I would like to discuss air conditioner condensate drains. I seem to see much confusion regarding drain configuration and termination.
First, a condensate drain is not required to be trapped by any of the Codes. Most manufacturers of air conditioning components require a trap on the primary condensate line to keep conditioned air in the air handler and to prevent insects from entering the air handler through the condensate line. A vent (some manufacturers call it a clean out) is usually required on the outlet side of the trap (install on the air handler side would negate the purpose of the trap) by the manufacturer. Most new models have a sticker on the air handler at the drain tapping showing proper drain configuration.
Most of the time condensate is gravity drained (meaning sloped to drain its entire length) to the exterior grade by a PVC or other pipe. The termination is usually visible, but not required to be visible. The secondary, or emergency drain, should be visible to alert the occupants of a problem. Since most layperson occupants have no idea water flowing from this secondary drain indicates a problem, many technicians are now installing a float switch in the drain pan and/or secondary line to shut down the system in the event of a primary condensate drain malfunction. A non-working system usually promotes a phone call to the service company instead of the usual ignoring the water flowing from the secondary.
Many jurisdictions have provisions for alternate methods of condensate disposal besides just draining to exterior grade or a floor drain. Any connection to the sanitary sewer system will require a trap. This is called an indirect waste by the Codes. Many times I see condensate drains terminating at trapped receptors (a standpipe, like used for the clothes washer) connected to the sanitary sewer system. Traps installed only for the condensate drain will require trap primers to keep them wet. Because of this problem the most common termination locations are routinely wetted traps such as lavatory sinks and bathtubs. The fixture keeps the trap wet to prevent sewer gas flow back into the air handler. The condensate is connected with a branch tailpiece on the fixture side of the trap, such as to the outlet of the lavatory sink or the overflow of the bathtub. Connection to the sewer side of the trap is improper and will result in sewer gas flow to the air handler since the condensate line trap (if installed) will dry out in winter. Another common location for condensate disposal is the clothes washer standpipe.
Condensate pumps are commonly used where gravity drain is not possible. These pumps accept the condensate and pump the liquid to an approved location once the tank is filled to a predetermined level. The outlet pipe from the pump is not required to be trapped. The inlet from the air handler should be installed per the manufacturer recommendations, which could be trapped or not.
Hope this clears up some questions about condensate.