Horizontal earth loops are lengths of geothermal loop piping that are buried 4 to 12 feet in the ground, in horizontally dug or trenched ditches. They are the most common type of loop used for DIY geothermal heat pump installations, because many people either have access to excavators, or can rent trenchers locally. Much of the "cost" of these loops is the labor (of hauling pipe around, moving dirt, etc.), which means they are DIY-friendly.
We only recommend that a single pipe is put in a trench. This is because horizontal earth loops absorb heat from underneath the pipe. If multiple pipes are stacked vertically into one trench, only the bottom pipe will do any significant heat absorption (the other pipes just fight with this pipe over who gets the heat).
The smaller the area that you install the earth loop in, the lower the temperature of the earth loop fluid will drop in the heating season, and the higher it will go in the cooling season. This causes greatly reduced efficiency, in both seasons, and possible shutdown of the geothermal heat pump in the heating season.
Horizontal bores are like vertical bores, except drilled horizontally. This does not do as much damage to the landscaping as installing the other types of horizontal loops. Some states also require horizontal loop pipes be marked for GPS locating.