I building my new home on a 4 acre lot. The house will set up on the highest point on the lot. We don't want to bore and or trench, but we were thinking placing lines behind our retaining walls. The walls will be built and then back filled and we thought it would be perfect time to bury the lines to the heat pump. Will this work? Building a single story slab on grade. Radiant heating and cooling for a 2200 sq/foot house. Full sun all day long 1450 plus elevation. Atascadero, CA Refer to the attachment for a visual explanation. Please feel free to comment with your Pro's, Con's or other questions. Thanks for your review.

If you're completely regrading the lot and are building retention walls to raise the level, then yes, with some planning and forethought, it would be a good time to lay out the geo lines.

Just a question though, how deep is this regrading going to be and what's the elevation change (top to bottom) on the lot?

You may still need to do some minor trenching to get the lines to a proper final depth.

Yeah so we're grading down 6 or so feet to the clay layer and bench back up 4 to 5'. I guess trenching in the area down another 2' to 6' more for the plumbing array may help with getting the best usage out of the system as well at a proper depth. I also need to figure out the usage for the house, but I just needed to see if I could place the lines behind the retaining walls. My frost line also is pretty high about 0'' to 10'' below grade Atascadero is located on the Central Coast of California and we won't see any major freezing depths. Our elevation is 1450 to 1500 above Sea Level. Which kind of Horizontal system do you think I should try? Slinky or single line?

Loop lines could cause some freezing/heave where you don't expect it.

Were in a warmer climate with a super short Frost line 0'' to 10'' below grade. Do you really think we'll get Frost Heave 6' to 12' down? Although I think I have plenty of room for a Single line run.

It's not so much about the climate frost line but how much soil depth in all directions around the loops.

Let's say you put the one slinky loop in and you extract heat from the ground all winter. The ground around the loops could get below freezing, especially if your loop is small for the heat load of the house.

That's where you can get frost heaving. Leaving water temps (LWT) in the mid 20's*F will freeze the ground around the loops.

I would think the loop needs to be back from the retaining wall 8'-10'. When my trenches were put in they had 10' of separation between loops.

To be honest I would look into Air to Water heat pumps for your mild climate.

Oh interesting ok I am educated on that possibility. Yeah the Geothermal is a Plan B of sorts I hadn't research it and I am looking at the options. The Air to Water was our first choice. Heck it's been 35 degrees at night and up to 85 degrees in the day here on the Central coast of California. Thanks for your input!!

In Atascadero, CA even a code-minimum 2200' house your heating loads at your 99% outside design temp of 32F (see: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf ) might not be high enough to warrant even a 1 ton GSHP, and in a some-what better than code house you can probably heat/cool the whole shebang with a 3/4 ton ducted mini-split. Even my 2400' antique bungalow + 1600' of basement with an inefficient shape and sub-code windows & insulation could be heated with 1.25 tons of air source heat pump @ 32F.

A right-sized better-class mini-split or reversible chiller (air to water heat pump) will hit geothermal type efficiency for seasonal averages in your location, and would run in the COP 3 range @ 32F. Even a 1-ton ducted Fujitsu hits an HSPF of 11.5 BTU/watt hour (=COP 3.37) in a bench test designed for cooler climates than yours, and can deliver ~20,000 BTU/hr @ +32F (16,000 BTU/hr @ +17F)

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/12RLFCD.pdf

In new construction you have the chance to design-out the load to minimize the cost & complexities of the mechanical systems needed to support that load. Spending the design time and money on the mechanical systems buys you effectively nothing in additional comfort, but spending it on the building envelope does.

As a starting point it's worth looking at Table 2, p10 of this document, the zone 3 row (you're in US climate zone 3C):

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

Note, those are "whole assembly R", not center-cavity R values. A code-min 2x6/R20 wall comes in at about R15 after factoring in the thermal bridging of the framing, and adding in the thermal performance of the wallboard, sheathing & siding, interior & exterior air films, etc. Adding an inch of foil faced polyiso insulating sheathing on the exterior of the structural sheathing, or using 1.5" Huber Zip-R sheathing gets you to R20+ whole-wall. Going with 1.5" of EPS under the slab gets you R6-ish for less money and less environmental hit than 1" of XPS (R5).

Paying close attention to the window specifications, and tuning how they affect the average heating energy consumption and peak cooling loads is important. Simulating the house with BeOpt (a freebie download from the DOE) is useful. As a general rule west facing windows need to have a lower SHGC and be smaller than south facing windows to limit peak cooling loads, and the south facing windows are best shaded from direct mid-day summer sun by roof overhangs. You may still want a SHGC>0.4 for the south facing windows, but the simulation would let you find the right balance of U-factor, window size & direction, and SHGCs, as well as insulation levels beyond code-minimum.

https://beopt.nrel.gov/

(The current administration is targeting NREL for major budget slashing- downloading and archiving the tool now might not be a bad idea.)