I have a 4 ton closed loop system in which the slinky coil was trenched into an elevated pasture behind the house (the site was originally terraced cropland). I estimate 20 foot head between the coils and the heat pump in the basement. The unit has failed after 12 years of use. The coax corroded and was leaking coolant. In my research of installation instructions I have not found any specs on the elevation of the horizontal field relative to the heat pump. I wonder if the head pressure in the coolant line contributed to the failure. What do you think?