I have an old gas furnance made by Trane. I had on heating cycle and I switched to AC and I noticed a water leakage coming out from the furnance. Do you know what cause the leakage? and is there something I didn't do before turning the AC?

thank you

Dominique

The leakage you refer to is the water that is being expelled by the a/c unit. I would investigate to see if the drain plug is clogged inside the unit and if it is, then unplug it. Water should be expelled in the normal course of running the a/c and should be funnelled away from the unit by a proper drainage line inside the machine to the outside.

where is the drain plug located?

It should be located on the bottom drip pan. You'll have to feel around with a finger to locate it if there is a lot of standing water there.