Several factors influence the depth of vertical geothermal wells: (1) average ground temperature, (2) HDPE pipe diameter (or rather surface area), (3) borehole grouting thermal conductivity, and (4) soil conductivity. The oft-cited rule of thumb is 150 feet per ton of GSHP capacity, but my understanding is that is based on a ground temperature of 52-55°F, 1" diameter piping, grout conductivity of 0.85 Btu/hour-foot-°F, and a soil conductivity of 1.2 Btu/hour-foot-°F. The factor that impacts the required depth the most is the soil conductivity and that is unknown, unless a cost prohibitive test is conducted. How does one make a reasonable estimate of the soil conductivity? In my case, the subsurface geology is almost entirely red shale overlying grey shale to a depth of 300 feet (according to the well driller's log); there is up to 2 feet of shaly silt loam on the surface. The thermal conductivity of shale varies from 0.5 to 1.8, depending on whether it is wet or dry and whether there is a significant quartz content (25%) or none. With regards to the vertical loop design, this means the depth of the bore needs to be 150±35 feet per ton of GSHP capacity. The contractors in my area seem to just run with the rule of thumb, 150 feet, but is there a more accurate (better) way? I'm thinking that taking the conservative approach 185 feet is more sound, albeit more pricey. Any advice would be greatly appreciated. Incidentally, the site is near Newport, PA, if that helps.