are reported in terms of system efficiency as a function of a fraction of full load. 2 Flow Other Flow From (8) − 0.48 × ηW −W = 0.125 + 0.924 × Heat Pumps Flow FL Flow FL 2 Flow Flow 0.125 + 0.924 × − 0.48 × Flow FL Flow FL A Reverse-Return to Other Loops Table 1 demonstrates the resulting power when Equation 8 is applied to a VSD system with full load conditions of 300 gpm (1130 L/m) and 80 ft of water head (239 kPa) and minimum load flow conditions of 25% of full load. This power is multiplied by the operating hours for various loads to compute the total heat added to the ground loop by the VSD pump, drive and motor. The total heat added to the ground loop is 54.5 × 106 Btu (57.5 × 106 kJ), which is 4% of the amount of heat rejected by the heat pump system. If the pump were cycled off for 12 hours per day (4,380 hours per year) when all units are off, the heat to the ground is 23.4 × 106 Btu (23.1 × 106 kJ) or 1.7% of the amount added by the heat pumps. An ongoing project to obtain field data (Kavanaugh, 2011) indicates the average installed pump power is substantially higher than the 8.7 hp/100 tons (6.5 kW/350 kW) used in Table 1. The average is nearly double this amount for VSD GHP systems. Table 2 is a repeat of Table 1 using a 50% larger pump (13 hp/100 tons [9.7 kW/350 kW]). The total heat added to the ground loop is 67.6 × 106 Btu (71.3 × 106 kJ), which is 5% of the amount of heat rejected by the heat pump system. If the main pump were cycled off for 12 hours per day (4,380 hours per year) when all units are off, the heat to the ground is 25.5 × 106 Btu (26.9 × 106 kJ) or 1.9% of the amount added by the heat pumps. It should be noted that none of the VSD systems surveyed had provisions to be turned off at no-load. To Other Heat Pumps Main Pumps Fluid Returns to Vertical Console Supply Line Heat Pump On-Off Pump U-Tube Bores With 20 ft Minimum Separation U-Tube Bores 20 ft Minimum Separation B Vertical Console Heat Pump Heat Pump in Closet Opening to Hallway Heat Pump on Mezzanine Over Hallway Three-Way Purge/ Isolation Valves Console Heat Pump With Sloped Top On-Off Pump C Heat Pumps with Two-Way Valves Located In Zones Central Ground Loop Circuit Valves in Equipment Room or Exterior Vault Central Loop Piping And Pump(s) with VSD 20 ft Separation Minimum Multifloor Building D U-Tube Bores (20 ft Minimum Separation) On-Off Pump Check Valve Purge Valves Close Header (Supply Shown) Central Loop One-Pipe Systems One-pipe heat pump loops (Figure 1a) typically incorporate multiple or variable speed continuously operating low-head pump(s) on a central loop with small on-off circulator pumps to provide flow through each unit.7 An option for 28 ASHRAE Journal Heat Pump Circuit Isolation Valves Larger Buildings Would have Multiple Sub-Central Loops Figure 1: A: One pipe loop. B: Unitary loop. C: Central loop. D: Sub-central loop. ashrae.org July 2011