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Basic question regarding Geothermal – how to calculate loop lengths

Alex S | Posted in Energy Efficiency and Durability on

Hi everyone,

I have a basic grasp on the general principles of Geothermal – capturing the consistent ambient temp below ground. 

It’s clear that there is a need for long lengths of pipe in order to equalize the temp – in simple terms, say water coming in at 40 degrees and exiting the loop at 50 degrees. 

How does one determine the needed lengths of pipe to achieve the full benefit of the underground temp? I realize there are a variety of environmental factors, was wondering if there’s a basic calculation to get some sort of approximate range.

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Replies

  1. Expert Member
    DCContrarian | | #1

    This isn't a simple question. You'll see rules of thumb like "500 feet per ton." The problem is that you never know what soil is like underground and it makes a big difference what kind of soil is down there. Digging holes is expensive, so you don't want to make a bigger hole than you need. At the same time if you make it too small you just dug an expensive hole that doesn't do what you wanted it to do.

  2. Walter Ahlgrim | | #2

    Is your question about a loop for a ground source heat pump or something more passive?

    Loop design is hyper local in that what works in one place with its soil is a total failure someplace else with different soils. The people that feed their family’s doing this work tend to be tight lipped.

    When I see people post there loop temps it seems like the differential is 4-8° and toward the end of the heating season the numbers are in the very low 30s very often below freezing.

    If you are considering a ground source heat pump understand that the return on investment numbers do not generally work in your favor.

    If you have drunk the geothermal CoolAid you will find like minded people on GeoExcange forum.

    https://www.geoexchange.org/forum/

    Walta

    1. Alex S | | #3

      Thanks for the input Walta. While I have a deep appreciation for building science, capabilities of Geothermal are a bit lost on me beyond the basic concept (capturing ambient temp from the ground).

      I don't know whether this would work on a project I'm looking at doing for myself - a building with 300 linear feet of footings, an 20'x30' insulated concrete driveway. The idea is to have loops run below the footings, which then runs through the slab, and are on a recirculating pump which would run continuously. I thought this closed loop system could bring the temp of the concrete slab the same as below the footings. This would help with snow build up. Am I completely mistaken here?

      1. Charlie Sullivan | | #4

        I don't know what climate you are in, but if it's cold enough to want to warm your driveway, it's cold enough that your footings are placed well below grade to make sure the ground under them doesn't freeze. What this system, run continuously, will do is use the driveway as a cooling system to cool the ground under the footings and freeze it.

        It's possible that you could install a system like this and run it for maybe a few dozen hours a year. After you get some ice buildup on the driveway, you could wait for a mild day, and then run this system to deliver a little extra heat to the driveway, making it easier to chip the ice off. If you also ran it during the hottest late summer days, that could help you go into the winter with a little extra heat under the footings.

        If you used just a little glycol, and monitored the temperature, you would be able to keep track of how much the hear reserve you have in the tubing under the footings and use it when it's most beneficial, and avoid freezing under the footings.

        I assume the insulation you mention is under the driveway? Note that if you want to use heat from the ground to melt snow on the driveway, the simplest way to do that is not to insulate under the concrete. You don't get much heat coming up, but early in the season you do get some and that can help a little. The small size of that effect can help give you perspective on how much heat you can expect from your footing tubing.

  3. Alex S | | #5

    Thanks. Would definitely be using glycol for sure. Not sure about the insulated slab under the driveway , would need to engineer this correctly. In any case, was more curious about whether the idea has some merit to it.

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