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Efficiency misconceptions........

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by heaterman, Feb 1, 2012.

  1. hobbyheater

    hobbyheater Minister of Fire

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    History. In the mid 70's, there was an article in Harrowsmith about a large sand filled bunker that had stainless steel tubing going through the sand to exchange the heat. It featured a fire box design much similar to that of the Garn.
    The Jetstream is fading into history but the Garn is still here :exclaim:

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  2. woodsmaster

    woodsmaster Minister of Fire

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    Speaking of efficiency how much power does it take to operate the motor on the garn ?
  3. heaterman

    heaterman Minister of Fire

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    Approximately 700-900 watts per hour depending on the model of Garn for 2-3 hours depending on load size.
    There is obviously no cost involved in moving combustion heat into storage. No pumps/piping required.
  4. Floydian

    Floydian Feeling the Heat

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    Lots of good stuff in this thread,thanks Hearth.com!

    So a Garn 2000 uses about 2100 watts to deliver about 1,275,000 btus to storage, thats about 600btus/watt.

    How about some of you folks with downdrafters+storage? It would be interesting to see some numbers on electrical consumption from boiler to storage.

    Heaterman, do you see ECM fan for motors for these applications in the not so distant future? Seems to be another place shave some watts.

    Noah
  5. bpirger

    bpirger Minister of Fire

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    I would be interested in hearing from Garn users using high temp water (i.e. not radiant). How low can they go on the Garn temp? How hot do they fire to?

    With my data from my 1500 back on page 4 of this thread, I don't really understand how high temp systems would work well with a Garn....at least for extended storage purposes (i.e. one fire a day).

    We love our Garn, but I've steered a few people away from it due to their need for higher temp water. I'd like to see/know that I'm wrong...
  6. heaterman

    heaterman Minister of Fire

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    If a given heating system consistently requires 180+ water then any storage system is going to be limited in its usefulness. A far better option would be to address the heating system itself or the building envelope in order to use lower water temperatures. There is a reason that by law (or papal edict?) the maximum system temperature in Europe is 167*. The other item of note that goes on in Europe is that they routinely design systems with delta T's of 30-40 and even 50* temp drop. Less pumping power is required and lower temp = higher system efficiency.

    I have however several Garn installations that work between 150-190 and work very well. They just fire more often.
  7. SmokeEater

    SmokeEater Feeling the Heat

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    I sure have a lot to learn about wood boilers and all their needs and specs to operate efficiently, but I do know a few things about energy, especially electric energy because I've been connected with producing it for over 30 yrs. A watt is a unit of power or energy use per unit of time. If the Garn 2000 has a total need of 700 watts to operate and it operates for 3 hours on a single "burn", then the Garn will use 700 watts x 3 hours or 2100 watt-hrs of electric energy. Power companies charge by the amount of energy used or "consumed" and a watt-hr is that, though usually to a kilowatt-hr by dividing the use in watt-hrs by 1000 and expressed as the unit kWh. So the Garn would use 2.1 kWh per burn to generate the 600 btu/Wh. If the power company charges $0.10/kWh, then the energy purchased would be 2.1 kWh x $0.10/kWh or $0.21. Just contributing my small share of knowledge with all.
  8. ewdudley

    ewdudley Minister of Fire

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    Not sure about your units, it should be reducible to a dimensionless number, e.g., power divided by power or energy divided by energy. At any rate my little boiler delivers 70000 btu per hour while consuming 165 W (fan plus circ), which is about (425 btu / hr) / watt, or a COP of 124 if you prefer. In my case heat production is intentionally limited to get better heat transfer, so I suspect other downdraft units with better heat transfer could do better.

    --ewd
  9. Kemer

    Kemer Member

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    I have a hot air system and run between 150 -190 I can go 24 hours in this kind of weather but I like to build a fire in am & pm.By doing it am & pm all I have to do is adjust my load with the weather conditions and how much wash and showers were done.Also I keep a higher temp and keep it in the "zone" for faster recovery.
  10. bpirger

    bpirger Minister of Fire

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    Thanks Harry and Steve. I guess two smaller burns a day would make this work. Agreed, if you need 180, any storage is going to be hard to supply for long. Harry, how often does your HX water pump run when it is cold outside? Obviously this is load/house dependent. When I think of hot air systems, I think of them cycling every 20 minutes or so....but I also lived in old, drafty houses. Do you run the water pump to the HX independently from the blower in the plenum?

    I know with those data curves, my output supply had dropped down considerably in just a couple of hours....and the pumps were on maybe 30 minutes during this time. Obviously there's a differnce between dumping in 200,000 pounds of concrete and using the water to air HX....so perhaps your cycle times are much more infrequent, and shorter, than I'm thinking...
  11. DaBackBurner

    DaBackBurner Member

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    Bruce are you mixing or doing a variable speed prior to your flat plate? I know I brought this up once before but it seems if it was mixed prior to the flat plate it would allow better/longer stratification. I'm hoping it will as I plan to do something this spring. Definately going to do an ODR for sure and I may bypass my FP and run my house as un-pressurized like my parents house and my workshop. In the meantime I've been making two smaller fires as opposed to one bigger one and limiting my max storage temperature. It seems it has made an improvement with wood consumption because it can heat that cooler water pretty darn fast. Time will tell I guess.
  12. jebatty

    jebatty Minister of Fire

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    This may or may not work. Notice the comment of DaBackBurner above related to Net Positive Suction Head. If there is enough head supplied by the Garn to the circulator to prevent cavitation, and if the circulator still can still move the hot water through the system, then it may work to skip the heat exchanger. I previously used an OWB for heat to a rental house, single floor, with the OWB at the same elevation as the first level of the house, plus a weighted vent tube that maintained about 2 psi pressure in the OWB. This worked fine to heat the house without a heat exchanger. The house also had an LP boiler in the basement, and that needed a pressurized system to work. Consequently, when using the OWB I had to valve off the LP boiler, and when using the LP boiler, I had to valve off the OWB -- thus keeping both systems independent. A bit of a hassle, but it worked for 7 years.
  13. goosegunner

    goosegunner Minister of Fire

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    What does that do to tank stratification?

    I have found if I run my econoburn pump at a low gpm I can top off the tank with hot water. If I run it at 18gpm med speed or 22gpm high speed it will mix the tank to about 160 degrees and then bring it all up from there.

    For twice a day burns I like to use the lower speed and keep the hot water on the top of the tank.

    gg
  14. Kemer

    Kemer Member

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    Before Heaterman educated me on Garn stratification I had my plumber put a relay on my Garn so if the fan is running my pump on the garn side is also running.
  15. bpirger

    bpirger Minister of Fire

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    Back burner....On the Garn side of the HX, I just have an 00R pump which is turned on/off with the call for heat. So when the Garn is hot, a call for heat (for radiant for example) will run the pump for about a minute or so...every few minutes. On the other side of the HX, the hot water is dumped into my primary loop.


    My mixing for the radiant is done with variable speed injection pumps....but inside the house at the floor manifolds.

    Some kind of mixing out of the Garn itself presumably would limit the temp of the water pumped out of the Garn. Now I essentially do that by limiting the time the pump runs. (Tekmar 363 controller is running the show...call for heat from it runs the Garn pump and the Garn secondary loop injection pump)
  16. woodsmaster

    woodsmaster Minister of Fire

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    My boiler has two fans. On high they use 160 watts the circulator on high uses 60 watts If I remember right. so 220 watts on high
    and it takes around 4 houres to burn a full load 60KW boiler.
  17. DaBackBurner

    DaBackBurner Member

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    bpirger-

    Thanks again for the explanation (forgot you are injecting) on the house side of the HX.
    Daaaanggg! Just looked up the price of the 363. That's more than I expected. Oh well, guess you can't take it with you.
  18. bpirger

    bpirger Minister of Fire

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    Yeah, when I built the house and designed the system, the 363 sounded like an outstanding controller so I bought it. At the time, 2001, I wasn't very aware of everything else that was available....somehow I got turned onto Tekmar. But it controls the mixing in the house floor, the DHW, boiler demand, boiler protection, primary pump, etc. I think it can do more as well....when I get everything hooked up in the addition, I'll have to sit down. I tried to set the boiler limit to 150, thinking it would shut off the garn and secondary pump (with the call for heat) keeping the water no higher than 150, no lower than what it needed....but it seems to NOT allow short cycling of the oil boiler. SO when the floor wants 120, the primary loop will cool down to 110 or below and it won't kick it back on. Likely I can read how to prevent this. But it didn't seem to work as I wanted a couple of weeks ago and without all my temp sensors up and running, I can't really tell what's going on.

    Sure is a fun hobby!

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