Want to convert our geothermal open loop to horizontal closed loop. Heat pump manufacturer has a publication that assists in deciding what type of loop, length, and selection of circulation pump. It briefly discusses depth of horizontal loop, and provides a graph showing greater depth a more steady state temperature is experienced annually. While searching for trenchers I could rent, the rental price between one that digs 60" versus 70" was practically the same. Am I understanding correctly then, that the 70" depth trench is preferred?