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Geo Thermal Sizing

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Henry_6
Henry_6 Member Posts: 32
> Hope the energy loads and soil conductivity have <BR>
> been checked. "Heating only" ge-exchange systems <BR>
> are notorious for creating permafrost in a couple <BR>
> of years with improperly sized systems in heating <BR>
> dominated climates. Quite a few coming to light <BR>
> up here in Canada, it takes a couple or more <BR>
> years sometimes, depending on the climate <BR>
> variations, but it can happen. <BR>
<BR>

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  • Brian
    Brian Member Posts: 285
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    Geo Thermal Sizing

    One of my contractors called me to lay the radiant lines in a 3200 sq ft utility shed.He said the heat source is going to be geo. The owner is going to be building a house about a 100' away from the shed.The two geo contractors that are bidding on the job say he will need two separate systems.Shouldn't you be able to install one system and just run insulated lines from one building to the other?

    Thanks,
    Dobber
  • [Deleted User]
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    Yeah but...

    and there's a butt in every crowd :-)

    Consider the transmission losses from the insulated lines. It's tough enough to deal with GEO as it is because you are commonly limited to 115 degrees F for a purebred system. Start dropping 5 or 10 degrees F in transmission losses and you're even deeper in dodo before the dodo hits the fan!

    Unless of course you can SUPER insulate the lines, but then you need to look at the economics of doing that, and compare it against placing the physical plant closer to the load.

    I suspect that you will find it cheaper and more efficient to put in two seperate systems...

    But YOU have to make the call. If the 3200 sq foot bldg has a loss factor of around 20 btu's per square foot, then you're looking at needing better than 5 tons anyway. Remember, those heat pumps dont put out the same energy for heating that they do cooling. Here in Colorados soil conditions, we typically "derate" the output by around 20% or augment it with high efficiency modcon boilers. So in your case, you'd need almost 7 tons of capacity just for the shed.

    ME
  • Brian
    Brian Member Posts: 285
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    Mark

    I'm just doing the lines.It's just that when the contractor told me their plans it seemed odd to me.Cost of insulated lines would be around $3500.00 for 1" double encased in insulation.They would be looking at about 2 degree loss over that distance.It will probably double the size of the equipment.I know it will save dollars I just don't know if it will make cents :)If we were talking about boilers then it would make sense to go with two, but I don't know if the same rules apply to geo.

    Dobber
  • Craig Bergman
    Craig Bergman Member Posts: 84
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    Limited tonnage

    Dobber;

    As far as I know, you are limited to six tons with residential Geo. Separate systems would be your best choice.

    Bergy
  • Dave_4
    Dave_4 Member Posts: 1,405
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    Energy balance and soil conductivity

    Hope the energy loads and soil conductivity have been checked. "Heating only" ge-exchange systems are notorious for creating permafrost in a couple of years with improperly sized systems in heating dominated climates. Quite a few coming to light up here in Canada, it takes a couple or more years sometimes, depending on the climate variations, but it can happen.
  • Henry_6
    Henry_6 Member Posts: 32
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    Geo

    Is this property going to have 3 phase power available? If it is available, a 10 ton water to water geo system with a mass tank and super-insulated pex lines would be a much better method of producing heated and chilled water with one vertical ground loop well operating in a closed loop method. This will allow latent and sensible heat production during the winter if you have an efficient evaporator (brazed plate). Units like this are available in Europe, because they realize that first cost is not the only cost. If you have 3-phase, then you will only need to adapt the voltage. I have used these systems successfully in New England. I can provide assistance if you would like.
  • Paul_11
    Paul_11 Member Posts: 210
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    could you explain this more

    I want to make sure I understand what you are saying. Where and how is permfrost being created. I'm not following you.

    Respectfully,

    Paul B. Shay
    pshay@arealgoodplumber.com
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  • Singh_6
    Singh_6 Member Posts: 19
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    Permafrost

    Could happen in the loop field. Most likely horizontal loops. Heat is extracted through out the heating season from surronding soil. Many ground source heat pumps can still be efficient when surrounding soil is 25* F - 32 * F (at tubing depths). Without the reverse cooling cycle dumping heat back into soil the ground can stay at permafrost conditions through out the short season summer, then back to heating mode again, pulling even more heat away, and so on through out the years.
  • Dave_4
    Dave_4 Member Posts: 1,405
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    What Singh sez

    The problem with selecting a geo-exchange system for a "heating only" application, in a heating dominated climate, means that you will be sucking heat out of the ground all the time, and not "re-charging it" with the heat dumped in from the cooling operation during the summer. If the soil conductivity is such that the local ground temperature around the vertical boreholes (or horizontal geo-coils) can't conduct the heat to/from the geo system faster than you are taking heat out, (or putting heat in for a cooling mode operation), then, over time, the geo system will gradually cool the whole block of earth off to the point where you'd have to leave the field dormant for a long time (many months or even a year) to recover. Or, inject heat back into the geo system in summer to re-charge it for the next heating season (solar panels, or use the geo for cooling as well, to dump the rejected heat back into the ground). There is at least one formerly award winning project in northern Ontario that has a frozen field of 400 foot deep boreholes even though they were separated according to the general rules of thumb. The mistake was not paying attention to the soil conductivity and energy extraction vs re-charge capability of the ground.

    I also watched a local project that was less than 10 years old here in my town where a horizontal geo field had to get ripped up and re-laid due to frozen ground. They had to add about 40% more geo-coil on a 7-8 foot deep horizontal slinky coil field to allow the geo system to operate at higher condenser temperatures so they didn't freeze the field again. Granted, in many climate zones of North America, a horizontal geo field can have the ground heat recharged by the summer ground surface penetration of summer solar heat gain - but that's part of the basic energy balance calculations of the geo system, which few folks seem to do properly.
  • [Deleted User]
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    Holy Geo Crisis Batman...

    Sounds like a job for SOLAR Man...

    I guess a lot depends upon whether you're dealing with a static aquafer or a moving,dynamic aquafer to eh...

    Enough to make you think twice about doing a heating only job.

    ME
  • Dave_4
    Dave_4 Member Posts: 1,405
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    Exactly

    My motto is "the wetter the better" when it comes to geo systems and soil conductivity. A static wet soil condition is not good though since the energy has to be transferred and absorbed somehow. A moving aquifer in the depths is best since the ambient earth temperature will be able to absorb and dissipate the energy transfer, and maintain a more stable geo-exchange temperature. The only way to know is to do a proper soil conductivity test, which is a big up front hit for small systems, but a small cost for larger commercial systems, which I am most familiar with. Once the actual on-site soil conductivity is known at the test borehole, the rest of the job becomes more of an engineering exercise rather than a guessing and assuming game.
  • Henry_6
    Henry_6 Member Posts: 32
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    Geo loops

    There is no question that heating only systems in a horizontal configuration MUST be buried at the frost line for the local area. If the idea is deeper is better, it is not and you will develop permafrost for sure. Also, damp clay is much better than sandy dry soil for thermal conductivity, but the best type of closed loop collector, without exception, has been vertical bore in hard rock like granite with moving aquifer for both heating and cooling. We have used propylene glycol inhibited at about a 30 % Solution.With the amount of demand for dehumidification and air conditioning, it makes a lot of sense to run a condensing temperature at 50-60F as compared to 120-135 F. The seasonal operating efficiency with 45-48F air delivery is excellent and the ground loop will recharge with only 500-600 hours of cooling per season. With a bedrock application and a brazed plate heat exchanger, we have successfully installed systems with 70-80 ft of borehole per ton. Don't measure once because you will be wrong. Accurate load calculations are a must and the application of the system is critical. What works in one area might not work in another. I agree that wetter is better and a static water level near the top of a 6" well will be much better than having to fill it with bettonite.Just my 2 cents.
  • Singh_6
    Singh_6 Member Posts: 19
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    May I also add

    Residential In-Situ soil testing will break the bank.
    I would try to keep maximum distance between loops, so as you're not
    pulling heat from between the two. This could make for a large field if it is horizontal. Also when excavating, install loops as quick as possible, then backfill quiclky. Having the exposed field and spoils from digging open to the air and sun will also deplete you from starting out with a good amount of moisture, and temperature, going into the heating season with less soil moisture than what is optimum.
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