Half the "my AC is completely dead" calls in an Austin summer trace to the least glamorous component in the system: a PVC drain line the diameter of a garden hose, clogged with the algae this climate grows for free. The physics are humble — cooling coils condense gallons of humidity daily, the condensate needs somewhere to go, and the float switch that guards against backups kills the whole system when the line loses — but the economics are the point: this is the failure that's a free DIY or a $100–$250 visit, standing between you and the belief that your AC died. Here's the whole story, vinegar included.
Dehumidification is half of cooling's job in Central Texas: the evaporator coil runs cold, the air's moisture condenses on it like an iced glass sweating, and the drip adds up — gallons per day per system through the region's muggy months. The condensate path handles it: coil to pan to PVC line to a discreet outdoor drip (the stub by the foundation you've seen dripping on hot days — that drip is health). The path's enemy is biology: warm, dark, wet PVC is an algae greenhouse, and the slime builds until the line chokes — faster in Austin's humidity, faster still with the mineral content our water contributes to every splash. When it chokes: the pan fills, the float switch trips (killing cooling to protect your ceilings), and the "dead AC" call gets made about a system that's actually fine.
The tells, in order: the system's fully dead (no cooling, maybe no blower) with breakers fine — float switch territory; water at the indoor unit — pan overflow announcing itself; the outdoor stub bone-dry on a humid day while the system runs — the line's blocked upstream; the ceiling stain under the attic handler — the defense stack already lost a round, address the water path before the paint. Confirm at the air handler: the float switch (a small device riding the pan or the line) sits tripped, the pan sits wet, and the diagnosis is complete. The warm-air guide's broader tree covers the alternatives; this branch is the merciful one.
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The classic Saturday fix, step by step: locate the outdoor stub (the PVC elbow near the condenser or foundation); seal the shop vac's hose to it — a hand-wrapped rag makes the gasket, and the seal is the whole trick; run 1–2 minutes — the satisfying slurp of the algae slug arriving in the canister is the sound of $200 saved; verify at the tee — pour a cup of water down the indoor access (the capped tee near the handler) and watch the outdoor drip resume; reset the float switch and restart. The refinements: distilled vinegar down the tee after clearing (a cup, seasonally, as the anti-algae ritual — skip bleach, which is harsh on pans and lines), and the pan sponged dry while you're there. The clogs that survive the vac — deep blockages, crushed or bellied lines, the pan's own drain fouled — earn the professional visit's $100–$250, still the cheapest line on the repair menu.
Attic installations make the stack worth auditing: the primary pan and line (the working path), the emergency pan beneath the handler with its own drain to a conspicuous soffit drip (the "why is water dripping over my window" answer — it's a warning light made of water), and the float switch(es) — primary line, emergency pan, or both — whose trip is the system protecting your drywall. The audit questions: does the emergency drain actually daylight somewhere visible, does the float switch actually kill the system (test it at the spring visit), and does the primary line include an accessible tee for the vinegar ritual? Older installs missing pieces of the stack are a $150–$400 retrofit away from ceiling insurance — the rare upsell worth taking unprompted.
The double-trip mystery: the switch that re-trips within days points past the clog at the slope — bellied or back-pitched lines hold water and re-grow blockages on schedule, and the re-slope is a modest one-time fix that ends the subscription. The condensate pump variant: installs that pump uphill (basements are rare here, but closets pumping to distant drains aren't) add the pump's own failure mode — its hum, its reservoir, and its check valve joining the audit. The frozen-coil crossover: the thaw that follows a freeze event delivers a flood the line must handle at once — the freeze guide's towels-and-pan protocol exists for exactly this handoff. The renovation clog: drywall dust settles in pans and lines like cement mix awaiting water; the post-remodel season's drain service is a scheduling note, not a coincidence.
One scheduling note that pays annually: the drain genre's calls cluster in early summer — the algae's spring growth meeting the first weeks of heavy condensate — which makes May the vinegar ritual's highest-yield month and the pre-season tune-up's favorite discovery. Beat the cluster and the season runs dry-ceilinged; join it and you'll meet the region's friendliest, most bookable repair anyway. As failure modes go, this one grades on mercy.
The sequence for the ceiling that lost: water path first (line cleared, pan drained, switch reset — the source ended before repairs begin), moisture assessed honestly (the small fresh stain dries and paints; the saturated or recurring one earns the moisture meter and, at scale, the remediation conversation), and the retrofit completed — whatever piece of the defense stack failed gets its upgrade in the same mobilization. The insurance footnote: sudden-discharge events read differently than long-neglect ones on claims, which is one more argument for the monitoring habit and the documented service history. The stain is tuition either way; the retrofit is the degree.
The climate never stops growing it, so the routine outpaces it: the seasonal vinegar cup (spring and mid-summer, down the tee), the spring visit's drain service (line flushed, pan cleaned, switch tested — bundled into the $100–$200 tune-up that prevents this article's whole genre), and the monthly glance at the outdoor stub on humid days — dripping means working, and the ten-second check is the cheapest monitoring in HVAC. The payoff stack: no float-switch summers, no ceiling stains, no $250 calls about $0 problems — and the humbling knowledge that the mighty system's most common failure was always a garden-hose-sized pipe asking for a cup of vinegar twice a year. Give it the vinegar.