I am a young contractor with a very viable opportunity to install my first ground source heat pump system in Chattanooga, Tn. (Big deal to me!!!) I will begin work on a 2story double brick exterior walled 1900sqft house. The house is within city limits but I have the adjacent lot available for a horizontal loop system. I am a little perplexed because I have had two different (qualified) contractors look at my project and i get different proposals. One is suggesting trench 2- 3/4'' pipes at 6' deep and 2- 3/4'' pipes at 4' for a total of 2400 ft of pipe, and another is suggesting 375' of trench at 5.5' with 6- 3/4'' pipe. Do not doubt either men are knowledgeable, guess im trying to make sense of the two very different styles of loop. can anyone shed some light on why i should consider one over the other (aside from the length of the trench). thanks!

Why do you need a GSHP in Chattanooga, TN?

average depth is identical (5.5') and length is fairly similar. So close I'd call them a wash. There are so many important questions with a new geo system are the loops your only concern?

Joe, have you looked into why your browser appears to post more duplicates than anyone else?

Both design will support a similar amount of heat extraction, it comes down to personal preference.

I am not a designer and don't have software or experience to quantify this but doesn't a lot of this depend on trench width and pipe spacing as well. Also wouldn't pump power be a good bit different? I'll cut Joe some slack on the higher math, but how much difference does a half foot makes (6+4)/2 vs. 5.5'.

lol thanks mtrent. i was thinking 5' average, but my point remains the same. OP is caught in minutae and ignoring every other pertinent question......hope this only posts once ICF, maybe I can break 4000 on this one comment

Mark is that really your only question on your first install?

I still would like to know why geothermal is indicated in Chattanooga.

Why not?

I'm 3 hours west of Chattanooga and for 2.5k more I got 27 EER/4.6 COP vs. 20.5 SEER/13 HSPF. I'm heating my house (2700 sqft (2500 sqft bsmt (unconditioned 63))) to 73 for the same price my neighbor (1900 sqft) heated to 68 is. His last electric bill + natural gas was $172 and mine was $178. Similar construction. 2x6 walls spray foam, etc... Carrier GT-PX 4 tons vs Carrier Infinity Greenspeed 2.5 tons.

Thanks, but I'm not following the comparison. You have a Carrier GT system and he has a Greenspeed and yours cost $2,500 more for almost twice the capacity? Where does the natural gas come into it?

I was quoted a Greenspeed 3.5 tons for 2.5k less (w/ 30% tax credit). So, my comparison was between a geothermal vs. heat pump quotes. Then I compared the electricity cost of my 4 ton geothermal vs. my neighbors 2.5 ton Greenspeed. The natural gas is the auxiliary/emergency. The comparison is for total electricity use, so it can skew the comparison. I have an electric water heater with a buffer tank and he has an endless water heater running on natural gas. 3 ppl live at my house and 2 at his.

Why would you install and maintain a natural gas solution for aux/emergency only? Very expensive option when you have electric strip heat, which is cheap to instal, usually maintenance free, and costs you less than $100/year to run?

I agree doc, that is why I have electric strips as my aux/emergency!

Back to this question of the loop, I agree they are a wash as far as efficiency, I normally install option 2, with 6 pipes instead of 4, because to me its easier to install than trying to staple to the walls of the trench and the potential for knocking the pipe off the wall, but this is really more opinion than anything else.
I also like option 2 because 6 circuits will give you less pressure drop and you might be able to use a smaller pump. If you look at most systems a 4 ton suggests a dual pump flow center, yet a 4 ton system loop can be designed to get proper water flow on a single flow center. When a flow center can take 250+ watts to operate per pump and your total energy for a geo heat pump is 2500 watts I feel that a 10% difference in efficiency is something to look at and loops should be designed for lowest pump cost by putting in the smallest pump available to get the GPM required.

I just performed a very quick calculation with my pressure drop software and show a standard single pump flow center might only be able to pump 11 GPM with loop option 1 with 4 pipes at 600' each. With loop two I show a single pump can flow 13 GPM and with a 3 ton system 12 GPM is recomended so I think option 2 is a better design, hands down.

"13 GPM and with a 3 ton system 12 GPM is recomended ....."

4 ton

Natural gas is so much cheaper than electric that is allows one to use considerably more aux heat and a smaller geo system. How it all works out dollar-wise depends on the circumstances, but keeping the geo system size the same and comparing electric to nat gas aux isn't the right question.

Even if natural gas is cheaper, how much more is a 3 ton versus 2 tons? Or 3 versus 4? We charge $2500 less per ton, or $2500 more per ton. Not worth it to downsize and pay more upfront for a gas back up.

The amount of money spent per year just to have a gas account is probably 3x more then what would be spent on elec aux!

"Natural gas is so much cheaper than electric that is allows one to use considerably more aux heat and a smaller geo system. How it all works out dollar-wise depends on the circumstances, but keeping the geo system size the same and comparing electric to nat gas aux isn't the right question."

Remember there is a dynamic change when you use a split system for fossil back up. As the geo and gas furnace can't run at the same time. Because of that, when we have clients around here that choose split systems (so that they have heat plants they can power by generator in our crappy grids) we tend to actually lean heavier on the design side. In other words we try to use less auxiliary or design for a lower balance point.
Ironically we also employ 2 stage furnaces to qualify for rebates, but wire them for 2nd stage with every call for heat so that we turn the furnace back off as quickly as possible.