Reading design guide from the manufacturer of our geothermal heat pump. It is currently in an open loop configuration, and am planning to convert to horizontal closed loop (in Ohio). On one page in the guide, they reprint a graph illustrating that deeper in the ground (12 feet down) the soil temperature swing is more constant throughout the year, with larger temperature swings closer to the surface. My assumption then is the deeper the better to bury a horizontal loop. However, they make a remark, "Pipe depths in the Northern Zone should be 3 to 5 feet. Excessive depth will reduce the ability of the sun to recharge the heat used in winter." What might they be referring to? I am confused by this. Why would 5 foot depth be better than 12 for horizontal loop? I suspect that maybe they are referring to one of two things: ... the yearly average temperature, which they don't discuss, maybe it is colder at 12 feet than at 5 feet in the winter? ... they spend a number of pages discussing multiple lines per trench, whereas I am only interested in one line in a long loop?