Over sizing vertical loop to reduce auxiliary heat usage

Discussion in 'Vertical and Horizontal Loops' started by dleroyg, Aug 25, 2010.

  1. dleroyg

    dleroyg New Member

    I have heard that over sizing the vertical loop up to 20% can reduce the auxiliary heat usage.
    How can I calculate the energy savings to determine if the added costs can be justified? I am located in Cincinnati, OH.
    Currently have two-gas furnaces/ac, one for the 45 yr old slab portion and one for 12 year old addition with walk out basement. I need a total of 5 tons (3 ton in basement for addition and a split system with heat pump in basement with refrigerant line through wall to air handler) for slab part of house. I could drill 6-150’ wells with ¾” pipe or 3-300’ wells that would require 1” pipe. I understand the 300’ wells have additional costs, but would the energy efficiency increase enough?
  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Ask your installer to see the design. It should calculate the number of degree days your system should require stage 3/ supplemental heat. From that info you can estimate the cost based on your electric rates to run stage three.
    Increasing the amount of loop in the ground does nothing to address the units operation in stage three. Increasing the amount of loop in the ground simply provides a buffer/ margin of safety. Installing more loop than the house load calls for only serves to lessen the swing of the exchangers ewt for any given season.
    Here in the mid atlantic we just survived the 100 year heat wave. That resulted in some of the highest ewt's we have ever seen. People who had "a little extra loop capacity" easily absorbed the increase in load that was not part of the design and life went on as normal. People whose designs were to tight or marginal in regard to the loop, suffered a decrease in ac performance because the house load exceeded the capacity that the loop design could handle.
    If the cost per foot to drill is not prohibitive I allways advocate "a little extra" to account for weird weather and odd patterns from mother nature. I had the displeasure of explaining to more than one cranky person this season that there is no stage 3 AC.
    Hope this helps
  3. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    To avoid Aux heat usage, you need to increase both the size of the heat pump and the size of the ground loop. This usually means that you need to increase the size of the ductwork as well.

    The short answer is that you can very easily spend several thousand dollars on increasing the geo system capacity and only save a hundred or so dollars a year by avoiding aux heat.
  4. dleroyg

    dleroyg New Member

    Thanks Waterpirate and Palacegeo for the information.

    Additional information: the GeoLink Design Studio from WaterFurnace provided Aux Elec kWh Total for year (504 hr for the 2 ton and 366 for the 3 ton system) both systems totaled 870 @ $0.11/kwh is about $95/year. The cost to drill another 150’ well for 6 Tons for my 5 Ton systems is $800. Both systems have a 2 Stage compressor and ICM Variable Speed DC Blower.

    I do not want to increase the size of the heat pumps and cannot change the ductwork. I understand that the well depths have to be the same. So, Waterpirate, how much do you consider a “little extra”?

    A Climate Master dealer proposed 2- 2 Ton systems, so it my 5 Tons is on the high end. If I added the extra well, could I ever see a decrease or eliminate the auxiliary heat cost? If I understood Waterpirate, I would only see a difference if the winter was extremely cold for long enough. Then my auxiliary heat would run less than if I did not have the “extra” ground loop.

    Or do I keep the $800 now to pay for the auxilary heat later?
  5. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    The size of the heat exchanger really has no input to the "capacity of your unit" within reason. 5 ton unit with a 5 ton loop field is the design. If the design has a balance point of 32 for winter and 76 for summer additional loop will only help to reduce the shock to your exchanger when design temps are exceeded. Your unit still will only perform as advertised at its 5 ton rating.
    I look at a little extra in terms of percentages. One more loop for a 5 ton loopfield would be a 20% fudge factor, more than plenty in my opinion.
    Your design will tell you what your balance point is and help identify any justification for an 800.00 overage. I advocate the geo covering 100% of the heating load, which is unpopular where the cost of drilling is high due to the stage 3 heat is cheaper than a bigger unit and more footage.
    My personal home is also 3 tons down and 2 tons up. I drilled 3 loops for up and 4 loops for down:D.. 50% more for up and 30% more for down, but drilling at my house on Saturday was cheap! lol The backlash at my house for inexpensive drilling costs were that I got to learn firsthand how important a correct manual j is. The down stairs is really a 4 ton load. Even with 4 tons of loop the unit will only produce 3 tons of heat. So when the outdoor temps hit 30 ish and the wind gets to 15-20 we require supplemental wood heat to maintain 72 indoor temps.
  6. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    I am way impressed that you can get a 150' well drilled and looped for $800.

    As WP stated, adding an extra loop will not avoid aux heat. A 3 ton heat pump with a 4 ton loop still only gives you 3 tons of heat.
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Sample case I'm working on would raise EWT 5* with 33% extra loop. that results in a ~$60 year savings in this case, but additional drilling, loop, grout etc add much more than $800 round here.
    To improve capacity of system; significantly lower balance point (where aux comes on) requires much more than a few degrees.
    Any who have dealt with open loop for instance know that higher EWT's lower balance point (but we're talking double digit improvements). I've told the story before of the fella who wanted to turn his 20 year old open loop to closed. Thing would not accept less than 45* entering water temp. Once we calculated the 5 miles or so of required loop to maintain such a temp:eek:, he began to consider other options.
    There is respectful disagreement on equipment sizing amongst pros. In heating dominated clims I support 100% loading......that employs electric auxiliary for the last 2-8%.
    For the once every 100 year heat wave, I would support the purchase of a big box store free standing AC that can be ducted out an open window before I would design it in. Running a larger compressor all the time to support the "blue moon load" will cost hundreds (or thousands) more than the price of auxiliary cooler/heater.
    Good Luck,
  8. SoundGT

    SoundGT New Member

    Strip Heat & GeoExchange

    I have to agree with AMI Contracting. I do a lot of work with hybrid GX systems. Here are two slides that I have found handy in explaining why we do not spend a lot of extra money on extra boreholes to squeak the last bit of goodies out of the ground. You, of course need to size the system to the load and be careful not to design it so short that you pull the field temperature down below the design point. The extra borehole will help keep the EWT up and make the Geo system more efficent during an Artic Freight Train but not necessarily be an economical choice.

    See the attachment. The first slide shows that 50% of the system load covers 90% of the year. The second is a bin distribution for SLC (Palace home). There are several residential loop design programs that plot load vs bin distribution vs system operating cost. It is worth reviewing these if they are available to you.
    Go Geo

    Attached Files:

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