We have a Coleman "Up Flow Hi-Efficiency Condensing Furnace" (Gas). I had never seen a Coleman furnace before this one. I must say that it is quite good, cooling and heating well from season to season. Recently, I have noticed a buidup of ice on the outside of the cabinet. It is on the left side and toward the top. My manual says this is the area where the "heat exchanger" is located. The insulated pipes which run to the outside unit drip water more so than any I have seen before. What is causing the unit to form this ice and "indoor rain" from the condensing lines?