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Garn: avoiding boil-over?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jklingel, Feb 5, 2008.

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  1. jklingel

    jklingel Feeling the Heat

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    I've recently read, again, about wood boilers boiling over when the power goes out and the fire is still cooking. That looks like a very dangerous activity, should one not be home at the time. Is it safe to say that a Garn would not be subject to such a phenomenon? Thanks. j

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    It's not all that common, IME. You could get a Garn to boil, but you'd have to work at it, I think.

    Garns are non pressurized, so in that sense they're more like an OWB than a regular pressurized boiler, which has a pressure relief valve and could potentially blow up if the valve quits working. I don't think a nonpressurized boiler like a Garn could explode.
  3. jebatty

    jebatty Minister of Fire

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    I experienced a boil over on my water jacket OWB, non-pressurized, forced draft boiler. The aquastat stuck "on" and kept the draft fan running. Imagine the surprise seeing a stream of steam shooting out of the top. I shut off the power and let the thing cool, replaced the aquastat, and never happened again.
  4. Jimxt88

    Jimxt88 New Member

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    That teaches me alot. Now I know what an aquastat does. It meaures the temperature of the water in the boiler and when that water reaches a certain temp, the aquastat shuts off the fan. When the fan goes out, oxygen is not being pumped into the combustion chamber, the flames consume the last of the oxygen and suffocate, the fire goes out and the water stops boiling. I suppose the water would keep circulating if the system was powered by thermodynamics alone and not electric pumps, valves and circulators. What is that called again?

    I would like to know from those who know - how the GARN works. Does it have an integral water storage tank? What capacity, 2,000 gallons? How does the heat from combustion transfer into the water? Radiation - I mean, is it just the close proximity of the water, as in a water jacket but here a water barrell? Or are there pipes running through the combustion chamber circulating water through a standing body of water in the barrell and heating it that way? Where does the exaughst exit? Is it the back end of the gasification tube? Does oxygen enter from the front of the GARN pass under the combustion chamber where it is heated and then come up around and enter the back of the gasification tube? Ia that a fan on the upper right of the door? What does it do? Are those blue types for circulation of something? If these things are described in another post, please refer me to it. Craig's photos are really inspiring and I'm sure no one would be better able to answer quesitons about the GARN.

    I've been trying to reach the people at GARN but no luck. I have to wonder if they've been EPAd out of business. Does anyone know of a GARN dealer in New England?
  5. jebatty

    jebatty Minister of Fire

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    Your water circulation likely is controlled separately. There may be a high limit aquastat controlling a draft fan, ie, shut it down when water temp hits 190F or so. Water/heat circulation likely is controlled by a thermostat(s) running the circulator pump(s).
  6. slowzuki

    slowzuki Feeling the Heat

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    Garn is a forced draft so yes, if power fails, combustion soon stops. Could you get it to boil over in a power failure? Only if you were right on the razors edge of boiling over before the power failure.

    The heat is extracted from the exhaust from the combustion process in the Garn.

    They are very slow to respond to inquiries btw so don't give up.
  7. Jimxt88

    Jimxt88 New Member

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    So the thermostat controls the water circulator and the Aquastat controls the fan/oxygen supply to the fire. The aquastat and the room air temp thermostat are not linked. So if there were no call for heat in the rooms and the water were not circulating but the fire was blazing, the temperature in the boiler tank would quickly rise high enough to triger the aquastat to cut off the oxygen to the fire. But that is not efficient burning, so there has to be a way of getting water heated and storing it. And how is that done? There must be some kind of open-valve/close-valve loop system.
  8. Jimxt88

    Jimxt88 New Member

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    What does non-pressurized mean?
  9. slowzuki

    slowzuki Feeling the Heat

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    In a Garn the temp does not rise quickly because of the huge water volume. You just continue charging your water tank battery.

    My boiler has a low and high aquastat. The low point turns on the circulator in basic installs, in more complicated ones it provides an input saying ready to pump. The high point shuts off the combustion fan irrespective of anything else.

  10. chrisfallis

    chrisfallis Member

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    The Garn does have an "overflow" vent hole at the top that dumps any excess pressure (water or air) to the atmosphere. The manufacturer's recommendation is that you pipe that overflow to the very front of the boiler. The theory is that if you are ignoring the danger signs from the thermometer right next to the loading door, all that hot water pouring on your feet should get your attention.
  11. deerhuntrer

    deerhuntrer New Member

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    An CB OWB will NOT boil over when the power goes out, it shuts down completely when the power goes out. (damper shuts)
  12. NickR

    NickR New Member

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    Jim- here is contact info for a very helpful Garn rep in Maine
    Chris Holley
    207 677 2509

    He does system design
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