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Garn wood consumption.

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by wardk, Oct 29, 2013.

  1. wardk

    wardk Member

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    I installed the Garn last year , got into the mode of filling it right up with wood and used a lot. This year I added zone valves for more control and hopefully less wood. I made weird discovery this fall, being shoulder season I only filled the firebox half full and found that that the Garn puts out the same flu temp for the same amount of time as a full firebox.I can see this saving me a lot of wood.When you think about it a fan driven fire will only get so hot and last so long.Am I breaking the law of BTU?

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  2. jebatty

    jebatty Minister of Fire

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    Same flue temp for same amount of time for a full load and for a half load doesn't make a lot of sense to me. Ideally, except possibly for a very small load of wood, the flue should reach a maximum temperature regardless of the amount of wood in the load, as the fire is oxygen limited, controlled burn. But, a larger load of wood should maintain a range of flue temperatures longer than a smaller load of wood.

    The only Garn with which I have personal experience is a fixed speed draft fan, so the same amount of air is being pushed through the firebox regardless of the size of the wood load. If the draft fan on your Garn is the same (fixed, non-variable speed), then physics suggests to me that it will take longer to burn a large load than a small load of wood, the range of flue temperatures should be similar, and the variable would be the length of time the various flue temperatures are maintained during the burn, assuming that the water temp in the Garn is the same for both burns. However, as the tank of water increases in temp, there likely will be a somewhat higher and rising flue temperature as the water temperature rises.
  3. woodsmaster

    woodsmaster Minister of Fire

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    Its going to be a little more efficient at heating cooler water temps. Mabey with the smaller load your only heating to 170 compared to 185 with a full load ?
  4. heaterman

    heaterman Minister of Fire

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    What you're seeing is the simple fact that you can burn X amount of fuel with X amount of air. The air flow is fixed on a Garn so when more fuel is available than air to burn it you just wind up with a less efficient and dirty burn. Flue temp is simply a function of how much combustion is taking place and how much heat transfer is going on.
    What were your flue temps running on the 1500?
  5. wardk

    wardk Member

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    All things being equal in this forced system I don't know if 8 splits would last any longer than 4. You guys are right , the main factor is the tank temp. increase between the two fires. Heaterman the flue is 400-450 at full burn.Thanks everyone for your input, I'll experiment with the different fires and find out if the tank temps are similar.
  6. TCaldwell

    TCaldwell Minister of Fire

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    Heaterman is correct, you will have more consistent burns if you can weigh your wood. My experience with a model 1900 size firebox suggests 70 to 85lbs to be optimun for heat output, if the splits are smaller or very dry, opt for the lighter load, if you still need more heat wait atleast 45 minutes to reload. Remember there is alot of heat in the char phase.
  7. Robert V

    Robert V New Member

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    I am in my first year with my garn 2000. I have found that the fire seems to burn better with a smaller load as well. I thinks this is due to the air/Fuel ratio. As an auto tech this make sense to me in comparison to an automotive engine. The leaner you can run an engine the more fuel efficient it will be. The difference between an internal combustion engine and a wood fire is the lean condition will damage the engine over time and create nox emissions above what the EPA would like to see. Does a lean wood burn cause damage to a wood burner?
  8. heaterman

    heaterman Minister of Fire

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    Running lean (low fuel to air) in a wood burner does not hurt it at all. The things missing in a wood burner as compared to internal combustion are the issues of compression and timing.
    As the fuel is consumed and runs out, every wood burner will go to an extremely lean condition until air is shut off also.
  9. Coal Reaper

    Coal Reaper Minister of Fire

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    i thought you cold hold you hand on the flu of a garn?
  10. jebatty

    jebatty Minister of Fire

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    The WHS3200 also has flue temps in the range of 400-450 at full burn (as does my Tarm).

    There must be a significant difference between the Garn and some other gasification boilers that have a primary firebox and a nozzle leading to a secondary ceramic tunnel or burn chamber, with the draft fan dividing the combustion air between the two, because a full load of wood in my Tarm takes a lot longer to burn than a half load. In fact, in the primary firebox the wood on top often is barely charred while the combustion in the secondary chamber is roaring blue from the combustion gases being created at the bottom of the primary firebox just above the nozzle.
  11. wardk

    wardk Member

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    The Garn has a fan downstream in the flu and pulls the make up air through the firebox and a ceramic ventury. The flu passes back and forth in the tank and the flu temp is measured before the last pass through the tank to the chimney , I would guess that the chimney would still be a least 350. If you look at my avatar you can see that all the wood in the firebox is burning equally.
  12. Coal Reaper

    Coal Reaper Minister of Fire

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    my varm concurs. consistantly 200-220*C. i brush the tubes when it jumps to 240.
  13. TCaldwell

    TCaldwell Minister of Fire

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    the flue temp at the thermometer at the beginning of the last pass is not the same as the temp leaving the appliance, if you monitor the temp in the first piece of flue pipe attached to the garn it is about 75deg lower than the thermometer. Also the hx design allows alot of residence time in a large body of cool water, enabling the ability to extract alot of temp, espically at the beginning of a burn.
  14. maple1

    maple1 Minister of Fire

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    I'm not so sure it extracts much more than other boilers? At least from the flu temps posted in this thread - if I was understanding those right. Mine also runs between 200-250c, although more often closer to 250 when it's really humping. I brush when it gets over that - mine does run a bit hotter than the fan Varms, apparently the nature of the natural draft units. I try to make a point of brushing before that, but I have my lazy periods. Would be interesting to see everyone's stack temps from all different units.

    On the lower boiler temps, I have started changing up my routine a bit - I don't think I'll try to charge up my storage as high as I was last year. I was pushing for something like 190/180 at bedtime, I think I'll try stopping 10 or so degrees short of that & just start a bit earlier the next day if needed - thinking cooler water around the HX tubes will get more heat out of them & up efficiency a bit. Might be a marginal effect though.
  15. TCaldwell

    TCaldwell Minister of Fire

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    maple, it takes less energy=higher effiency when starting at a low storage temp, than trying to get that last 5 deg into storage at the end. Since most of us have to work, higher storage temps afford a extra time period that probably outweighs a few extra splits, as nofo says the waf needs to stay high at all costs.
    hobbyheater likes this.
  16. heaterman

    heaterman Minister of Fire

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    Routine flue gas temps at the outlet of a Garn are in the 275-325*F range at full burn.
    flyingcow likes this.
  17. wardk

    wardk Member

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    I need a moisture meter, I dought the pine is at 7% that being said my usual load is a combination of Fir and pine. I have always been under the impression that fir was better but lately the pine seems to be hotter and easier to get.
  18. Robert V

    Robert V New Member

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    While we are on the topic of storage temps, what is a good goal temp for the garn? I know it says not to fire over 200 deg due to water loss, steam, ect... What are you garn owners using for a high end temp goal? I have only been running mine about a month or so and so far am happy with it. My flu temp on the controller seems to run around 320-380 on average. Is this normal?
  19. heaterman

    heaterman Minister of Fire

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    I usually tell people to try and stay 180-190* if they can. Too much risk of pump cavitation when water temps get to crowding 200* or above.
    The flue temp you are seeing is at the beginning of the last pass on the heat exchanger and the temp at the end of it will be 60-80* less than what you are reading.
    I'd say you're in normal territory. Reduce your load size or clean your flues if you see sustained temps of 400* plus for a good part of the burn.
  20. jebatty

    jebatty Minister of Fire

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    The math doesn't work. Garn controller readings of 400-450 at full burn translates to 330-380 at the outlet, which still is good (using 70 as the average). Most users will rely on the controller gauge instead of installing their own, so they shouldn't be surprised to see 400-450 on the controller gauge at full burn. I think it is important to take into account that user hx tubes are not whistle clean with every burn, that they are burning a variety of woods at variable moisture content, that they have variable wood loadings, and that they have variable tank temperatures. A hot tank temperature, say 170F will result in higher flue temp than 120F tank temp, other things being equal.
  21. heaterman

    heaterman Minister of Fire

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    You may see 400 - 450* on the readout/gauge for 15 minutes or so at peak burn but the average is about 100* less than that over the course of a normally loaded Garn.
    Very true that the flue temp will be affected by the water temperature. This is why any boiler is more efficient when it can directly fire into low temps.
  22. ewdudley

    ewdudley Minister of Fire

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    Again, this is only true under certain conditions for certain boilers that are not sophisticated enough to control flue temperature to the minimum non-condensing flue gas temperature throughout the burn cycle.

    Any boiler is more effective when firing into low temperatures, but as you well know efficiency is measured according to the temperature and composition of the flue gases, not the temperature of the water at the heat exchange surfaces.
    Last edited: Nov 2, 2013
    TCaldwell likes this.
  23. jebatty

    jebatty Minister of Fire

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    Various sources state that a 100F change in flue temperature represents about a 2% change in efficiency.

    My Tarm has constant draft which does not vary, much like the Garn; however, I can manually adjust the draft by an adjustable damper.

    The Froling with lambda and variable speed draft is interesting to watch in operation. The data sheet on the Froling says that the FHGL-50 can vary efficient output between 85,000 and 170,000 btuh. For example, as storage water temperature rises and delta-T closes, the Froling draft fan slows down to reduce output, reduce the likelihood of idling episodes, and I presume maintain efficiency.
  24. Gasifier

    Gasifier Minister of Fire

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    How much difference would you be talking on efficiency if the water temps were lower, say a tank sitting at 130 when firing begins, compared to a tank that is sitting at 160 or 170 when firing begins? Wouldn't it be a very small difference? I know there are a lot of variables involved.
  25. maple1

    maple1 Minister of Fire

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    Some things in this wood heating thing I still have a hard time getting my head around. You just brought up one. If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.

    EDIT: That was replying to jebatty's post - gasifier got in ahead of me on the reply.

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