I have a climatemaster 6 ton unit with a 3600ft horizontal closed system pressureless loop. Its about 10 years old. It worked great for years but the last couple years it seems that it has become less efficient. If the outside temp gets below freezing it cant keep up. If the the outside temp gets too high it cant keep up. When it was working properly no matter what the outside temp was the inside temp was always what it was set to on the thermostate. Also, I noticed a black film in the loop water. Is it possible that I have some kind of buildup inside the loop or the coils inside the unit that is causing the unit to be less efficient. There is also ice buildup in side the unit around the coil. The unit is running non stop when it gets cold and can not maintain the set temp in the house. My installer has long gone out of business so I'm having a hard time finding someone who is familiar with geothermal, so any help there would be appreciated. I live in the Atlanta area north east of the city. Any suggestions or references would be greatly appreciated.