During a lot of my Home Inspections I discover water or stains in the pan below the attic air handler unit. Most new houses have a split HVAC system installed, meaning that the condensing unit is located on the outside of the house and the air handler, which has the cooling coils inside of it, is located inside of the house. The air handler is typically in the attic on newer Phoenix area homes but may be located in the garage or a closet. There are two PVC drain lines ran from the attic unit to the exterior of the house. One of the drain lines are connected to the air handler unit and drains any condensation that forms on the coils to the exterior of the house. The second drain line is attached to the safety drain pan below the unit. The drain pan is installed to prevent water leaks at the unit from causing water damage to the ceiling. If the system is working properly, no water or stains should be present in the pan. Below are the two main reasons for water or stains in the drain pan.
Smaller stains in the pan can be due to condensation dripping from the suction line. The suction line is the larger diameter copper pipe that carries refrigerant as a gas between the exterior and interior units. If this line is not properly insulated than condensation can form on the pipe and drip into the pan or the attic. Condensation forming on the suction line is an easy and inexpensive repair. You can purchase pipe wrap insulation at any home center—simply wrap it around the suction line. None of the copper piping should be exposed.
The other reason for water in the pan is the air handler may not be properly draining the condensation from the coils as intended. This can be due to the unit not being level, the drain line not being properly slopped or the drain line being clogged or blocked by debris. In rare cases you may actually have a hole in the coil drain pan. In which case the coil or the pan will have to be repaired or replaced. The primary PVC drain line being blocked is the number one reason for water leaking into the pan below the unit. There are a couple of ways that you can clean out condensate lines. One way would be to use a wet/dry vacuum and suck the line out. Try running a plastic snake or wire through the pipe to remove any solid particles. You can also use a garden hose to flush the line but this can make a big mess so I would be careful when using this method.
When I do find large stains in the pan during my Home Inspections I recommended having a licensed HVAC technician service the system to be sure that it is functioning properly. The coil section of the unit is typically sealed and is not inspected during a home inspection.