try to keep it a secret. They don't know what they are looking for really, just trying to poke around hoping someone will figure it out for them. Eventually it comes out that the idea is an old idea and is only new to the newbie. At least Whitsett didn't let the cat out of the bag only to find that his great project and idea has already been done before and only his ignorance makes it a new idea to him.
    He is not the only one to have this happen to him. I think we all have, but he is just more rude about it than most I have seen.
  10. kofkorn

    kofkorn Feeling the Heat

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    It doesn't matter what conversations lead up to it, there's no place in this net for statements like that. Even when people disagree on issues, there's no reason to get personal about it. That just speaks to the writer's maturity.

    J, you've been one of the most helpful people on this forum, and your testing proves that you are anything but lazy.

    While I enjoyed the discussion, I am very disappointed in that comment.
  11. tjnamtiw

    tjnamtiw Minister of Fire

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    What he is, turbotech, is a college kid who, for a few credit hours, goes to a factory for a semester and basically gets in their way asking stupid questions that he could find the answers for himself if he took the time to dig in and get his hands dirty. So to get rid of him, they assign him some useless project and hope they don't see him again before he leaves. But instead, like the Corporate Executive he thinks he is shortly going to be, he has others do his work for him yet doesn't understand the principles involved in the manufacturing process. You will notice in the discourse of this thread, he didn't have the vaguest idea of how the pellet stove even worked or how it was constructed. AND he didn't want to know! He will be the one sitting in the office and having no idea of what is happening on the other side of the wall. When you come to him with a problem, he'll give you that blank stare that says he doesn't understand, could care less, and wants you to just go away and fix what ever is wrong. It's only when things go VERY wrong that he suddenly wants to know why YOU didn't keep the plant running.
    Total worthless jerk and he's our FUTURE!!! God help us!
  12. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    BTW whitsett2014, what are the error bars on your results?
  13. Countryboymo

    Countryboymo Feeling the Heat

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    I just stumbled in to this thread and all I can say is ..... WOW.... whitsett2014 you are totally out of line. Keep in mind as you travel through life that no one cares what you know until they know that you care.
  14. tjnamtiw

    tjnamtiw Minister of Fire

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    Very well said, countryboymo!
  15. mkling

    mkling New Member

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    I also just stumbled across this thread and just read through the whole thing. It started off great but then when some advice wasn't taken for what the advice giver thought it should have been his/her tone started to change to condescending. The right thing to do is to post your advice and let the person requesting it do what they will with it, he might have used it eventually once the full scope of issue was being examined. Whitsett was obviously looking for a mentor in an area that was completely new to him and if you know anything about working in the real world you know that being a great mentor is an extremely important thing and that doesn't include getting mad at someone for not taking your advice. I would not be where I am today without someone I have worked with that I consider to be a phenominal mentor. Remember, this was completely new to him and there was a lot to absorb and he was trying to tackle the small pieces he had the resources for right now. Lastly, while I don't think personal insults have any place on a forum like this, I also think that generalized statements that insult an entire generation don't either (because there was a comment that I took more offense to than Whitsett's). People need to get a grip and calm down. It's too bad this thread had to go downhill like that, I found it pretty interesting being an engineer myself, albeit with only minor experience with thermodynamics. Next time lets all try to do a better job at this... take some lessons from kofkorn as he was oustanding.
  16. turbotech

    turbotech Feeling the Heat

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    Looking at the overall pellet stove efficiency give (99+98+40)/3 = 78, That means the heat exchanger is approx 40% efficient. Assuming a bag of pellets is 40*8200 = 328,000 BTUs. Then the approx. output per bag is 328,000 * .4 = 131,200 BTUs. Not much higher than a gallon of burned oil output.
    For a wood stove (96+40)/2 = 68, or 68% efficiency as given by the EPA for a secondary burn stove.
    Overall, it looks like a value of 40% heat exchanger efficiency is the norm. That seems sadly low when compared to an oil furnace and propane. Is it that the propane rating of 93% is NOT the actual heat exhanger?
  17. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    Most heating devices rarely extract as much energy as the EPA and other testing groups say.

    To get the most out of the system the exhaust temperatures must be as low as possible and the volume of exhaust gases must be as low as possible.

    It is not useful to be able to actually burn 99% of a fuel if in so doing you send most of the heat up the flue. This is true for all heating devices.

    Also the last I knew when measuring things one also provides information on the accuracy of the measurements and the associated error bounds (bars) on any of the calculations based on the measurements.

    But then I'm a woodland critter so I don't have any clues on doing back of the envelope calculations.
  18. Delta-T

    Delta-T Minister of Fire

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    whenever a math situation arises that I have no clue as to what I need to input....safest to use a single digit prime number.....7 is my favorite.....3 never hurts...whats the worst thing that could happen?
  19. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    3 was tried by a group of political hacks to simplify calculations involving that lovely irrational number that relates the circumference of a circle to its diameter. I believe it might have resulted in a bumpy ride.

    I prefer 42.
  20. kofkorn

    kofkorn Feeling the Heat

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    Best number of all time!!!
  21. mkling

    mkling New Member

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    Turbo, based on your comments if I used 800 gallons of heating oil per winter before switching to my pellet boiler then I would need to burn 800 bags of pellets to equal the same heat output. This is not the case however as I only burn 5 tons of pellet fuel (250 bags) per winter to equal or surpass the amount of heat output equal to 800 gallons of heating oil. Also, I believe that heating oil contains about 130k BTU per gallon but that doesn't take into consideration the efficiency of the boiler/furnace burning it, which are typically >=80% efficient these days. Common sense says that 1 bag of pellets in my PB150 generates 3x more heat output than 1 gallon of heating oil; of course this is if I assume that 5 tons of pellets generates an equal amount of heat output as 800 gallons of heating oil over the course of an entire winter (seems to be about the case for me).
  22. mkling

    mkling New Member

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    But then again heat exchangers are heat exchangers, there can't be that much different between those in pellet appliances vs. those in oil appliances, the principles are the same. So 130k BTU per gallon of oil would really equate to 130k*0.4 or 52k BTU of heat output per gallon of oil, using the same exchanger efficiency. If this is true then the pellets have about 2.5x (close to my 3x guess) more BTUs of heat output per bag vs. gallon of oil.
  23. kofkorn

    kofkorn Feeling the Heat

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    In the long run, if you've got experience that says that 5 tons works about the same as 800gal oil in your situation, this is about the best data you can use. It really gets rid of all of the hassle of efficiencies and calculations.

    Empirical data is always much better than theoretical calculations.

    I am sure, as Smokey has mentioned, that if you combine all of the error in the calculations that are in this thread, you will find that the error tolerance far exceeds the calculated output value.

    a number like 48,000 BTU/hr +/- 40,000BTU/hr is not really a good answer in anybody's book.
  24. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    There are plenty of differences in heat exchangers and they aren't even close to being equal to each other.

    That is one of my primary gripes about pellet stoves the trade offs made in the heat exchangers on both the hot side and the cold side.
  25. turbotech

    turbotech Feeling the Heat

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    I agree with what you are saying. I don't think an 80% efficient oil burner takes into account the heat exchanger. Then add the ductwork or pipe loss on top of that. A pellet stove doesn't have that loss.
    That is some good data you posted. It makes sense that 1 bag = 2 to 3 gals of oil. My guess was that the oil/propane efficiencies did not take the heat exchanger into account and your data and others with data back that up.

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