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Garn Low Temp Storage Question

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by eauzonedan, Sep 17, 2013.

  1. eauzonedan

    eauzonedan Member

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    Loc:
    Bayfield Co. Wi
    I'm ramping up a Garn1500 and need to make some design decisions as to how low I can practically pull my storage temp. This number would drive the sizing of a few panel radiators that will cover a couple areas that I didn't heat with either in-slab or Warmboard. I also plan to use the Garn for pre-heating well water for domestic hot water in a conventional water heater. There will also be an LP fired backup boiler in the system for "just in case". The rads will be the most "temp hungry" of the lot, but get pricer as you upsize them to take advantage of the lowest stored water temps.

    Current thought would be maybe pull the Garn as low as 125 with a deltaT of 5 thru the main HX. This would provide 120 to the rads and appears to be about the practical limit without getting too huge (and also expensive) for the rads.Running anti-freeze in the entire secondary is also in the plan and will further impact their sizing.

    Any input as to considerations as far as pulling Garn storage temp that low as well as practical limits on sizing rads would be appreciated.

    Dan

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  2. heaterman

    heaterman Minister of Fire

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    Lots of variables there Dan but basically speaking if you have a rad rated for 6,000 BTU @ 200* water temp and you want to find the output at 120-130* range, derate it by a factor of 3.5. So that would net you approximately 1700BTU. If you keep it at 140 or so that derate # will be right around 2.75 or near 2,200btu. Those are rough numbers and do not take into account actual room air temp variations that can influence output by 10% between 65*-72*. Lower room air temp = higher output because of the greater temp differential.
    That help?

    www,hydronicalternatives.com has a chart on their website for low water temperature performance that will go through all the math if you are so inclined.
  3. maple1

    maple1 Minister of Fire

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    You can get a lot more mileage out of simple fin tube baseboard at lower temps just by putting more in than would be sized for hotter water. They'll give up heat at lower temps, just not as much. I quite often run my storage down to 140 or so when heating, and my house stays warm. I think I remember reading the rad specs way back & they listed output down to temps hotter than 140 - so rad specs alone might be a little misleading, making a person think they won't work below a certain temp when they really will, just that you need more of it.

    Not sure that circle talk is applicable here. ;?
  4. eauzonedan

    eauzonedan Member

    Joined:
    Jan 21, 2011
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    Loc:
    Bayfield Co. Wi
    Thanks Guys!

    Uncertain what "circle talk" is however - maybe you can educate a newbie.

    I think I have a conceptual handle on the conversion tables to size the rads once I settle on a supply temp. My preliminary estimate of de-rating to 1/3 of output at their standard conditions appears to be in the ball park. My in-building locations just don't lend themselves to baseboard and rads will better fit the physical locations.

    Any thoughts on my target low storage temp of 125 degrees at the tank / 120 degrees in the secondary - based on real world experience? I understand there are a ton of variables, but I'm always looking for those un-intended gotcha's that seem to lurk out there for the inexperienced (like yours truly). I attempted to upsize my system (underground pex and future HX) so far - to allow for greater gpm flow rates (at lower temps) yet still be able to transport the btu's 'and thereby allow for utilization of the low end storage temps. Low temp emitters were chosen for the same reason. The thought of only pre-heating the domestic was with the same intent as well keeping things as simple as possible . A standard electric water heater is fairly cheap, would only be used to "top up" the domestic hot water temp therein and would also function on its own as a stand alone - if the boiler was not run in the summer.
    Dan
  5. leon

    leon New Member

    Joined:
    Feb 3, 2013
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    In my case I have a pressurised Switzer 100000 BTU coal and wood
    boiler that I have had for 31 years.

    I filled half the firebox half full of full fire brick and I should have done it 31
    years ago. as it has helpe tremendously with batch buring and as a heat
    sink.

    I have left my boile temp at 160 degrees since my house is so
    poorly insulated and old.

    Is there any real reason you cannot leave the settings at 180 degrees for
    the high end and 160 for the low as you have the benefit of storage in a large volume?????????
  6. maple1

    maple1 Minister of Fire

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    'Circle Talk' = talking in circles around a topic but not right at it.

    (My definition). ==c

    Leon - what settings are you talking about? No reason to have a backup boiler set that high if you will be pulling storage down to 120-130. Your backup would needlessly cycle in that case.

    I see not much wrong with figuring on temps down to 120 - if that turns out not to be quite hot enough, you can always just burn a bit more often to keep a higher temp. I think you're way ahead of the game in oversizing radiation - way better than the alternative on cold winter days.
  7. leon

    leon New Member

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    ======================================================================================================

    The high limit (160)limit aquastats on my wood boiler the and oil boiler low limit(140).
  8. BoilerMan

    BoilerMan Minister of Fire

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    Ever thought about Cast Iron radiators? If you like the looks or they fit the décor. They can be had for cheap in most cases and are usually HUGE lending well to low storage temps.

    TS
  9. Jim K in PA

    Jim K in PA Minister of Fire

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    Dan - Achieving a delta across the HX of just 5 degrees is going to require a pretty big exchanger. Also called the "approach temp", it is tough to get that close to 96% transfer efficiency (120/125=0.96). I sometimes get down to 7-8 deg F delta, but am usually closer to 10.

    It is all about transfer surface area, wherever the heat is being handed off from one media to another. Low temp panel rads are rated for lower temps based on high surface area/volume ratios. As mentioned above you can do something similar by adding more linear feet of finned tube BB radiator, or large surface area cast iron radiators. Your peak heat loss calc is always a good place to start considering what will be "enough".

    HTH.

    Jim K in PA
    Last edited: Sep 23, 2013
  10. woodsmaster

    woodsmaster Minister of Fire

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    from my experience once the water gets down to 120 you don't get a lot of heat from the rads. 120 is about as low as I let mine get before charging. You could go lower on days that are calm, cool, and sunny if your not heating your dhw
    all the way with the boiler. On days that are coldest, windy, and cloudy I might have to keep my water at 140 to stay toasty.
    skfire likes this.

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