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.
you can run the numbers yourself for a loop design here: https://geoconnectionsinc.com/ The LoopLink RLC is for residential systems. The site allows free access for something like 30 days before you have to pay to use it/get a printout of your results. The calculations are based on the first book in this link https://igshpa.org/manuals/ which is the standard method for design in a residential system. I can confirm that spreadsheets built based on the IGSHPA book provide the essentially the same results that I get via the geoconnetioninc website. If you don't want to use a contractor's 150ft/ton mantra, this is the real way to engineer a ground loop that is 1) specific to your site conditions and 2)specific to the installed equipment at design conditions that is loading the loop. You will have to make an assumption in the calculations about the thermal conductivity of the ground and grout (if you don't know them) in order to get an answer but you appear to have at least researched this aspect. If you don't know wet/dry or quartz %, error to the low side so you don't end up underlooped (easy to install more now, more difficult to fix/put in additional loop later). For what its worth, following the above procedure got me ~183ft/installed equipment tonnage at design conditions for my loop. Careful with the "ton" nomenclature as equipment nameplate tonnage is not the same as actual equipment tonnage mainly due to EWT for the design hour (assuming closed loop).
Hi and welcome! Contact local hvac contractors and drillers. Ask them at what depth they have had the most favorable results. Problem solved. Hope this helps. Eric
Bonjour! My perception is that the local hvac contractors and geothermal drillers are all using 150 foot per ton, even though the subsurface geology varies significantly in this part of Southcentral PA (some areas are underlaid with limestone, mine is all shale). I have had 5 different hvac contractors onsite and, of the 3 that provided proposals, not a single one had any details about the wellfield. When pressed, one stated 2 wells @ 225 feet each for a 3 ton system, with Bentonite grouting of the top 50 feet, not the whole borehole, even though the well driller claims to be IGSHPA certified. Another told me I could get the details of the wellfield AFTER I signed a contract and it had been installed! As I told them all, it's not about the cost, it's about proper design for efficient operation.
Mosts important is the amount of BTUs you are planning to pull out of the ground (house load, thermostat setting etc), linear feet per ton of capacity means not mach. Including the load factor, how much surface area the pipe has, volume of water in there, antifreeze mix, and pulsing (do you have a constant load on it, like a variable speed heat pump, or a single speed, which shuts on and off much more, resting the loop), which for example also affects the turbulence in the loop. Things get complex, loop software and IGSHPA guidelines do not account for all those factors. The 150ft/per ton installed capacity is a rule of thumb which has worked pretty much under all occasions. Also, there is really not much need for more expensive safety factors. In case you design for 30 degree minimum temps, and you are 20 % short, and your loop drops to 28F, your heat pump will still be very happy if you feed it with the correct flow.
