c327 wrote:Probably the drain holes or piping in the evaporator are clogged and not draining completely. What you are smelling is stale water that is left in the evaporator after it has shut down and not drained all the way. If the AC system is a year or older the evaporator could be dirty and needs to be washed out.
If it's a house AC there should be a pan under the evaporator, the pan should have two drain pipes, one low level and one safety for high level. This pan should also be cleaned once in a while.
14 years as an HVAC tech and the only time I've had to clean a coil is when the owner or renter was too lazy to keep an air filter in the system. If you change your filter on a regular basis, you will never
have to have the A-coil cleaned (evap coil). Where I live, only one city requires the use of both drains from the A-coil. The low drain is piped to the floor drain or condensate pump and the high drain is piped one foot above the floor next to the furnace (if installed in a basement. Attics and crawl space installs are piped differently). The purpose of the secondary drain is to let you know when the primary line is plugged. The A-coil in an upflow configuration is designed to allow water to remain in the pan. That water is there to create a liquid seal between the bottom of the coil and the pan to prevent the air from going around the coil rather than through the coil.
If the odor persists, you can purchase odor tablets that sit in the pan and dissolve slowly over time. Sometimes it's not worth the trouble to open the sheet metal just to drop a tablet in the pan. Some systems will have a removable front panel if the coil is a factory cased coil. Some companies buy uncased coils and build their own sheet metal plenum around them. Those are the ones that are hard to get into for a home owner. If you have a humidifier on the system, make sure it is shut down for the summer. Below are two pics of the system in my house (I rent). I installed the A/C onto the existing furnace. I used a Trane cased expansion valve coil, it has a removable front panel. The city I live in requires both drain lines to be connected. You can't see the primary line in the first pic because it is behind the secondary line. The two other plugs are to be used if the coil is mounted in the horizontal position (crawl space and attic installs). The second pic shows the primary line going into the floor drain and the secondary ends next to the furnace. The rust on the furnace bottom was most likely caused by a major water leak or drain back up before the A/C was installed.