Quick Answer:

If a musty or sewer-like smell shows up only when the air conditioner runs, the likely causes are a dried trap, the AC condensate line tying into a drain, or HVAC-driven pressure changes that pull sewer gas through a trap. Start by identifying which drain area smells, check nearby floor drains and trap water levels, and inspect where the condensate line connects to the drain system.

Why This Happens

There are three common mechanisms that make a drain smell worse when the AC is on:

• Dry traps: Floor drains, seldom-used sinks, or shower drains can lose the water seal that blocks sewer gas. Running the AC changes pressure or airflow and can make those smells move into living spaces.
• Condensate tie-in: The AC condensate line sometimes drains into a nearby floor drain, utility sink, or building drain. If that connection is poor or if the condensate creates suction, odors can be pushed back out through nearby drains.
• Negative pressure: HVAC systems can create brief negative pressure in a home or room (you might notice doors pull closed). This negative pressure can siphon water from traps or draw sewer air through gaps around drains and vents.

If you see related problems such as slow draining only with hot water or bubbling when fixtures run, those clues point to related drain or vent issues like partial blockages or venting problems. See also Drain Bubbles When the Toilet Flushes and Drain Is Slow Only With Hot Water for more on related symptoms.

Step-by-Step What to Do

1. Identify which drain area smells

• Walk the house while the AC is running and sniff near sinks, floor drains, laundry drains, showers, and utility sinks.
• Note whether the smell is strongest at one fixture or in a hallway/room near an HVAC return or condensate line.

2. Check nearby floor drains for dryness

• Find any floor drains in basements, garages, laundry rooms, or utility areas near the HVAC equipment.
• If a floor drain is dry, it will often allow sewer odors to escape when pressure changes occur.

3. Pour water to refill traps

• Slowly pour about a quart (around 1 liter) of clean water into each suspect drain to restore the water seal in the trap.
• For toilets, flush once or pour water into the bowl; for shower/tubs, run the drain and refill with a small bucket.
• Check again with the AC running to see if smells return.

4. Inspect the AC condensate line tie-in

• Locate the condensate drain coming from the air handler and follow it to where it discharges or ties into the building drain.
• Look for sloppy connections, open fittings, or a gap where air could be drawn through the drain connection.
• If the condensate drains into a trapless receptor (common in some installations), that can allow odors unless properly vented.

5. Confirm negative pressure signs

• With the AC running, gently let a door close from a slightly open position to see if it pulls shut — a sign of negative pressure.
• Check for drafts near vents, and notice if exhaust fans or other appliances run at the same time as the AC causing pressure changes.
• If pressure is part of the issue, consider balancing vents, adding make-up air, or consulting a pro to evaluate return and exhaust flows.

6. Re-test and monitor

• After refilling traps and addressing obvious condensate connection problems, run the AC and re-check the smell over several cycles.
• Keep a note of when the smell appears (start, mid-cycle, only at high demand) — that helps a technician diagnose the cause.

What Not to Do

• Do not cap a drain/vent to stop smells; capping vents or drains can create bigger plumbing or health hazards.
• Do not ignore HVAC-driven pressure issues that can siphon traps — the trap loss is the symptom, the pressure change is the cause.
• Avoid pouring large amounts of harsh chemicals into drains without knowing the cause; they can harm plumbing seals and create toxic fumes if combined.

When to Call a Professional

Call a licensed plumber or HVAC technician when:

• You’ve refilled traps and the smell persists only with the AC running.
• You find a questionable condensate tie-in or suspect a trapless condensate receptor that needs correction.
• There are signs of sewer backup, continuous slow drains, or persistent negative pressure that you can’t safely diagnose. A pro can test venting, trap integrity, and HVAC air balance.

Safety Notes

• If the smell is strongly sewer-like, work with gloves and avoid inhaling for long periods; sewage gases can be harmful.
• Do not mix household cleaners, especially bleach and ammonia; that creates dangerous fumes.
• If you need to access the air handler or electrical panels, shut power off first and follow safe practices — when in doubt, hire a professional.

Common Homeowner Questions

• Q: Why does the smell only happen when the AC is on?
  A: The AC can change pressure or add airflow that draws sewer gas through an open or dry trap, or the condensate line can disturb a nearby drain.
• Q: Will pouring bleach fix the smell?
  A: Pouring water to refill traps helps. Bleach may mask odors temporarily but won’t fix pressure, venting, or condensate tie-in issues.
• Q: Should I call a plumber or an HVAC tech?
  A: Start with a plumber for trap and drain checks; if the condensate line or system pressure looks involved, include an HVAC tech to inspect the air handler and returns.