I've had a formidable challenge for 5 yrs and I'm at my wit's end.
10 yr. old house, bought 5 yrs ago. Orig. owner hid problem and left state. Hi-effic. furnace in attic, another in walk-out basement. Basement has condensate pump, sends water up across ceiling, down ext. wall, vents outside. For attic furnace (on its side), water gravity drips into PVC pipe, runs down ext. wall and outside next to other drain pipe.
In 2003, these 2 lines attached into 1 line in basement and went out. In Jan '04 while at Disney, line froze, water backed up knocking out furnaces. Came home to a bigger problem. Mold had been growing on basement walls under insulation and plastic sheeting orig owner installed. One expensive mold remediation treatment, plumbing bill, and insurance settlement later, all is not solved. When temp drops below 20, the lines "creatively" freeze. I'm in Massachusetts.
A plumber split single drain pipe into 2 lines (so both furnaces won't go out), then plumbed from 3/4" PVC to 2" PVC and left open-- 3/4" pipe goes into 2" about 1/2" but isn't sealed-- if pipes freeze, water overfolows into basement-- still won't knock out furnaces. Went from one 3/4" pipe venting outside 5" off ground (angled at 90 degrees) to two 2" pipes 2 feet off ground at 90 degrees. Pipes stick out 3" from siding.
Basement drain pipe purges all water via condesate pump, but drips over siding outside and freezes. Discovered the wall is wet again under insulation inside basement (w/some mold). Frozen water outside on siding builds up substantially. Water also freezes in the two 90 degree bends going to outside.
Attic drain pipe drips constantly onto ground outside and an icicle forms from ground up, to the pipe, sealing it, freezing water in the pipe, then sends it into the basement. We can't go away for a few days in the Winter! Drives my wife and I crazy!
At Home Depot, I showed photos and explained issues. Told me to keep lines at 3/4", send them out at 45 degrees and extend pipes farther out away from siding to get ice off. Did that last weekend to one pipe to see if it worked. It doesn't. Instead of a 2" thick icecube, I get a 3/4" thick one in 2-4 hrs. (it's been below zero).
It's too far to run attic condensate line to basement condensate pump or washer standpipe. There's no electricity in this area of basement to put in a pump for attic line. I'm going to Home Depot again, put the 2" pipe back and try to extend it farther outside. Furnaces produce lots of water, leading to 4" thick ice outside, soaking foundation and siding. Builder did this on north wall where there's no sun.
Any suggestions? I'm desperate! Check out the photos. HVAQ guys says "call plumber," plumbers just scratch their heads.