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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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    Hello all,
    I am an Electrical Engineering Co-op working with a renewable energy company this quarter. The company has tasked me with determining the actual amount of energy that the two wood pellet stoves are producing. I have been searching all over the internet to no avail as to how to come about this number. If anyone has any thoughts or suggestions on where I should begin it would be greatly appreciated.
    Some thoughts I have had...
    1) Weighing the pellets and knowing the exact change in weight would tell us the flow rate, and knowing the BTU rating of the pellets that we are burning could give us the amount of BTUh. This would be assuming thought, that the pellets actually burn at the rated BTU (highly doubtful). This wouldn't be viable because of the tedious task of weighing the amount of pellets for the coarse amount of data that I would like to have (approximately every 15 minutes).
    2) The stoves have a three fan speed variable, and through the work day are always on. The building I work in is currently striving to be one of the first Net Zero Energy buildings in Ohio, so the thermostat is kept a couple degrees slightly below average to try to accomplish this. The second thought that I had would be to put a temperature sensor on the stove which could calculate a temperature differential. Somehow deriving the information I need from this...

    Thanks in advance.
    Whitsett

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  2. jtakeman

    jtakeman Minister of Fire

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    The thing that's gonna get you is the BTU output of the pellet. The only way to get the actual BTU is have them tested or purchase a Boom Calorimeter(approx. $25K). You need to know the route of the fuel. How much it consumes and Then you would have to calculate the heat loss(efficiency) of the appliance. This will give you the output BTU's.

    There is a lot of guess work in the pellets and the actual BTU output of the stoves. But the end user only needs to be somewhat close. Don't go by most manufacturer's as most only rate them as input BTU's. Some give both, But don't state the BTU of the test pellet. Some have the EPA's tested efficiency, But again not stating the input BTU leaves big holes in the output. I have figured out what my stove consumes in volume per hour on the medium heat setting(where my stove spends most of its time heating). So if I knew the BTU input and weight by volume. I can get close. As the EPA rating of my stove is 79.5%.

    I am very interested in this and hope you can share your findings with us here.
  3. whitsett2014

    whitsett2014 New Member

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    Hey,
    I'm back to work on this today. I will keep you posted with anything that I can come up with, thanks for the reply.
    Whitsett
  4. tjnamtiw

    tjnamtiw Minister of Fire

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    I think who ever gave you that assignment has no idea of what he/she is demanding of you. You would not only have to know the btu content of the pellets but also how many btu's are lost up the flue since you can't assume that the factory rating is anywhere near correct given the cleanliness of YOUR stove's heat exchanger and your operating conditions. You would also have to know the heat loss of your building through the floor, walls, and ceiling as well as the average heat gain/loss of the building volume. It's basically impossible. Since the person asking hasn't a clue, then my suggestion would be to bury that person in impressive looking data that they can't understand.
  5. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    No, don't do that a lot of the folks that hand out these kinds of assignments know a lot more about it than an outsider might think they do.
  6. whitsett2014

    whitsett2014 New Member

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    Tj,
    I am beginning to agree with you more and more, as I continue my research on the subject. It seems that there are too many variables to consider, and not enough reliable information readily available. It may come down to me calling the manufacturer of the stove and asking some questions to get the information I need. Even then there will more than likely still be too many variables. I think I will more likely bury my boss in unanswerable questions. ;) I'm still plugging away though.

    Never give up,
    Never surrender.
    Whitsett
  7. kofkorn

    kofkorn Feeling the Heat

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    Since there is a relatively small amount of heat lost through the exhaust pipe due to the double wall construction, you could use a short exhaust setup and measure the temperature of the exhaust.

    Use a wind speed gage or gas flow meter on the intake to get your mass air flow. Then you can use the pellet weight to estimate your BTU input and the temperature and exhaust flow to calculate your BTU output. You'll probably be in the 10-15% accuracy range. If you have a data logger, you could chart the mass air flow and temperature over the whole bag. This should increase your accuracy significantly.

    Good luck and keep us posted on your results
  8. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    A couple of the national labs have conducted a number of such measurements. Maybe contacting Sandia or Brookhaven National Labs will provide you with a lot of leads.
  9. whitsett2014

    whitsett2014 New Member

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    Yes, Thanks I will most definitely look into this.
  10. tjnamtiw

    tjnamtiw Minister of Fire

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    %-P I spent 40 years in manufacturing engineering management trying to answer questions from upper management types who had no idea of what happened on the 'other side of the wall' and only looked at the bottom line and keeping the stock holders happy. It gets quite frustrating when you give them facts and answers that they don't want to hear and they just crawl back into their inner self. 'Screw up and move up' was the corporate motto. Sad but true. Once in a while someone who actually knows that they are doing gets promoted but it doesn't take long after that to see him realize he's surrounded by idiots.
    Am I bitter? Yep, when I go to finance.yahoo.com and see that they all have voted themselves a million dollars worth of stock options twice a year........ and at the same time they cut the employees' benefits because the company can't afford it. :snake:
  11. tjnamtiw

    tjnamtiw Minister of Fire

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  12. kofkorn

    kofkorn Feeling the Heat

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    Since it is a closed system (air leaks are negligible), the air flow into the intake is equal to the airflow out of the exhaust, so my comment above of measuring the airflow of the intake will give him the flow of the exhaust.
  13. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    We hope that's the case ;-) .
  14. kofkorn

    kofkorn Feeling the Heat

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    I agree. From the number of people complaining about black soot and unburnt pellets, we know that it's not always the situation. But for calculations, it should be a fairly safe estimate. When it comes to running real world numbers on any system like this it's EXTREMELY difficult to get highly accurate numbers.

    This takes me back to my college days when every problem was done on a system that looked like a kidney bean. Cause that's what the professor could draw and it was also about how accurate the calculations can be :)
  15. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    TJ,

    I doubt if that is the case here. I used to hand out funny assignments to interns, co op, and newbies. Used the results among other things to do some weeding.

    I'm also more than aware of the C level, their direct reports, and the games played in today's environment.
  16. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    Plus or minus x, depending on LOD and $ allowed to do the measurements ;-P .
  17. whitsett2014

    whitsett2014 New Member

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    Just wanted to keep you all posted on some newly gathered information.
    The two stoves are the same make and model: HarMan Advance (pellet only stoves)
    The fan speed is not variable by any switch on the outside of the machine. (though I cant assume it is always constant)
    The fuel flow rate is variable on speeds 1-6. (it has always been set on 3 for winter) Max rate of fuel input is 5.7 lb. fuel/hr.
    The stove has a built in thermostat, and can also run off of a external thermostat, which will lower the flow rate I believe. Though this only comes into play in the fall and spring seasons as the climate changes.
    Blower is rated 135cfm (in the handbook though I expect this number is a little high).
    Hopper capacity 60lbs.
    Outside Air Size: 2 3/8 inches. (this must be the intake size? please confirm)
    BTU range: 0 to 48k
    Feed Rate: 1.00 lb/hr min, 6 lb/hr max.
    The insulation in the room is above average.

    **EDIT**
    Sorry, I forgot to add the information regarding the pellets we are using.
    Pellet: Cleanfire Premium Grade <1% Ash content.

    Also, I just had a conference with my Project Manager, his thoughts were to put a temperature sensor directly outside of one of the blowers, and a temperature sensor elsewhere in the room. We would also measure the CFM of the blower (which through winter would remain constant, because the stove is always on full blast), and use these three pieces of data along with air density, and specific heat to determine the BTU's that the stove is producing. Open to thoughts or comments on this please :).

    Never give up,
    Never surrender.
    Whitsett
  18. kofkorn

    kofkorn Feeling the Heat

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    Tough to measure just the air temp from the blower and temp in the room. There is a significant amount of radiant heat generated off of the sides of the stove that won't be accounted for. Additionally measuring the temperature of the room won't give you much unless you have a solid idea of heat loss from the room, which now means you also need to accurately measure the outside temperature and do a heat flow analysis through the walls. Additionally, it will make it more difficult if you use the thermostat because it will cycle the stove high and low, or on and off, which will mean that you will need to also account for your stove cycle times and measure temps in both conditions.

    If the purpose is to find out how many BTU's the stove is producing, then for the testing period, keep as many variables as constant as possible. Don't use the thermostat, don't change the blower speed, don't change the feed settings, and if at all possible, try to regulate the temperature of the incoming air. This will simplify your calculations and improve your accuracy.
  19. whitsett2014

    whitsett2014 New Member

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    I agree with you on the first point, though I believe there is a calculation that can be done to determine the amount of heat transferred through radiation. On the second point, for the purposes of simplicity at the moment, let us assume that we will only be doing the calculation in the dead of winter. Which means that the stove will never actually reach its set thermostat temperature. This will hold the fuel flow rate constant, and thus the air flow of the blower constant. So if we can calculate the heat transferred by radiation, and the heat transferred by the blower, simply add them and this will give a rough (more than likely very rough) estimate of the BTU's that the stove is producing. As always, thoughts comments and criticisms are welcome. Thank you!

    Never Give up,
    Never surrender.
    Whitsett
  20. kofkorn

    kofkorn Feeling the Heat

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    With those assumptions you should be able to get some numbers.

    I'd love to see the results!
  21. tjnamtiw

    tjnamtiw Minister of Fire

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    I know, Smokey. I guess I better go back to bed. :)
  22. tjnamtiw

    tjnamtiw Minister of Fire

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    If you can confirm what the btu rating of the pellets are and can determine how much heat is going out the exhaust, then you know the total heat gain from the stove by subtracting what's going out the exhaust. Like Smokey said, it depends on how much money they will give you for test equipment to measure the btu's going out the exhaust. If you make enough assumptions, you could get a number. Assume the btu's are correct as quoted by the manufacturer, assume the cfm of the combustion blower is as stated by the stove company (not likely because it's probably free cfm, not restricted), assume 100% combustion, and accurately measure the exhaust temp over several bags of pellets using a data logger.
  23. BDPVT

    BDPVT Member

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    Whinsett, I used to do this sort of testing for a living. As a starting point, refer to EPA Methods 28 & 28A for test procedures. You will find this level of testing is well beyond those with limited resources. You may be able to develop your own test method and produce some general data, but I would question the value of this. If the pellet stoves in question are EPA certified, and I assume they are, then the manufacturer(s) will have the test results you are seeking. I wonder why your employer is going to all this trouble? Good luck.
  24. whitsett2014

    whitsett2014 New Member

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    The website of the company who makes the pellets claims that the pellets range from 7900-8100 BTU/lb. So let's make up some numbers shall we?

    Lets say that the stove is set at rate 3 (on a scale of 1-6) and 6 is the maximum feed rate. That puts me at 3/6*5.7lb/hr=2.85lb/hr. Over the course of the day the stove runs at a constant speed for 9 hours. It would burn 25.65lb of pellets. If all of the energy created by the pellets is converted to usable heat, all we have to do is multiply the weight of the pellets used over the course of the day, by the BTU/lb rating (assuming it is correct) to get the amount of BTU's the stove has produced in a day. Which comes to 205,200 BTU's (using a 8000 BTU/lb average). According to what I have been reading, the only other place that the heat energy can go is through the exhaust(if there are other significant sources of heat loss let me know :)). So now I need to calculate the heat lost through the exhaust and subtract that from what I found above.

    Another thing that concerns me is that at 205,200 BTU's in 9 hours, comes out to 22,800 BTU's/hr which is far below the maximum value of the given range.
    The given range is 0-45000 BTU's. I realize that this number is not in BTU's/hr as mine is.... So I wonder, how did they get this number?
    Something which I realized after writing the above statement... I am running at half the maximum feed rate (3/6) so does it make sense that I am achieving half of the maximum BTU rate?
    Something to note is that the feed rate of 3/6 is optimal for this stove, meaning that the wood pellets are being completely burned before reaching the point where they will fall off the tray.

    Another note to make is that this is not quite what I am looking for, just wanted to throw some numbers around to try to verify that I am heading in the right direction. What I'm looking for would be a way to calculate the instantaneous BTU output of the stove.
  25. whitsett2014

    whitsett2014 New Member

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    Thanks for the information, I will look more into this :). I'm not really sure if I am technically permitted to tell you why... Sorry... a vague answer would be that we are just trying to gather all the possible information about how much energy we are using vs. how much we are creating (or saving in the case of the pellet stove) via renewable solutions, and put all of this data in one spot with pretty and easy to understand graphs!

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