Pipe spacing from Vertical loop

"Heard people say that the heat is from the sun ... and even that loops should be close to the surface for this reason." You may have heard that loops benefit from solar gain, but that is horizontal loops. Loops also benefit from depth due to higher mean soil temps.

 

The heat is mainly from the sun, the true geothermal component is minimal in the first few hundred feet. The deep ground temperature down 100-300ft follow pretty reliably the average outdoor temperature, plus a couple degrees for the absorbed solar radiation. Loops closer to surface (within a few feet) do not work well in the winter, they get influenced by the colder ambient outdoor temperature. The entire concept is that we tap into the "stored" solar heat.

I agree, the designing factors are complex but are also fascinating.