Customer said furnace was leaking water on the floor. Found water in furnace on and below trap. Reseated trap. Checked tube clamps. Turned on furnace. Within 30 seconds of turning on furnace water started dripping out of the fernco on the vent in the furnace. Fernco not seated properly on pipe and not tight enough. Reseated pipe and tightened fernco. Watched for 10 minutes. Found no more leaks. Confirmed operation.