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Sizing new boiler

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jrod770, Nov 3, 2013.

  1. Nofossil

    Nofossil Moderator Emeritus

    Joined:
    Oct 4, 2007
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    3,398
    Loc:
    Addison County, Vermont
    Couple of rules of thumb - not always dead-on, but usually close:
    • Average heat load is typically about 50% of calculated peak load
    • Average boiler output over the course of a fire is about 75% of rated output
    In my case, a peak load of 30kBTU per hour and an average load of 15k. I have an Eko 25 rated at 80kBTU/hr with an average output of around 60k.

    Average winter day: 24 hours at 15kBTU/hr = 360,000 BTU
    Average winter fire: 7 hours at 60k = 420,000 BTU

    Where does the extra 60kBTU go? Domestic hot water and hot tub.

    For me, 7 hours of burn time per day works well - 7 hours heating from the boiler and 17 hours from storage. I don't have enough storage, so if I had a bigger boiler I'd be all done after a 3 hour burn and I'd have a hard time heating the house from storage for the remaining 21 hours.

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  2. kopeck

    kopeck Minister of Fire

    Joined:
    Mar 24, 2011
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    524
    Loc:
    Maine
    Something else to keep in mind, and it's somewhat dependent on what kind of storage you use but in some situations you can only add heat to storage so fast. I use unpressurized storage with an exchanger (copper coil). During a "normal" winter day my Tarm Solo Innova 30 will load the tank with out any/very little cycling since any extra heat is taken up by the house's heat load. Tonight I threw my first load in of the season it's it's cycled a few times since there's very limited heat load from the house so nearly 100% of the heat is going into the tank. Obviously with my setup sizing the exchanger to the boiler is key, my exchanger seems to to be effective at removing the heat (plenty of temperature difference), it just doesn't flow fast enough to keep up with my boiler output. The solution in my case is in the shoulder season shouldn't try and push my tank up to it's max temp. A smaller load of wood will make better/more efficient use of the wood.

    I would guess this would be less of a problem with pressurized storage but could still happen if you pushed it hard (ie large boiler, no load, everything going to storage).

    What I'm trying to get at is bigger isn't always better, that is if I 40kw unit will work a 80kw unit wont necessarily work better. :)

    K
  3. jrod770

    jrod770 New Member

    Joined:
    Oct 22, 2013
    Messages:
    63
    Loc:
    granville, ohio
    Yes, this is what I'm trying to figure out. I am almost certain the eko 25 will be too small for my needs. I am not home all day long, nor is anyone else, and need something that will not require constant attention. Am I thinking right that by putting in a eko 40, or similar brand with 1000 gallons of storage, I will have longer spans in between burns?
  4. kopeck

    kopeck Minister of Fire

    Joined:
    Mar 24, 2011
    Messages:
    524
    Loc:
    Maine
    Just re-skimmed the thread I and I think you are in line with a 40kw unit and 1000 gallons of storage.

    My 30 and 820 gallons of storage needs to be fired twice a day during the winter. That takes care of a decent size house plus our DHW needs (2 3 year olds = plenty of laundry).

    On the super cold nights I throw a few extra sticks in before I go to bed, not a big deal, we're talking a handful of nights a year.

    K
  5. stee6043

    stee6043 Minister of Fire

    Joined:
    Aug 22, 2008
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    2,286
    Loc:
    West Michigan
    If you spend too much time thinking about these things you will either 1.) go crazy or 2.) spend more time trying to define the system than you will spend actually using the system.

    Your time between burns is primarily a function of the size of the tank, the temp range your emitters will work within and your average heat load. The only time boiler size impacts time between burns is when you consider how long your burn cycle lasts. During a burn cycle you're heating the house and the tank with the boiler. Once the boiler shuts down you're on the tank exclusively until the next burn.

    So if you want your head to really hurt you could consider arguing that a smaller boiler might give you longer spans without a fire because it takes longer to charge the tanks, thus you rely exclusively on the tanks for "less time". While this may be true on the face of it the reality is that the smaller boiler will require more loading, more burning and more work to charge those tanks so is that really better? Not really since one big goal of thermal storage is convienence for the operator. Like you said above you don't want to babysit it so smaller is not better.

    And to push ourselves to the verge of head-exploding you could argue that a larger boiler would reduce the span between burns because they would charge the tanks so stinking fast that you'd end up with a much larger gap between peak tank temp and refire. Would a larger boiler make matters worse? Ohh the agony.

    The cold hard truth is that nobody on this board is going to be able to answer your questions with 100% confidence, sadly. Every house has different loads. Every load of wood will burn uniquely. Every boiler make/model/serial # will have it's own personality. Every different weather pattern will impact your performance. Etc and so on. Burning wood is as much an art as it is a science. As long as you develop a system that should work in the "theoretical" envelope of what your home needs, go for it. You'll quickly learn in your first season that all the math in the world won't make that EKO burn in September the way it does in February. And you'll develop a deep love for oak or cherry and might grow to loathe pine. All part of the experience.
  6. stee6043

    stee6043 Minister of Fire

    Joined:
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    West Michigan

    I feel like maybe my post above provided you with zero help. So I'll offer this.

    I'm heating 3200 square feet with the EKO 40 and 1000 gallons pressurized storage.
    During a typical winter day I start my fire in the EKO after dinner, say 7ish. I start with a very small load, an hour burn max. At say 8ish I throw in a half load or so. Enough to get me to 10pm or so (bedtime).
    Right before I call it a night I will go down and throw a final load in the EKO. If the temps are going to be well below zero overnight I will fill it 100% full. If the night is going to be in the 20's I may throw in another half load. Depends on the day and the mood.

    Depending on the size of my last load, the weather, and the # of beers I had the night before I will usually see my tanks at or near 180 degrees top to bottom when I leave for work in the morning. On MOST days this is perfect to get me through until another post dinner fire start while keeping my tank top temps above 140-150 +/-.

    During a design-load type of day I may have to set my timer to shut my EKO down before the fire is completely out (1AMish) so I can throw another load on in the morning before I go to work. This has happened 3 or 4 times for me in 5 years or so. Rather rare. The flip slide of a design-load type day is a winter like we had two years ago. I think I survied 90% of the winter without ever having to throw in my "bedtime load" of wood. It was a warm winter. And I had wood left over in April.
  7. jrod770

    jrod770 New Member

    Joined:
    Oct 22, 2013
    Messages:
    63
    Loc:
    granville, ohio
    Yes you're right, all this info will make your head spin, or is that from the beer?? Sounds like your set up is what i'm leaning towards, with forced air HX and all. It's about time to just buy the dang thing and get started putting the pieces together.

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