By, Doug Carruthers 
All HVAC equipment is subject to some form of rating system that attempts to put everything on a level playing field so that system designers, engineers and consumers can make an apples-to-apples comparison of how they work in a specific set of conditions. The problem is that actual performance can vary significantly from rated because installed HVAC equipment doesn't work in a controlled environment.
By virtue of how heat pumps work (air source and ground source alike) and their inherent dependence on the temperatures of their source/sink, they are particularly susceptible to this variability. LoopLink already adjusts for adverse effects on geothermal system efficiency based on entering water temperatures (EWTs) that differ from rated conditions but because we don't model air-source equipment, we only use the user specified efficiency for our economic comparisons.
When clicked, you are prompted to adjust the rated efficiency specified to reflect more realistic performance in the installed location for the system. The regression equations we apply are from "Climate Impacts on Heating Seasonal Performance Factor (HSPF) and Seasonal Energy Efficiency Ratio (SEER) for Air Source Heat Pumps" and are meant to be applied to the rated efficiency (HSPF or SEER) of the air source equipment being used in the LoopLink Operating Cost Comparison.
The next time you are bidding a project where the homeowner is choosing between geothermal and air-source (including air-conditioners), make sure you give them the whole story by adjusting the air-source efficiency according to actual operating temperatures. Not only can it help you to sell a geothermal system, it will help them make an educated choice based on realistic cost projections.
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This blog post has an accompanying white paper that details the methods employed and further expands on the content of this article.