Making the case for proper water flow

Discussion in 'Maintenance and Troubleshooting' started by Bergy, Jan 10, 2011.

  1. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I compared a few loop systems recently when I was having trouble with a new vertical system (bad thermometer).
    To get an idea of whether a loop system was performing correctly, I switched systems from heating to cooling and watched EWTs. What I found was a steady climb in temperature that varied not only based on speed, but also length of loop in the ground. My vertical system actually climbed in temp at twice the speed of the horizontal (which has twice the pipe in the ground).
    The temperature rise was fairly quick in both occasions and proportional to loop length. 1* every three minutes for the vertical and 1* every six minutes for the horizontal. It is obvious that I didn't heat up the core of the earth that quickly (as no snow melted) yet I saw no decline in the pace of the rise even after an hour.
    In geo folks imagine the temp of the soil surrounding the loops to remain relatively constant with a slow decline throughout a heating season.
    In reality a heat pump has this decline in every cycle only leveling off when the Delta T (between LWT and earth) is such that the heat is conveyed by the soil further from the loops as quickly as we are removing it. If you accept that, then you can accept that a higher gpm (in heating) can create a lower EWT with a relatively static amount of BTU's (net HE).
    This is why Earthlinked systems call for down time vs high percent of hour operation. They take the heat from contacted soil more quickly than more distant earth can convey it to the loops so the ground has to "catch-up" (thaw). The net extracted heat is the same over the course of an hour, but the HE initially (when running) is very high for the system.

    In sum I percieve an HE calculation as a snap shot in time that does not necessarily reflect net btus a system can extract from the ground in a given hour. Since soil- EWT's can change dramatically in a relatively short period of time, whenever a system shuts off, surrounding soil temps will "rise" quickly and initially show a higher HE when unit comes on. If you accept soil temp as a wave (moving target) than HE necessarily would be as well. We always say soil is the limiting feature, ergo more GPM would not improve soil conductivity (btu's delivered).

    Like Mark I'm less interested in many of the "rules" that some take for granted.
    I think more a long the "see it to believe it" line of thinking.
    I'm not suggesting that laws of thermaldynamics are in flux, simply that our measuring techniques do not offer the whole picture.

    Last edited: Jan 18, 2011
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    So your thinking is that the faster pump operation cooled the soil nearest the bores faster, essentially deepening and steepening the temperature gradient in the soil.

    Using an automotive metaphor, is it better to drive 80 for 30 minutes and stop for 30 or drive 40 mph continuously.

    If the soil conductivity is on the high end it might support faster design pumping rates (higher fluid velocity in the tubes) than low conductivity soil
  3. Looby

    Looby Member Forum Leader

    The rate at which the thermal gradient deepens and steepens
    depends on the soil's heat capacity as well as its conductivity.
    That's where soil diffusivity comes into play.

    diffusivity = thermal_conductivity / volumetric_heat_capacity

    ...I have now told you far more than I know about diffusivity,

  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    ROFLMAO at difusitivity

    I allowed that my thinking coupled with my poor typing and non-existance spelling ability would get me into trouble on HE.

    Lobby: My point was I think that the HE is fixed by the refrigerant system. The numbers may change as EWT and LWT do but the total product, (BTUH moved) is fixed.

    If that is still foggy I will try again. As always since my degree is in Journalism, I am an amature engineer by choice, I am willing to learn and open to being taught.
  5. Looby

    Looby Member Forum Leader

    I agree that gpm has very little effect on HE -- for any given EWT.

    OTOH, changes in EWT have a significant effect on BTUH (regardless
    of gpm). For example, WF NDV038 specs show about a 15% increase
    in HC and a 20% increase in HE for each 10°F increase in EWT.

    Not sure whether we have a difference of opinion or a violent agreement.

  6. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    We have been

    doing this together awhile so...

    My guess is both.

  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    That is my thinking.
    I think we take for granted that our EWT is close to the ground temp. I think it's more along the lines of (brine v soil) delta t X loop length X length of cycle......or some such.

    Mark and Looby you crack me up:D

  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Incidently, this is why I didn't think Bergy was crazy when he first brought this up. It was my experience (because a loop problem caused me to be a student of loop behavior). That temps take more than an hour to stabilize after a speed or function change (remember the steady rise when switching from heat to cool).
    BTW I also amended my prior post on the temp rise it was 1* every 3 or 6 minutes.
    Last edited: Jan 18, 2011

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