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A Question about exhaust temps and efficiency

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by NW Walker, Dec 5, 2012.

  1. NW Walker

    NW Walker Member

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    Hey gang, I'm a new guy here so go easy on me! I've been wondering about efficiency ratings, BTU's, and exhaust temperatures. When stove ratings are calculated do they take into account EGT's? Take two stoves with an identical firebox size. Say that Stove A burns 80% efficient and has an average exhaust gas temperature of 500*F and Stove B also burns 80% efficient but has an average EGT of 200*F. It follows that Stove B is putting much more BTU's into the space to be heated. Does this figure into stove ratings in any way? This is probably a really dumb question, but it's been bugging me as I look at newer stoves and their ratings.

    Thanks in advance, and thanks very much for all the great info I've gleaned here since finding you guys a few weeks ago. You'll be happy to know that although I've been heating solely with wood for almost a decade this week marked the first time I've gone and cut wood this early in an effort to continue to get ahead. You did that! Thanks again!

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  2. MnDave

    MnDave Guest

  3. NW Walker

    NW Walker Member

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    Wow! I just looked up a QF2100 for a comparison. EPA numbers Eff. 63% BTU/hr-9,300 - 39,300. MFG's numbers Eff. 79.15% BTU/hr 40,800.

    Surprises me too! Not picking on QF, it looks like all the stoves are either 63% or 72% efficient according to the EPA. What gives?

    Also, back to the original question; Btu's, are they calculated with a formula using potential in the fuel and efficiency numbers or do they take into account actual heat into the space?
  4. blwncrewchief

    blwncrewchief Burning Hunk

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    I believe during the EPA testing they are running the stoves on a weigh table to be able to calculate the burn rate. They also run them with a "fixed" draft so the can calculate btu loss up the chimney. A couple other things to keep in mind. Only some manufactures actually pay the testing lab to get real efficiency numbers. Those that do not just get designated "default" efficiency numbers assigned by stove design, either cat or non cat. Also all EPA tests are done with a fixed test load that is the same amount for all stoves tested. This may or may not be close to an actual optimum load for the stove being tested. They are tested with kiln dried pine so the numbers you get with say 60 pounds of hardwood cord wood versus maybe 20 pounds of pine can be quite different. Hence the reason you may see 30,000 btu's on the EPA test but the stove may be rated by the manufacture at 80,000 btu on cord wood.

    As far as measuring stack temps, you need a whole lot more information than just the temp to be able to even begin to actually calculate any even semi accurate efficiency numbers.



    Well, not without all the rest of the valid information. The reason is for the stack temp to mean anything you would need to know the "flow" at that temp to calculate how many btu's are actually going up the flue. Example: Stove "A" may be 500* at 5 cfm and Stove "B" may be 200* at 50 cfm. 500* at 5 cfm would be about 2,322 btu's and 200* at 50 cfm would be about 7,020 btu's. Then we would need to know the actual burn rate to calculate efficiency. Even with that difference in stack temp vs. efficiency being backwards from what you may be thinking if Stove "A" is burning at 11,610 btu's and stove "B" is burning at 35,100 btu's they are both burning at 80% efficiency. Clear as mud? :p
  5. Huntindog1

    Huntindog1 Minister of Fire

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    Those efficiency ratings listed for the EPA is the minimum required to meet the standard. So all the non-cats are listed as 63% and all the cat stoves are listed at 72%. The actual efficiencies are not listed on the EPA published document..

    I think for the EPA test they use a sub par wood so as to make it tougher on the stove designers.

    The higher efficiency ratings and Btu ratings listed on the manufacturer websites are what the stove did with some really good wood and ideal conditions. Most are at max settings which a hotter stove is for sure gonna burn more efficiently.

    But then the question is, is that max efficiency rating the efficiency of burning the wood efficient or is it a rating for how efficient you get the heat out into the room. I would say its number that shows how efficient the stove burnt the wood and all its by products.

    The max btu rating number is most likely different on some stoves as its a function of whats the max air your primary air can be set at the more air you can let in, the hotter the fire can burn as they are measuring the out of the stack exhaust. But most people dont burn a stove at its max setting but rather at its lowest setting that will allow secondary burn to sustain for the longest burn time. As once the house it warm its thermal mass of the house helps keep temps up and a properly matched size stove to the house size will be able to keep temps up in an already warm house operating at the lower settings.
    MnDave likes this.
  6. NW Walker

    NW Walker Member

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    Very, very helpful guys. Thank you so much for the great replies. Blwncrewchief, great info there. Lots to think about.
    This is very succinct and is what I was clumsily trying to ask I guess. Thank you.
  7. MnDave

    MnDave Guest

    I went to a long established stove shop and discussed efficiency numbers with the guy who manages the place.

    Everything that Huntindog1 said above.

    He said that some manufacturers do many test burns and publish the best numbers.

    I asked him about burn times. He said that he always subtracts 2 hours from say a 12 hr stated burn time. He sells Lopi and Avalon.

    I also stopped by the Quadra Fire dealer and talked to the salesman who sold me my QF 5700.

    He also had the same explanation for the EPA vs mfg efficiency numbers.

    When I told him that I installed a stovepipe damper because there was no way I would get an efficient overnight burn he didn't say anything.

    I asked if his installers ever put a damper in and he said no. I asked if they ever get complaints about short burn times. He said no.

    He said that Quadra Fire does not endorse damper usage.

    I carefully did not ask if it voids the warranty. I don't see how a damper could ever cause an overfire. I see where not using one if you have a strong draft will cause and overfire.

    MnDave
  8. wkpoor

    wkpoor Minister of Fire

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    I read more and more on here about people in our position with tall chimneys that give way more draft than our stoves where designed for. The dealer near me that sells QF and Hearth says they have installed 2 dampers in tandem for real tall chimneys. I know there are days my one dampers doesn't slow it down enough.
  9. MnDave

    MnDave Guest

    Rumor has it that the EPA tests with a 15 foot chimney with outside temps in the 80's. So, yes, that affects the design of stoves.

    I think that my stove needs a good draft for a solid start. The stove damper in my mind is a simple solution to a problem that many would love to have.

    My last non-EPA stove in this location would overfire if I left the air control open more than half way. When I shut that control it could totally choke the flame out. I did not have nor need a stovepipe damper then.

    My new EPA stove on the same chimney would surely overfire with the control open half way. This is because there is so much uncontrolled air allowed to enter these EPA stoves. If I shut my stove air controls all the way down, my stove will not get anywhere the burn times that the mfg lists.

    Solution, stovepipe damper, $3. I put one in and just the presence of it (vertical) made a noticeable difference.

    This is really the first time I have ever heard of someone not choosing a particular installation because of too much draft.

    I would first get a length estimate before I started worrying about too tall a chimney. The roof pitch also plays into this.

    MnDave
  10. Todd

    Todd Minister of Fire

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    Last time I looked at the EPA wood stove site there were a few stoves that weren't thrown into the typical default efficiencies. BK King was the highest rated followed by the new Woodstock PH. They also had the new Lopi Cape Cod in there. Maybe they changed something and are now testing for efficiency instead of using the old default?

    IMHO I do believe stack temps play a roll in efficiency but I'm not sure how they calculate it. It just makes sense to me the less heat that goes up the chimney the more heat that stays in the room.

    http://www.epa.gov/compliance/resources/publications/monitoring/caa/woodstoves/certifiedwood.pdf

    Looks like they have both tested efficiencies and defaults.
  11. Highbeam

    Highbeam Minister of Fire

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    For another angle, efficiency ratings are not important. They just don't matter. All of the stoves that are being sold meet EPA emission requirements so you know that they are generally good. Other than cat vs. non-cat we do not generally see members here experiencing a reduction in wood consumption with different stoves of the same size. Firebox size,operating skill, and fuel quality have a lot more to do with efficiency than these ratings.

    This ain't a gas furnace. Ignore the efficiency ratings and do a good job of sizing the stove and preparing your wood.
    Huntindog1 likes this.
  12. Highbeam

    Highbeam Minister of Fire

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    Actually, the BK gets #1 with the king and ties for #2 with the BK princess matched by woodstock's PH. Did you note the the EPA has a new default efficiency for the PH of 78? and the cape cod is listed as a cat stove.
  13. begreen

    begreen Mooderator Staff Member

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    Agreed, a lot of efficiency is in the hands of the operator. EPA testing a long while back proved this. Lots of decisions like the wood burned, how the stove is loaded and when, blower operation, stove maintenance, damper use or not, can affect the amount of heat the stove produces and how cleanly it burns.

    EPA testing simply establishes a baseline with a set of controlled parameters. This makes it easier to do side by side comparisons of stoves but is not intended to reflect the full capabilities of the stove nor the variables possible due to region, wood quality and user operation. This is similar to EPA published car gas mileage. They don't test how fast the car can go or what mileage you will get if you have a lead foot.
    Huntindog1 likes this.

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