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Wood boiler size

cutter
cutter Member Posts: 282
A while back there was a discussion about a wood boiler size. I am heating 2375 square feet (basement and main level combined). I am using a 140.000 BTU Buderus discontinued coal and wood boiler. It is piped like the picture in Idronics #10 with the hydraulic seperator. Pumps on each boiler and another pump pumping to the zones. I have oil back up piped in also.
With the -25 and -30 temperatures in Minnesota right now it takes quite a while to bring the house back up to 70* after the fire dies out overnight. Once the house reaches 70* upstairs and with continued stoking of the fire every 1/2 hour or so and maintaining 170* to 180* water the house temperature upstairs will not get above 70*. So my thought is a little over size of boiler on the wood side might be a good Idea. To say it another way a 100,000 BTU wood boiler might not be equal to a 100, 000 BTU conventional conventional. I think an 85,000 BTU conventional boiler would heat this house. Just my thoughts

Comments

  • Robert_25
    Robert_25 Member Posts: 361
    The BTU rating on a lot of wood boilers is based on a clean boiler burning small splits of dry wood. If that is not how you are running the boiler, you will not see the same output.
    GrallertSolid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 15,029
    Do you have enough heat emitter to keep up on those cold days? is the boiler catching up and cycling off ever?

    You really need to keep a good hot fire going to get even close to rated output, almost hourly tending.

    Which model do you have?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • GroundUp
    GroundUp Member Posts: 1,282
    Typically, a 100k BTU boiler is rated with a 100k max output at the highest point in a manipulated cycle. Sustained output over, say, an 8 hour period might be 20k based on firebox size and water quantity.
  • cutter
    cutter Member Posts: 282
    hot rod_7 said:

    Do you have enough heat emitter to keep up on those cold days? is the boiler catching up and cycling off ever?

    You really need to keep a good hot fire going to get even close to rated output, almost hourly tending.

    Which model do you have?

    Hot Rod, I was not really complaining about my boiler being to small or not doing it's job. With that -25 and -30 outside temperature and the wind I feel this boiler did a good job with the conditions outside. I was sort of referring to another post where the home owner was given a lot smaller wood boiler to heat a larger house than mine. But then again where in the US was he located?

    This Boiler is a Buderus, Logana 02.40. On the boiler door it says 140,000 BTU and also 125,000 one rating is steam and one is wood. On the paper work that came with the boiler it looks like this boiler is 125,000 BTU with wood. I was stoking this boiler every 1/2 hour or so it would keep up with the outside temperature.
    I feel I have the heat emitters, I need to replace a Becon Morris Twin Flow that the squirrel cage fan Died.
    The boiler caught up and maintained the room temperature. Never had to dump any hot water.
  • cutter
    cutter Member Posts: 282
    Robert_25 said:

    The BTU rating on a lot of wood boilers is based on a clean boiler burning small splits of dry wood. If that is not how you are running the boiler, you will not see the same output.

    Robert, Looks like this boiler is 125,000 BTU wood and 140,000 BTU steam. That statement does not make sense but that is what I interpret from what is on the boiler door.
    I am burning dry wood, but they are not small splits. some rounds and different size splits.

    Read what I sent Hot Rod, there is more detail there.
  • hot_rod
    hot_rod Member Posts: 15,029
    Manually fired wood boilers are probably the toughest device to get a good handle on as far as performance. So many variables as you know from fuel type and condition of the fuel, to operator skill, how often they are cleaned, etc.

    Only by weighing the fuel and adding a BTU meter would you get a good idea of your actual performance.

    In the real world most boilers are over or under-sized as mother nature never presents the exact same conditions.

    When you have a fixed speed, manually fueled boiler it REALLY becomes and owner/ operator sensitive appliance.

    I'd agree with a tad of oversizing as keeping an ideal burn is next to impossible without constant care. I have a tough time getting up several times during the night to stoke :)

    Of course extended periods of below design conditions would require enough heat emitters to move the excess the boiler might have.

    This is why buffer tanks come into play and properly applied they really smooth out the operation.

    The first best place to store your heat energy is in the un-burned wood in the pipe :) Once combusted it starts slipping away from you.

    Within the heated envelop or a well insulated buffer tank would be the next best place to park that energy.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • Nimrod66
    Nimrod66 Member Posts: 13
    Wood boiler output is often rated as maximum output not average output. Between lighting the fire and the point that the fire is nothing but cooling ashes again, output will vary. Typically, up-firing boilers with cast iron grates like the Buderus will have a bell shaped output curve with the highest output occurring after moisture has been driven from the wood, but volatiles (smoke) still remain.

    Modern boilers have an output curve that is shaped like a plateau. The wood is burned in a much more controlled fashion. Modern boilers also have a total output test as part of EPA approval, so it becomes easier to know what you are buying. Having said that, wood fuel energy value varies greatly depending on density and moisture content.

    I imagine your Buderus has a maximum output of 125,000 Btu/hr. at its peak and the output is lower at the beginning and end of a fire. Average output over a burn, if you let the fire go out without re-fueling might be as low as 50,000 Btu/hr. If you keep re-fueling the boiler on a 1/2 full load of already burning wood, I imagine that total output will hover near the 125,000 Btu/hr. rating.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,304
    To really know what a boilers output is you would have to:

    Accurately measure your supply and return temps. Then figure the average water temp over a given time.

    Then you must get as exact a number as you can of your system flow. Then you can fairly accurately put a BTU/HR number on any given piece of equipment.

    I've installed many and used 4 different wood boilers in my lifetime. If you've heated with a wood stove its fairly similar, the bell curve with a flat top seems to be heat is emitted.

    I have a "45Kw" or 153,500 BTU/HR gasification wood boiler and have my emitters and thermal storage controlled in such a way to leverage its output pretty well. Back when I had oil my Toyotomi OM180 held 5 gal of water and was very low mass fire tube S.S. with an output of 145,000 BTU/HR. It would hit high limit even with all my emitters calling simultaneously.

    So even with a glowing hot clean burning refractory the output is nowhere near the rating of 153k. I believe it's in the neighborhood of 100-120k now that I have a mod/con as backup and can get a pretty accurate output of each zone based on the modulation of the gas boiler.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!