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Pellet Stove BTU Rating are worthless

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by gscreely, Feb 2, 2008.

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  1. gscreely

    gscreely Member

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    This is a little rant about the BTU ratings of Pellet stove's. Before I begin let me say I love my pellet stove and have owned several. The BTU rating on almost all stoves is simply the max feed rate per hour multiplied by a BTU rate of a typical pellet (and I would guess not all manufacturer's use the same figure). So all told a 40,000btu unit can burn 40,000btu's worth of pellets in an hour. To that I say so what !

    A number that would be far more useful would be the output btu's, and the actual heat exchange efficiency. Many stove makers list an "overall efficiency" which is a bit of a mystery, but to get the rating they are claiming I believe they take the combustion efficiency (nearly 100%), the electrical efficiency(nearly 100%), and finally the heat exchange efficiency and average them for the overall efficiency. So a stove with a heat exchange efficiency of 45% could have and overall efficiency rating of 81%. I had a quad santa fe and I am almost certain there was more and hotter air coming out of the exhaust then out of the heat exchangers. So of the 30,000btu's it could burn I was probably getting 15,000 or less. My Harman advance is much better, but it has two heat exchangers and uses less air then the quad. The exhaust of my harman is of far less volume and a lower temperature then that coming out the heat exchangers.

    I am not bashing the quad, it was well built and ran great, but it was just very inefficient.

    One of the most common questions on here is what size unit should I get, but an important factor is the stove efficiency because the units ability to keep up will be related to its output. This is particularly important because so many folks get these stoves to save money. Interestingly a Harman dealer I spoke with didn't know that the btu rating was not an output rating, so there are many involved in this industry who don't know it them selves.

    I suppose the bottom line is selling and sizing stove's based on their btu input, would be a little like selling and sizing air conditioners based on their electrical usage.

    I would be interested if any one else has noticed this, and is there reform on the horizon??

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  2. sylvestermcmonkey

    sylvestermcmonkey Member

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    You're totally right. For what it's worth I started a new thread on this question. To their credit Quadra-Fire's website has a link to calculate relative fuel costs. Reverse-engineering it reveals they're using an overall efficiency of 70%, but who's to say that isn't a wild-a$$ guess.

    I think it's pointless to take any combustion or electrical efficiency into the equation, but I'd rather doubt the 70% is realistic.
  3. newpelletstove

    newpelletstove Member

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    Guys, I am not an expert on this, but somehow I believe that an efficiency of 70 - 80% is realistic for a well-designed combustion heater (like a good pellet stove or an oil furnace). The overall efficiency I doubt is an average of three different efficiencies (heat exhange, combustion and electrical), but rather how many btu's go out compared to how many go in. I don't have any proof of this either, but I bet at least the larger stove manufacturers have tested their stoves in a lab to come up with the efficiency figures.
  4. Corie

    Corie Minister of Fire

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    I can only speak on this topic from personal research experience, which is relatively short term.


    If the pellet stove is EPA certified (few are), the BTU numbers on the EPA hang tag are 100% correct numbers that were achieved in actual testing. EPA certified pellet stoves can choose to take default 78% efficiency, or choose to go through the costly efficiency testing procedure.

    The "average effiency" you speak of is not a familiar quantity, and not one that is used, as far as I know. If I ever quote an actual effiency from a stove I am tesing, it is the TCC (total combustible carbon) efficiency, which is an actual measure of effiency. It is calculated using the stack loss method, and takes into account the total amount of carbon (and thus, potential heat) as well as the amount of heat which is up the chimney (based on gas temperature and gas volume). The moisture of the fuel is measured and is taken into account in the equation.

    I can tell you right now that the stove I'm developing right now is not by any means a technological revolution in terms of heat exchange system, is between 80% and 83% efficient (TCC).

    I hope that sheds some light on the situation for you.
  5. sylvestermcmonkey

    sylvestermcmonkey Member

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    Good stuff, thanks! I'll use 70% as a conservative estimate for now.
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