New Pond Geothermal System Design

Discussion in 'Surface Water Loops' started by nd96, Jun 21, 2012.

  1. nd96

    nd96 New Member

    I had the system sized by this company: . They seem to be highly recommended and very good at what they do. I wouldn't go down on the system size. I know it is a bit more up-front cost, but I would rather have the extra power if needed for above average summers (this was a very average year). Also, please correct me if I am wrong, but aren't the systems more efficient running in first stage vs second? So, efficiency wise, isn't it better to have a over-sized system always running in first stage than an under-sized system always running in second stage?

    My main goal in this thread was the loop design in the pond. It is a non-standard loop design I was trying to do myself for my particular situation/needs. The size of the system I left up to the experts. The pond loop design, however, I did myself. Other than my heating/cooling cost, how can I tell if the loop design is adequate?

    Thanks for all the feedback.
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Some rue using Rue...

    The best (really only) way to evaluate waterside performance is via old school pressure temperature taps. I hope your system includes "Pete's Ports". Two temperatures and two pressures easily measured with <$50 worth of instrumentation tell how the waterside is performing.
  3. geoxne

    geoxne Active Member Forum Leader


    At first glance the AHRI ratings suggest that 1st stage is significantly more efficient (maybe 0.5 COP). However, this increased efficiency rating is caused by the 9f degree increase in entering water temperature during 1st stage testing conditions.

    In reality you are not going to see a magic 9f (or any) boost in loop temps just because you are running in 1st stage. If you look up in the capacity charts at equal EWT's the difference is 0.1 to nil COP between Part Load and Full Load. A 0-2.5% increase in efficiency is not significant enough to design a system that "cripples" a 2stage 5 ton HP to 3.6 tons. All the while losing the advantages and comforts of a properly sized 2stage system.
    Last edited: Nov 8, 2014
  4. nd96

    nd96 New Member

    This is what I am looking at for my Tranquility 27. I have the TT064. COP at first stage is 5.7 . If I was using the downgraded TT049 in second stage the COP of 5.2. As far as I can tell that assumes the same entering water temp (why would it be different for a water loop)?

    Same thing for the EER. My TT064 in first stage is 5.2 rating vs the TT049 in second stage at 4.6.

    I know the smaller unit wouldn't always run second stage and the bigger unit wouldn't always run first stage, but overall it seems first stage is preferable (unless I'm reading this table wrong). Can you please clarify for me?

    EER and COP.jpg
  5. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    One needs to know that all tables are lab generated. They remove all variables, but one at each test point. So the tables are a screen shot of the test. That is not to say tables are useless, but here is what happens in real time.

    As the pump moves heat in either direction things change. So by the time one reads EWTs and LETs the space has changed, up or down. Now the readings mean nothing. They need to be taken again and while doing so the space continues to change, up or down.

    The actual state of a heat pump, especially geothermal heat pumps, is a moving target. Unless one has all of the test probes read at the same point in time one is sort of guessing. No one ever taught me this. I learned it from messing with heat pumps since Jimmy Carter was President. We had geo back then. I commissioned a closed loop system in Amherst, Ohio in 1974. BTW it is still in service.

    All I use tables for is direction.


    I agree with your reading of the crystal ball of the tables. I think you chose well.

    If any one wants to poke me in the eye with a stick about this, please start a new thread. I give webinars, but only here. I am a Browns fan and know anything is possible.

    Nd96 the majority of the folks that found out I was using PEX-A for loops went nuts until the systems worked well.

    I agree with your thinking and welcome further discourse.

  6. geoxne

    geoxne Active Member Forum Leader

    First, I hope you don't have a "Water Loop HP" installation. They do not qualify for Energy Star and thus do not qualify for the US federal tax credit. If you do, you are over looking and missing out on the TT049 first stage 6.2 COP. We all wish we could heat with 68F EWT.

    Second, I believe you have a "Ground Loop HP" installation (even if it is in the pond). That would be all the way to the right on the chart you snipped from.

    Third, my snip below from the capacity charts (does anybody look at these?) shows very little difference in COP (0.04 COP difference at 40f EWT) in the between part and full load at equal EWTs. The difference might just be from pumping penalties at the different flow rates published. One could argue first stage may be less efficient as most systems are over pumping at while in 1st stage. I believe greater first stage efficiency is a myth and most certainly, systems should not be oversized to discourage 2nd stage operation.
  7. nd96

    nd96 New Member

    Looking at those numbers, for cooling (most of my energy use here in Dallas), the EER at Partial Load is quite a bit higher at each temperature point than at Full Load. Say EWT of 70, partial load has EER of 23.0 vs EER of 18.2 for full load. That seems like a significant difference to me. Overall, it still seems to me that running at partial load is more efficient (not to mention less wear and tear to the system components and probably prolonged system life).

    Back to my loop performance. I got some numbers tonight that may help assess the loop performance. I turned off my system and let the house get down to 71 deg, then turned on the bottom floor unit heat and set the thermostat to 74. The bottom floor unit ran for about 50 minutes before turning off. I recorded the system information (available from the thermostat) at different points: 0, 5, 15, 20, 25, 30, 35, 40 and 45 minutes running. The unit ran in 2nd stage for the first 30 minutes until the house temp reached 73 (within 1 degree of target temp), then went down to 1st stage. At 45 minutes the house temp had reached the set point of 74, and it turned off at about 50 minutes.

    Here are some stats at each point:

    Tranquility 27 data.jpg

    From what I can tell the loop seems to be doing fine. The entering water temp doesn't change despite running 45 minutes straight (EWT at the 5 minutes and 45 minutes points are basically the same). Also, the pump doesn't seem to be pushed very hard, running at 55% /80 watts in second stage and 39% / 35 watts while the unit is running in first stage.

    Any thoughts?
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Keep in mind that cycling means more wear, you also need more pumping power in real life to run a larger unit, some of that is factored in the COP. You also need more air volume moved over the coil, some of which is not factored in the COP. Given that usually the second stage runs only 15-20% of the time, and higher upfront costs, turning a 2 stage unit into a single stage larger unit is not understanding how the units work. Converting lab data to real life operational data is not a very valid approach.
  9. nd96

    nd96 New Member

    I agree. I am not an expert and am not trying to suggest people should intentionally oversize their systems. I was just responding to posts suggesting that my choice of system size was poor, and therefore the system was less efficient. I just don't think anyone can definitively claim that without knowing all the details. I left the sizing up to a company that specializes in that area, and I am satisfied with their recommendations. Given the choice between being a ton short vs a ton oversized, I'd pick the ton oversized system every time. But that is just my opinion.

    Any thoughts on the loop? I don't have any experience with other geothermal systems, and was wondering if the loop data looks good, bad or typical of other pond loops.

    Thanks again.
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    you have about a 9F delta T which is typical for cooling mode, temp looks good too. If anything, your flow is a bit low, (11.8 gpm in second stage), but still within range.
  11. RogerH

    RogerH New Member

    nd96 - I followed the thread with interest as I am in NETEX and have built an ICF house 6500sq with basement - my house is clear story, passive solar and has a 16 ft ceiling on the north side of the house with a row of windows along the top south facing wall - we get full sun access in the winter and are completely shaded in the summer. My AC guy calculated my tonnage at 3 for the south half of the house with a duel zone control for upstairs and downstairs and a 4 ton for the north side which has the increased cubic ft of air space due to the high ceiling. He sells and installs Climate master and ran the duct and installed the unit but told me upfront that he would not install the loops for the system, I do a lot of construction so I am not scared of the loop install but need to learn a bit more about it.
    As I was reading your posts something started sounding vaguely familiar. When I saw the pictures of the update then I knew that I had been there before. The ICF contractor who was building your house took me there when they were framing the roof joists on top of the second story stem wall. A very nice house and design and I would have to concur that the storm shelter is very over engineered ! I would be interested in an update now that you are several years into the operations of your geothermal system. I am wanting to put in a pond loop, my pond is 35' deep with 3.3 surface acres when full. I am curious if the 2" loops are still what you think they should be or would you go with a smaller diameter. I am planning on running 1200 to 1500 ft loops for each unit. Others can comment here to if they wish.

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