What is the best thing to pour down the AC drain?

The absolute best and safest substance to pour down your air conditioning condensate drain line for routine maintenance is a simple solution of equal parts warm water and distilled white vinegar. Pouring about one cup of this vinegar solution down the T-shaped vent opening once a month effectively neutralizes and kills algae, mold, mildew, and bacterial slime without introducing harsh chemical compounds into your plumbing system or damaging the PVC piping. Another excellent option for long-term prevention is placing specialized HVAC condensate drain tablets or algaecide strips directly into the drain pan; these slowly dissolve with each moisture cycle and keep the entire line sanitary. You should strictly avoid pouring heavy household bleach in high concentrations, as repeated use can degrade plastic joints and cause unpleasant chemical vapors to circulate through your home ductwork.