Can over-sized circultor pumps overheat a geo loop?

Discussion in 'General Discussions' started by cjfox, Aug 3, 2011.

  1. cjfox

    cjfox New Member

    Can over-sized circulator pumps overheat a geo loop?

    I have a project I am dealing with, where a pump specified at 15 gpm @ 40 ft head was installed. The actual flow of the system is 12 gpm, the actual pressure drop is 32 ft head. Is it possible that the circulator pump is partially responsible for overheating the loop?
  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    The load is what imparts the heating or cooling to the gshx. Loops become to hot or to cold when the load applied to them exceeds the designed load they were meant to carry.
    Hope this helps
  3. GCI

    GCI Member

    Big Pump=Waste Heat

    If you only need 15 gpm at 40 ft. of head but had a 40 hp pump supplying 300 gpm to the loop, yes you could overheat a loop. All of the electrical energy you consume through the pump ends up in the system in the form of heat energy.

    But no, a reasonably-sized pump will not affect the system enough to matter. Typically, software programs include pump energy in the model used for design.
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Third time:

    No, No, No.
  5. GCI

    GCI Member

    No to which part?

    The 40 hp and 300 gpm numbers were out of thin air and not intended to be taken literally. The point I was making was that pump energy ends up in the ground loop in the form of waste heat.

    It's funny the way things work out sometimes. I just got a chance to look at my copy of the July 2011 Issue of the ASHRAE Journal last night and found this article by Dr. Kavanaugh. I scanned the first page (attached).

    The title of the article says it all. Kavanaugh makes a good case for the K.I.S.S. principles in pumping/controls system design. Well worth the read if you get the publication.

    Attached Files:

  6. cjfox

    cjfox New Member


    Thanks Ryan, that is what I was thinking. The waste head has to go somewhere. Like you said, it would need to be grossly over-sized to really make a big impact on the system though.
  7. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Agree with the kiss principle entirely. But 15gpm at 40' of head is a long way from 40hp and 300gpm. The waste heat from 15gpm at 40' of head is going to have to be measured with a micrometer.
  8. Looby

    Looby Member Forum Leader

    Kinda surprising that your "15 gpm @ 40 ftwc" pump is delivering
    only 12 gpm into a 32' head -- but the water doesn't care about the
    pump ratings. It only cares about actual flow and pressure drop:

    flow_rate x delta_p = mechanical power (at 100% efficiency)

    12 gpm x 32 ftwc = 72.4 watts = 247 BTU/hr

    (units and conversion factors left as an exercise to the student)

    A typical circ pump will be only about 30% efficient, but most of
    the wasted energy passes from the pump motor into the air, and
    is not injected into the water.
    Last edited: Aug 4, 2011
  9. GCI

    GCI Member

    Energy which must be absorbed by the heat pump (when in cooling mode) to be rejected to the loop.
  10. Looby

    Looby Member Forum Leader

    The mechanical energy of pumping, (i.e., P*V work), is transferred
    directly from the pump impeller to the loop fluid (and subsequently
    dissipated as heat -- also directly into the loop fluid). Thus, it never
    passes through the heat pump -- although it is rejected into the soil.

    Of course, waste heat transferred from the pump motor to the air
    adds to the cooling load -- IFF the motor is in a conditioned space.
    In any case, it's at most 200-ish BTU/h additional load in a system
    that's moving tens of thousands of BTU/h.
    Last edited: Aug 4, 2011
  11. GCI

    GCI Member

    So it's safe to say that pumping energy ends up in the loop. Residential pumping systems are typically in conditioned space.

    It's also safe to say that the amount of heat generated by a reasonably-sized pump would be a drop in the bucket.

    See post #3 above.
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    A pump with 2 many GPMs may create an artificially elevated EWT in cooling: we've spoken before that in lengthy cycles EWT's may come to a very slow drop or climb but often don't remain completely static.
    This is not a true measure of the deep earth temp, but a snap shot in time that can be impacted not only by loop design and soil, but GPM as well.

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