A set of recommendations for loop sizing, piping guidelines, piping arrangements, and antifreeze precautions is provided for Residential & Light Commercial geothermal heat pump installationsThese residential closed-loop ground source heat pump (a.k.a geothermal or groundcoupled heat pumps) design guidelines were developed in conjunction with a project sponsored by the Tennessee Valley Authority in conjunction with a TVA Ground Source Heat Pumps promotion.
Previous guidelines were verified and adjusted according to data gathered during this effort and previous similar projects. A set of recommendations for loop sizing, piping guidelines, piping arrangements, and antifreeze precautions is provided. They attempt to balance the conflicting constraints of installation cost and efficiency. These guidelines have recently been extended to regions beyond the TVA service territory.
Table 1 provides recommendations for ground loops in the Tennessee Valley Authority service area and beyond. The values use results from this TVA sponsored project coupled with a previous Alabama Power project and the resulting recommendations for Alabama (Kavanaugh, 1991). The length of the trench or bore must be based on the amount of pipe in the trench, burial depth, and average ground temperature and conductivity. The table provides extremes of pipe length per length of trench from 10 ft/ft (i.e. 1000 ft. of pipe in a 100 ft. long trench) to 2 ft/ft. The table was developed for an average burial depth of 5 ft. and ¾” high-density polyethylene (HDPE) pipe. The extremes of normal ground temperature are 44 F for the northern continental US and 70 in southern USA (not including southern Florida and Texas and all of Hawaii). These lengths should provide a maximum loop temperature of 90 F entering the heat pump in
normal applications. In homes with excessive run times this temperature will be 3 to 5 F higher.
The table also includes recommendations for vertical ground loops in ft. of bore per ton for ¾” and 1-1/4” HDPE, which will operate about 5 F cooler than the horizontal loops.
Table 1. Recommended Lengths of Trench or Bore Per Ton For GCHPs
Multiply length of trench by pitch to find required length of pipe.