more loop vs pump costs

Discussion in 'General Discussions' started by Marty G, Nov 2, 2018.

  1. Marty G

    Marty G New Member

    Hello,

    Before I begin I need to mention that I’m not an engineer or an HVAC contractor, just a general contractor with over fifty years of experience building and renovating homes.

    I posed this question about seven years ago and had one response and was not convinced that the response was correct.

    My GSHP systems were installed in March of 2011. Later the following winter my HVAC contractor called me during a cold spell and asked me to check the incoming water temperature on my air handler. I was surprised and concerned that the temperature reading was 49 degrees, about ten degrees below the ambient ground temp in Wake County North Carolina (60 degrees). All of my reading about GSHP systems before installing the system talked about using the constant temperature of the ground to provide a heat sink for heat and A/C.From that, I thought the loops were sized to maintain a near constant ground temperature, retuning the loop water to the heat pump at near the same temperature as it entered it.

    My thought is that the lower the temperature of the ground the more the system needs to work to produce the same amount of heat and therefore is less efficient (using more electricity) as the ground temp falls. Just like using an air to air system when the outside temperature falls.

    The responder to my previous post said that it was a question of the law of diminishing returns. That if I did have more loop or pipe length any gain in efficiencies from a higher ground temp would be offset by the additional cost of running the circulating pump for the larger loop. I’m not convinced. With a “warmer” loop it should take less time to produce enough heat to warm the house and therefore less run time for both the compressor and the circulating pump.

    So, seven years on my questions are:

    · How much more loop would be required to maintain a near constant ground temperature, say within 4 degrees, at 10 degrees outside temperature?

    · Can the same size circulating pump be used for the increased loop size?

    · How much more electricity would be consumed pumping water through the larger loop than the smaller one.

    Some specifications: I’m using a 3-ton Climate Master 27 horizontal package unit, installed in a “closed crawlspace”. I have two - 290’ deep, 6”, bore holes backfilled with bentonite grout and the bore holes are about 100’ from the house foundation. There is 17’ of overburden at the bore site and the water table is 64” below the surface.
     
  2. arkie6

    arkie6 Member Forum Leader

    The Climatemaster website has easy to use Geothermal Design and Loop Pressure Drop programs you can download and answer all of your questions. I have posted links to them on this site many times. It is under their Residential Contractor page.

    The first issue I see with your current design that conflicts with your goals is the use of low thermal conductivity bentonite grout (assuming no sand was added during the mixing to enhance its thermal properties). You've installed a thermal insulator between your loop pipes and the earth (solid rock bore below 17'? what kind? sandstone? shale? granite?)
     
  3. Marty G

    Marty G New Member

    As I mentioned at the beginning of this thread, I’m just a GC. I did not interfere with my HVAC contractor’s installation but as I recall, bentonite grout was “the” thing at the time. The best grout available. Yes, solid rock below 17’ and lots of water, the rock is granite and slate.

    Please don’t misunderstand, I am very happy with my system, just surprised that it was designed to allow the temperature of the rock to decrease. In 2011 North Carolina offered a 35% tax credit on top on the Federal credit for a total of 65%. It really made the fixed costs of the system cheap. And it would not have been very expensive to add more loop while we were working. The before tax cost for drilling and filling (not pipe) was $13.00/foot, my net cost was $4.35 per foot! So another 100’ per bore, or 30% more loop, would have only cost $900.00 net.
     
  4. arkie6

    arkie6 Member Forum Leader

    The temperature of your loop water will never be the same as the deep earth temperature in your area due to the thermal heat transfer characteristics of the soil / rock, grout, and the plastic loop pipe. None of these materials are perfect thermal conductors (far from it). All of these limit transfer of heat from the water to the earth. You can realistically minimize the effects of these heat transfer limitations by 1) using high thermal conductivity grout, 2) increasing pipe size which increases the surface area, or 3) increasing the length of pipe in the ground. A pretty good system design would limit loop water temperature to say +/- 20 deg F of the earth temperature, i.e. if your deep earth average temperature is 60F, then the minimum loop entering water temperature would be 40F or higher in the winter and in the summer the maximum loop entering water temperature would be 80F. To take that pretty good system and make it where it would limit entering loop water temperature to say +/-10 F of earth temperature would probably take almost twice as much pipe in the ground as the pretty good system. This is due to the fact that the amount of heat transferred from the loop water to the earth diminishes as the differential temperature between the water and the earth gets lower. You have to have a temperature difference to move heat and the lower the temperature difference, the less heat is moved. Using the net costs you listed above, I estimate it would cost you at least $3500 to double your loop (well drilling, grouting, and pipe cost). If you borrowed that amount @ 5% interest, it would cost you $175/yr in interest. Or assuming you used cash to pay for the extra loop, you could invest that amount in the market and make >5% annually (S&P 500 return past 20 years) or >$175/yr. The increase in heat pump efficiency going from 40F entering water temperature to 50F water temperature would probably be in the low single digits percentage wise considering the additional pumping power needed to move the extra loop water and it may save you at best $25/mo in electricity costs on a 3T unit during the 3 coldest and 3 hottest months of the year (likely little to no benefit during mild spring and fall months). That is $25 x 6 mo = $150/yr savings. But it cost you more than that per year in interest on borrowed money or opportunity cost of using cash to install the additional loop length.
     
  5. Marty G

    Marty G New Member


    Thank you for your detailed explanation of how heat is exchanged in the ground. I agree with you analysis of costs vs benefits with the caveat that the cost of electricity is likely to increase steadily over time, offsetting the cost of money, but that's splitting hairs. I will hunt down the CM calculator and play with the numbers.
     

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