Arrived at the home and the homeowner spoke about the issue with water backing up. This is a common drain line which is connected to the second floor unit. It seems that it is also connected to the drains for the washing machines for two floors. It also is connected to the sewage system. 1. Began diagnostics of the equipment drain line and vacuumed water from the drain line and it is clear, no debris in the line. Filled with water and backed up again, removed water . 2. Forced air into the drain and water drained and heard gurgling sounds inside the wall where the common drain line is located. 3. Heard water dripping on the other side of the wall and seen it coming out of the drain for the washer. 4. Reconnected the drain line again and it backed up to the unit. 5. Tried to look for any wet spots outside but could not find any. 6. The condenser disconnects have padlocks on them and the equipment are not marked to indicate which unit goes to which condo. If diagnostics were to be p