Size of GHX based on building loads - not heat pump capacity

There is a common misperception that the capacity of the heat pump is what determines the size of the GHX. Many times I’ve heard a contractor say that he installed a “5-ton loop” (17.5 kW loop) for a home. What if he installed a 6-ton (21 kW) heat pump in the home rather than a 5-ton (17.5 kW)…would the size of the GHX have to be increased? Or if a 4-ton (14 kW) heat pump was installed, would a smaller GHX work as well?

Let’s look at a typical home with a peak heat loss of 56.0 kBtu (16.4 kW) and annual heating energy load of 158,167 kBtu (46,356 kWh), and peak cooling load of 55.0 kBtu (16.1 kW) with annual cooling energy load of 2,700 kBtu (791 kWh). If a 4-ton (14.7 kW) heat pump is installed for this building, is the size of a horizontal slinky or vertical borehole GHX smaller than the size required for a 5-ton (18.8 kW) or 6-ton (20.5 kW) heat pumps?

Ground Loop Design (GLD) sizing software was used to calculate the length of borehole needed for this building and connected to three different heat pump capacities. We found that the horizontal slinky GHX required for each heat pump was virtually the same when the same minimum and maximum temperature parameters were used for each system.

For the vertical GHX, the borehole length was kept the same  and the minimum and maximum temperatures resulting for each heat pump varied by one tenth of a degree for the three heat pumps. The difference can be accounted for by slight variations in the efficiency of the three heat pumps. 

The point of this exercise is to illustrate that the size of the heat pump installed in a given project must be based on the building heating and cooling loads...not the size of the heat pump you are installing in the building. In other words, there is no such thing as a "4-ton loop" or "5-ton loop".  This raises an interesting questions: Should you install the same size GHX for the 5-ton (18.8 kW) heat pump you installed in a home last week be used in the 5-ton (18.8 kW) heat pump you are installing this week?