Oversized/Undersized Ground Loop ?

Discussion in 'Geothermal Loops' started by SReg, Jul 9, 2014.

  1. SReg

    SReg New Member

    I have decided to spend the money and go with a geo unit in our home. We are replacing our LP furnace as it is on its last leg and propane bills are enormous.

    So far I have decided to go with a 4 ton 7 series WaterFurnace unit.

    I have received 3 seperate quotes from individuals and each of them spec out a 4 ton system for my heating demands.
    I have elected to go with subcontracting the loop install as I can save a substantial amount of money doing it this way rather than combining the whole package.

    I have also had 3 drillers come out to see the house and I have told each of them I am installing a 4 ton Series 7 system.
    Here is what they recommend:

    Driller 1: One 600' vertical
    Driller 2: Two 320' vertical = 640' total
    Driller 3: Two 350' vertical = 700' total
    Driller 3 also said that he would do two 375' wells for a total of 750' for a few hundred bucks more.

    My questions are .....
    What is the actual depth that the geo well should be for a 7 Series 4 ton system with desuperheater?
    I have read that you need more loop for a 7 series rather than a 5 series. Is this true?

    Is it worth oversizing the loop? And if so, by how much? 50 feet? 100 feet?

  2. SReg

    SReg New Member

    Forgot to mention. Location of install is in Maryland. Soil is rocky.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    We need building loads. Equipment run times. Etc. You're skipping a step in here. Which can be done, and more often works out based on "what has worked before in your area". But this is a public forum, and I just don't want somebody else reading this a few months from now getting the same idea.

    Ground loops are not sized to "tons". Keeping in mind a "ton" is just an arbitrary term for 12,000 btu's in the first place. And different companies label their units somewhat arbitrarily as well.

    What you have likely got is a series of quotes for equipment and labour. And are trying to get the drillers to quote based on that. You still need building loads. You still need a design. I would like to think they would have pointed this out to you. Or asked for a design.

    And yes, you can skip the step and guess. Our industry is rife with that crucial missing step. Just read the forums here.

    There is nothing you can oversize/undersize without knowing the size.
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I have dug up yards in MD. Where in MD? Why do you think there are rocks? MD is mostly a delta mostly silt.

    Tell us what you have in the way of heating and cooling equipment BTU ratings currently.

  5. SReg

    SReg New Member

    Location is in Monrovia MD which is still Frederick County.

    I am not sure what my cooling needs are but my heating needs are 44756 btuh at a 70 inside set point and 16 outside temp.

    I do not have the manual J in front of me as I do not have a hard copy of it yet. Just saw it on the computer and that was the only number I wrote down.
  6. SReg

    SReg New Member

    BTW installer of system just took their normal 64o' of vertical loop for a 5 series 4 ton system and added an extra 100' to cover the difference between a 5 series and a 7 series.
    I would just like to make sure that my loop size is correct for my btuh.

    If I can provide any other info please let me know.
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    4 ton for a 44mbh load? have your guy do an op cost model between that and a 3 ton.
  8. SReg

    SReg New Member

    No they have not I will ask today when I meet with them again. Are you suggesting that a 4 ton is a bit oversized or not the best bang for my buck?
  9. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    "Are you suggesting that a 4 ton is a bit oversized or not the best bang for my buck?"

    Yes. Almost certainly is.
  10. SReg

    SReg New Member

    So after talking to my installer we decided to go with a 4 ton, 7 series WF system at 740 vertical feet of loop. (two 370' wells)

    My question.... Is this the correct amount of loop for my system as well as a my heating needs?

  11. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    It depends.

    There are a lot of variables.

    To make it tougher the target moves.

  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The amount of loop sounds good. How about the pipe size. The 4 ton 7 series likes to see 13.5 gpm. Grouting? length and size of header pipe to the 2 boreholes?
  13. SReg

    SReg New Member

    "Install two 1.25'' geothermal loops to 375' each, inclusive of siz-inch casing through overburden, SDR-11 U-bend closed loops and geothermal bentonite grout.

    As far as the length and size of the header pipe to the two holes I am not sure what that will be. I have about a 80-100 foot run from my wells into my house. Any suggestions?
  14. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    1.25" pipe header will work for 100ft each, as long as there is 1.25" pipe in the holes. then you only need 1 pump flow center. With 1 variable speed pump you should get up to 14 gpm of flow, good for a 4 ton 7-series unit.

    Joe is right, 44 KBTU/h heat loss might be well covered by a 3 ton heatpump and a bit of supplement heat, might saving you some upfront money. But a 4 ton will work well, too, especially a 7-series.
  15. DavidCraig

    DavidCraig Member

    Just a word of caution. Make sure your contractor for the heatpump is satisfied with the loop design. When you DIY and there are problems, you might get stuck in finger-pointing as to what is to blame ... sometimes there are hiccups or even worse problems in a system. For both troubleshooting, watching performance, and silencing finger pointers do this: make it simple to know the ground source water temperature during operation (EWT), the delta T (temperature drop during operation), and the flow rate (GPM).

    In one sense, there is no such thing as 'oversizing'. It is always 'undersizing' that is a problem. Loop/field size is a compromise between 'plenty' and cost. Field size (in this case a vertical loop) affects heat extraction rate. It takes time for heat to 'migrate' to the pipe.

    You might compare it to how much insulation you put in the walls of your home. A 2x4 wall holds R11 insulation. If you double that to R22, the heat loss is cut in half. Double again to R44 and heat loss is cut in half. But notice that the 2nd doubling improves half of a half (just 1/4). At some point it isn't cost effective to make thicker walls for thicker insulation.

    So using some imaginary numbers for a vertical bore loop. Say ground water/temp is 50 degrees and you start with a 300' loop that provides 30 degree water and costs $5000. Double that to 600' that provides 40 degree water for $10000. Double that to 1200' that provides 45 degree water for $20000. Once more to 2400' to provide 47 degree water for $40,000. You can see that at some point the cost is very high for just a slight improvement.

    There are other factors beside loop length. For example, if you put a 300' loop in a 50 degree, fast flowing river, it would probably work great. But in the still ground it is too small. While not knowing what your actual bids were, 'error' on the longer loops.

    Another simple thing to watch is pipe diameter. I used 1 1/2" pipe for a single loop that is 1200' feet long (500' vertical + 100' header). This was DIY and cost 10 cents/foot more that 1 1/4" pipe. This reduce head loss from 24' to 10' and allowed the use of a circulator that uses 150 watts less - it pays for the larger pipe in 2 years. Note: the performance claims of all geothermal heat pumps excluded the cost of pumping the water. Some poorly designed systems have used large circulators that cost a lot to operate and therefore degrade the overall performance of the installation.

    Lastly do not overlook the importance of the design of the heat delivery system in your home. The ductwork for force-air heating in older homes was designed for the high temperatures of fossil fuels. High efficiency systems (geo or otherwise) use larger ducts and more air flow. Installing a wonderful WF 7 but choking the air flow will degrade performance.

    I have radiant floor heating instead of forced air. But the same principles apply. The original system was designed for 130 degree water provided by an oil boiler. The first Geo quote showed an detailed computer printout estimating an average COP efficiency of 3.4! It is not possible for any water-to-water unit that is fed <50 degree water to provide 130 degree water with that efficiency. The solution has been to add aluminum plates to the pex in the floor to speed up heat transfer at lower temps - a laborious task. In your case, if you don't already have ductwork for high efficiency, you will need to add more/larger ducts.

    All these issues are addressable by good design and money properly spent. It can be done DIY but you benefit by a good contractor.

    Wish you success with your new system.
    waterpirate likes this.
  16. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Well said, David.
  17. Calladrilling

    Calladrilling Member Industry Professional Forum Leader

    I would ask about the mix of the "Geothermal bentonite grout". Thats is a very broad grout description.
    The mixture of the grout is a big factor in design ( believe it or not).
  18. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    indeed Dan,

    Grout TC can range from .44 to2.0

Share This Page