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Boiler efficiancy

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by bbb123, Dec 21, 2007.

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  1. bbb123

    bbb123 New Member

    Joined:
    Dec 6, 2007
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    Loc:
    Coxsackie, NY
    Trying to figure out how many btu's I'm getting outa X lbs. of wood.
    900 gal. x 8.33 lbs. = 7497lbs.
    I burned 3 -3 1/2 hours raised tank temp from 135 - 165
    7497 lbs. x 30 deg = 224910 btu
    I burned 60 lbs of wood the question is btu/lbs is 8600 but when you take moisture content out there is only 7000?
    So 60 x 7000 = 420000
    224910/420000= 53.5%

    Made sure no zones or DHW were used.
    Burned around 100lbs for the day.
    Wood is 1 - 1 1/2 year seasoned mixed hardwoods.

    Ok tear it up!

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Loc:
    Addison County, Vermont
    Love the analysis. I don't know your setup, but one major possible source of error is measuring the tank temperature. You almost certainly have a lot of thermal stratification before and/or after heating the tank, so your starting and ending temps might not be true averages.

    I think the number for air dried wood (20% moisture) is a bit less - around 6500 BTU/lb. Depends on species a bit, as well. Moisture content will hurt you twice - makes the wood heavier per actual pound of fuel, and you have to boil off the water and send that heat up the chimney. Do you know your actual moisture content?

    You're also calculating full system level efficiency, not just boiler efficiency. You're doing the right thing, but an 85% boiler will deliver a much lower system efficiency because of the energy to heat the boiler itself, the losses in the plumbing, the energy to heat the tank structure, and so on.
  3. bbb123

    bbb123 New Member

    Joined:
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    Loc:
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    I have 3 temp. guages in tank top, middle, and bottom when I started they were all the same. After the burn I had to do a little "averaging" but the middle one was pretty close. I am losing some heat with all the plumbing. I don't know the moisture content but theres no SIZZLE. I figured the tank was 800 and the boiler and plumbing another 100.
  4. Nofossil

    Nofossil Moderator Emeritus

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    Loc:
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    SO tell me a little bit about the storage tank and how it's plumbed. You can get a very sharp thermocilne in some cases where the temp changes quite sharply in a small distance.

    I believe that wood that's stored covered with air circulation will get down to about 20% moisture in one to two years depending on the species and the length. Drying happens pretty much from the ends inwards.

    If you're deeply into this whole measurement thing, you can cut a chunk of wood from the middle of a representative log, weight it, and bake it at 200 degrees until it stops losing weight. That will give you a real moisture percentage.

    In efficiency calculations, moisture becomes the overwhelming variable as you start getting up around 80% boiler efficiency.

    How clean and odorless is your flue gas? Basic rule: If you can smell it, it's unburned and flammable. There are unburned and flammable things that you can't smell (CO, for instance), but they tend to correlate with the detectable gases.

    Efficiency losses, in no particular order:

    1) Unburned fuel going up the chimney
    2) Water going up the chimney as steam
    3) Heat carried up the chimney as flue gas
    4) Heat lost from the system components to the outdoors (not much of a problem if all components are indoors and only fired during the winter)
    5) Energy required to heat combustion air from outdoor temperatures - there's about 6 pounds of air required to burn one pound of wood. It comes from outside - through gaps and cracks in your house if not through a dedicated cold air inlet.

    Flue temp and odor gives you a good idea as to whether #1 and #3 are a problem. #2 is strongly affected by wood moisture.
  5. bbb123

    bbb123 New Member

    Joined:
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    Loc:
    Coxsackie, NY
    I have the stss tank they sell with the tarm. 2-3/4x180' HX and 1-3/4x180' DH. I basically followed the 2 boiler 2 cirulator plan tarm sends with unit. Cooking the wood in the oven is a good one I'll have to wait till wife goes back to work for that one (teacher). I'll prolly need my postal scale so I can get to the ounce. My stack internal temp runs around 350-400 with turbulators in. No smoke, I havn't got up on the roof to smell the chimney but theres not much odor on the ground. System is in the cellar there is a garage door I can see light so I'm getting some air in there.
  6. Nofossil

    Nofossil Moderator Emeritus

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    Loc:
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    Sounds like a nice system. What's the standby loss rate from the tank?

    I did a few back-of-the envelope calculations and they all came out with about 70% overall system efficiency for mine, based on 26mm BTU/cord for the mix of wood that I'm burning. I haven't weighed the wood, though. My storage tank is outdoors, so heat loss from the tank is truly loss from the system.

    I'm a data junkie - it's good to see real data. Thanks for taking the time and effort, and for sharing your results.

    If you haven't visited my site, please feel free to do so. Link is in my signature below. I've got a complete data set going back over a year with a system snapshot every 30 seconds. I'm adding combustion temp and stack temp soon, and I'm hoping to add an oxygen sensor to it as well. I've done lots of analysis, some of which might actually be valid.
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