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Wood Usage Calculation

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Dec 2, 2010.

  1. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,295
    Loc:
    Northern MN
    For anyone interested in fine tuning their firewood needs, or just learning more about their wood heating system, what follows may be a methodology that is useful and informative. During the 2nd half of November I logged outside temperature, which has allowed me to calculate Degree Days (based on base temperature of 60F). I also maintained constant my radiant floor temperature at 63-64F, which resulted in inside shop air temperature of 58-64F, and I weighed the wood burned, including kindling. All calculations are based on the assumption that my wood had effective heat value of 6,050 btu/lb (20% MC and 400F flue temp). The shop is 1500 sq ft, R19 nominal walls and R39 nominal ceiling, fairly typical for much newer construction.

    During the test period Nov 17pm - Dec 1 am (14.54 days actual):
    Degree Days (base 60) = 558
    Wood burned = 765 lbs (1,288 lbs burned during the entire month)
    Wood heat value = 4,628,250 btu
    Btuh = 13,261
    Wood/hr = 2.2 lbs (weight a small log to see how small this actually is)
    Wood/day = 53 lbs
    Btu/Degree Day = 8,294
    Wood/Degree Day = 1.37 lbs

    Firewood burned during this time was all jack pine, which seasoned (20%) typically weighs 2,670 lbs/cord.
    Wood burned for whole month: 0.48 cords
    Wood burned during data period: 0.29 cords

    Annual normal Degree Days for a reporting station 5 miles from where I live are 9,017 (recalculated to base 60 from base 65). If 1.37 lbs wood/Degree Day, then my annual wood needs are 12,353 lbs, which is, for example:
    Aspen: 5.38 cords
    Jack Pine: 4.63 cords
    Cherry: 3.96 cords
    Red Oak: 3.39 cords
    White Ash: 3.35 cords
    White Oak: 3.08 cords
    Hickory: 2.85 cords

    Besides Jack Pine, Aspen is the other primary wood I burn, so right now it looks like I will need about 5 cords of firewood per typical year, and my goal is to always burn 2nd year wood, so I will need 10 cords on hand for the start of each heating season.

    These numbers, and other data I collected, can be worked to provide other interesting information, but this seemed most useful. Nov - Mar are the normal cold months where I live, and I hope to continue to log data to better understand how btuh changes with differences in the dT between inside and outside temp, and therefore how actual wood usage/needs may vary from this initial calculation.

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  2. Pat53

    Pat53 Minister of Fire

    Joined:
    Aug 21, 2010
    Messages:
    568
    Loc:
    UP Mich
    That is pretty amazing considering where you live. My degree days are probably similar to yours, but I expect to burn about 7 cords hardwood, mainly oak. I have a total of about 3500 sq ft to heat . I do think however that I need more attic insulation, I believe there is only about 6" up there. In addition, the "warden" (aka wife) always wants the house around 72F, so I oblige, for obvious reasons ! LOL
  3. in hot water

    in hot water New Member

    Joined:
    Jul 31, 2008
    Messages:
    895
    Loc:
    SW Missouri
    Great info, thanks. At first blush I thought 5 cords seemed excessive for 1500 square feet. But living in SW Missouri I have become somewhat spoiled, as I have a much lighter heating season and plenty of dry hardwoods available.

    I hope to heat 1800 sq ft with 2- 3 cords this year. My shop is sprayed foam with 6- 8" everywhere, that helps a lot with both r-value and infiltration.

    Your data does show how much difference there is in location, insulation and fuel. You have some time on your hands? :)

    hr
  4. nate379

    nate379 Guest

    If all works out should use no more than 3.5 cords this winter. Been heating for 2 months and have burned about .75 cords, lot of that was cottonwood, old old flooring and punk wood, so pretty much "junk". Not sure how the temps are in Minnesoda but it's pretty mild here, coldest we see is maybe -20*
  5. SteveT

    SteveT Feeling the Heat

    Joined:
    May 21, 2008
    Messages:
    335
    Loc:
    West of Boston
    This is an interesting calculation; thanks.

    I am curious as to why you decided to go to base60 for the degree day calculation as opposed to the conventional base65. My understanding for using base 65 is that it represents a zero fuel day; 65 degree air temperature plus "typical" heat rise from radiant solar and most people will not use their heat at all.

    Are you just saying that since you like your shop somewhat cool, that you wouldn't need any heat on a day that the temperature averaged 60 degrees? If so, I think others could use your exact methodology but use the baseXX dependent upon what they feel is their zero fuel day.
  6. jebatty

    jebatty Minister of Fire

    Joined:
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    4,295
    Loc:
    Northern MN
    I used Base60 because that's about the temp I like to maintain in the shop. Any warmer, and it gets too warm to work. Even 1t 60, it can easily be short-sleeves. Radiant from the floor feels warmer than the 60 air, which is one of the reasons radiant is so good - it heats the surfaces, not the air..

    For others who want to try this, the easy way to get your degree days is simply average the hi and lo for the day, subtract that from your BaseXX, and that's the degree days. Then weigh your wood, use the 6050 btu/lb, and the rest is pretty easy. A lot of home in/out thermometers have a daily hi/lo feature, which would be helpful. For your annual degree days, look here: NOAA.

    I calculated degree days using the average of my data points, which log every 5 minutes. That probably is A little more accurate because it takes into account longer cold or warm periods rather than assuming a linear temperature curve.

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