DAP,

Typically, it's figured in that the COP will diminish. If sized and installed correctly, your COP (efficiency) should not drop below what the Design Temp. COP listing is. For 50 deg. earth and a drilled (100' per ton) loop, the 4 ton EarthLinked system shouldn't see a COP lower than 4.2. That's still a 420% efficiency, instead of the likely 500%-600% the system started the season with.