To get an accurate installed efficiency rating, factors such as flue gas heat losses, cycling losses caused by oversizing, blower fan electrical usage, etc., must be included. It's a scientific way of determining how much energy the system produces versus how much it uses. Most geothermal heat pump systems have COPs of 2.5 - 3.5. Where a fossil fuel furnace may be 50-90 percent efficient, a geothermal heat pump is about 300 percent efficient. Some geothermal heat pump manufacturers and electric utilities use computers to accurately determine the operating efficiency of a system for your home or building.
All heat pumps can handle temperatures in the moderate to warm ranges.
A closed loop system, on the other hand, will encounter EWTs below freezing. Not all geothermal heat pumps will operate efficiently at those temperatures. It's important for you to know what EWTs your heat pump will handle. Don't be afraid to ask for references from dealers. A reputable dealer won't hesitate to give you names and numbers to call to confirm his capabilities.