You spot a brown, ring-shaped stain creeping down the wall right below your split AC's outdoor unit, or a damp patch spreading from the corner where the indoor unit sits. First reaction: "the waterproofing has failed." Second reaction, after a contractor quotes ₹40,000 for exterior waterproofing: panic. Before you sign anything, check the AC. In a large share of Bangalore homes, this exact stain has nothing to do with your terrace or wall membrane — it's a clogged condensate drain pipe, and the fix costs a fraction of a waterproofing job.

This mix-up costs homeowners real money every year. Here's how to tell the two apart and fix whichever one you actually have.

Why AC units cause wall stains that look like waterproofing failure

Every split AC produces condensate — water pulled out of humid air by the indoor unit's cooling coil. That water is meant to run out through a thin PVC drain pipe, usually routed alongside the refrigerant piping through a wall sleeve to the outside. In Bangalore's dust and pollen, three things commonly go wrong:

  • Algae and dust clog the drain pipe. Over 12–18 months of running, a slimy build-up narrows the pipe until water backs up and overflows inside the wall sleeve.
  • The pipe was laid with the wrong slope. If the installer routed it flat or uphill even slightly, water pools instead of draining, and it finds the nearest gap to escape — usually straight into the wall.
  • The wall sleeve was never sealed properly. The hole drilled for the AC pipes is often left with a loose PVC collar and no sealant, so both AC condensate and rainwater can enter the same gap.

How to tell it's the AC and not the wall

The 3-day test: Switch the AC off completely for 2–3 dry days. If the stain stops spreading and the wall dries out, the AC drain is your problem. If the patch keeps growing or the wall stays damp even with the AC off, you're looking at genuine seepage — from the exterior wall, a cracked sill, or a plumbing line — and it needs a proper inspection.

Other tells: an AC-caused stain sits directly below or beside the unit and gets visibly worse after long AC sessions (think: after a work-from-home day with the AC on 8 hours straight). Genuine wall seepage tends to worsen after rain regardless of whether the AC ran, and often comes with a musty smell the AC leak doesn't produce.

What to check, step by step

  1. Trace the outdoor drain outlet. Follow the thin pipe from the outdoor unit and make sure it actually discharges away from the wall, not against it.
  2. Check the indoor unit's drain tray and pipe for clogs. A gentle flush with water (or a wet/dry vacuum on the outlet end) clears most algae blockages in minutes.
  3. Inspect the wall sleeve where the pipes pass through. If the PVC collar is loose or unsealed, both AC water and rain can be entering there — this is the most commonly missed spot.
  4. Run the 3-day dry test described above to separate an AC issue from a structural one before you spend on anything.
  5. If the wall stays damp with the AC off, book a proper leak/seepage inspection — don't guess between the two causes, because the fix for each is completely different.

What NOT to do

Don't repaint over the stain and hope. Whether it's the AC drain or the wall, paint alone doesn't fix a moisture source — it just hides it until the next monsoon or the next long AC session.

Don't assume it's "just the AC" without the dry test. We regularly find both causes active at once — a clogged drain pipe and a cracked wall sleeve leaking rain — and fixing only one leaves the stain coming back.

Don't ignore it through a full season. Constant moisture behind an AC bracket corrodes the mounting anchors and can seed mould inside the wall cavity — a ₹1,000 fix left too long turns into a ₹15,000–₹20,000 wall repair.

What it costs to fix in Bangalore (2026)

  • Condensate drain clearing / re-routing: ₹500–₹1,500 (AC technician, same day)
  • Resealing the wall sleeve / PVC collar: ₹800–₹2,000
  • If it's genuine wall seepage — exterior waterproofing: ₹35–₹60/sq ft
  • Free diagnostic inspection: LoklFix checks both causes on one visit before recommending anything

Not sure if it's the AC or the wall? We check both on one visit and give you a written diagnosis before you spend a rupee on the wrong fix. Call +91-8123083393 for a free inspection.

Frequently asked questions

Why does my wall have a brown stain near the AC unit?+

Most often it is a clogged or wrongly sloped condensate drain pipe backing up and overflowing near the indoor or outdoor unit, not a waterproofing failure. Algae and dust build up inside the thin drain pipe over months and slow the flow until it overflows.

How do I know if it's the AC or a wall seepage problem?+

Switch the AC off for 2–3 dry days. If the stain stops growing and the wall feels dry, it is the AC drain. If the patch keeps spreading or feels damp even with the AC off, it is genuine wall or external seepage that needs a waterproofing inspection.

How much does it cost to fix an AC drain leak in Bangalore?+

Clearing and re-routing a condensate drain pipe typically costs ₹500–₹1,500 through an AC technician. Resealing the wall sleeve where the pipe passes through costs ₹800–₹2,000. If the wall itself is also seeping, exterior waterproofing runs ₹35–₹60 per sq ft.

Can an AC leak cause permanent wall damage?+

Yes, if ignored for months. Constant moisture behind the AC bracket corrodes the mounting anchors, blisters paint, and can seed mould inside the wall cavity. It is a small fix early and an expensive one if left through a full season.