讲得非常好，补充一下

割开的管道是air duct, not the copper pipes.  Typically the contractor can cut a hole in the plenum box to gain access to the coil, and wash the clogged coil.  Some people use compressed air to blow away the dirt.  

The other important thing to check is the air handler motor.  If the air handler doesn't produce enough air, the coil wil freeze up. 

I was told that another possible cause of coil freezing is low refrigerant.  This is a bit counter intuitive.