What are the common problems with AC condensate drain lines?

Air conditioning condensate drain lines frequently experience several common issues that can compromise system performance and cause property damage. The single most prevalent problem is a physical blockage caused by the continuous accumulation of dust, algae, mold, and slimy bacterial growth inside the dark, damp PVC pipes, which prevents water from escaping and leads to overflowing drain pans. Another frequent issue is improper initial installation, such as a drain line lacking a proper P-trap or sufficient downward slope, which causes water to pool, trap air locks, or fail to drain correctly by gravity. Additionally, external factors like crushed or disconnected outdoor drain pipes, broken or burned-out condensate pumps, and rusted or cracked drain pans often lead to severe water leaks that damage ceilings, subfloors, and drywall if not caught and repaired promptly.