AC leaking condensation leaking into furnace

jrf0022

My 2nd furnace / AC unit is leaking into the furnace. I cleaned the drain hole to make sure it wasn't blocked turned it on and it worked fine for about 6 hours then started leaking again. i noticed the coil is frozen solid. Its icing up the drain hole? I checked the compressor outside and i don't see any icing anywhere else. It a carrier SX58 vintage 92.

Any idea as to why???

jogr

Frozen coil is usually due to dirty air filter, dirty coil or low freon.