Does my AC drain line need a trap?

Yes, an air conditioner drain line almost always requires a P-trap, and in many jurisdictions, it is explicitly mandated by standard residential plumbing and mechanical building codes. The primary purpose of the P-trap—a U-shaped bend installed near the indoor air handler—is to hold a small amount of standing water that acts as an airtight seal. This water seal prevents the air handler's powerful blower fan from either sucking unconditioned outdoor air backward through the drain line, which disrupts proper water drainage and causes messy indoor leaks, or pushing foul sewer gases and harmful bacteria from a connected sanitary plumbing stack up through the PVC pipes and into your home's air duct system. However, it is important to note that if your system features two separate drain connections, such as one on the evaporator coil and one on the secondary pan, they generally require independent traps rather than being tied into a single shared trap.