Quick answer: A sewer smell in one room is most often a dry drain trap: the water seal in a little-used sink, shower or floor drain evaporated. Run water for 30 seconds to refill it. If the smell comes through the vents, check the AC drain line, which can pull sewer gas into the air handler when it’s tied into a plumbing drain.

In this guide

Why does a room suddenly smell like sewer?
Which drains are most likely to dry out?
How do you refill a dry drain trap?
When should you call a plumber?
Can your AC make the house smell like sewer?
Why does the AC smell musty instead?
What does an HVAC technician check when the vents smell like sewer?
What should you do next?

Why does a room suddenly smell like sewer?

Usually because a drain trap went dry. Look under any sink and you’ll see a U-shaped bend in the pipe. That’s the P-trap, and every tub, shower and floor drain has one too, even where you can’t see it. The trap holds a small plug of water that seals sewer gas in the drain line and keeps it out of the room.

When a drain goes unused for a few weeks, that water slowly evaporates. A warm Texas house speeds things up. Once the seal is gone, sewer gas drifts straight up the drain, and the room smells like a gas station bathroom.

Which drains are most likely to dry out?

The ones nobody uses. The usual suspects in a Central Texas house:

The guest bathroom sink and shower
A spare tub that’s become a storage spot
A laundry room or garage floor drain
A wet bar or utility sink
Every drain in a house that sat empty during a trip, a sale or a move

Back from a few weeks away and the hall bath smells off? Start there.

How do you refill a dry drain trap?

Run water. That’s the whole fix most of the time.

Run each sink, tub and shower for about 30 seconds, including guest baths.
Flush toilets that don’t get used often.
Pour a bucket or two of water into floor drains.
Do it once a month. A phone reminder helps.

For a drain you almost never use, a spoonful of mineral oil poured in after the water floats on top and slows evaporation. If the smell fades within an hour or so, a dry trap was your culprit.

When should you call a plumber?

When water doesn’t fix it. Call a plumber if:

The smell comes back within a day or two of running water
Drains gurgle, or more than one drain backs up at the same time
You see water under a sink or around the base of a toilet or tub
The smell is strongest near a wall, a toilet base or up in the attic, not at a drain

Those point to a cracked pipe, a failed toilet seal, a blocked plumbing vent or a main line problem, which are plumbing jobs. Every house has plumbing vent pipes that run up through the attic and out the roof. A vent pipe that came apart in the attic can let sewer gas into the attic, and that brings your AC into the story.

Can your AC make the house smell like sewer?

Yes, in two ways, and both matter if the smell seems to come out of the vents.

The condensate drain. Your AC pulls water out of the air and sends it down a small PVC line. In many Austin homes that line runs outside, but in others it ties into a plumbing drain, like a bathroom sink drain or a standpipe (the open pipe a washer drains into).

When the connection isn’t made correctly, and especially when the trap in the AC’s condensate line dries out, the air handler can pull sewer gas right back up the line. The blower sits in the air handler and creates suction there, so the smell gets spread through the ducts.

This shows up most in fall and winter. When the AC isn’t cooling, it isn’t making water, so the trap in the drain line can dry out just like a guest-bath sink. The first time the heat runs in November, the house smells like a sewer.

Leaky return ducts. Return ducts pull air in. If one is leaking in the attic near a disconnected plumbing vent, it can draw that sewer gas into the system and blow it into every room. In that case the plumber fixes the vent, and an HVAC company seals the return.

Why does the AC smell musty instead?

A musty or dirty-sock smell when the AC starts usually comes from the indoor coil, the drain pan or the drain line, where water sits in a dark, warm spot and things grow. It’s a different problem from sewer gas.

Common causes:

A drain line clogged with slime, so water backs up into the pan
A drain pan that holds standing water or has rusted
A dirty coil, often from a skipped filter
Mold growing in the air handler or on the insulation inside it

A clogged line can also trip the float switch (a safety switch that senses water in the pan) and shut the AC off, or overflow into a secondary pan and onto the ceiling. Our guide on clearing a clogged AC drain line covers what you can safely do yourself, and warning signs of mold in your HVAC system covers when it’s more than a clog.

What does an HVAC technician check when the vents smell like sewer?

When the smell comes through the vents, a technician works back from the air handler:

Where the condensate line goes, whether it ties into a plumbing drain and how that connection is made
Whether the drain line has a proper trap that holds water, and whether it has a cleanout (a capped opening for clearing clogs)
The primary and secondary drain pans, and the float switch
The coil and blower for buildup, and the inside of the cabinet for mold
The return ducts and plenum (the box where the ducts meet the air handler) for leaks, especially near plumbing vents

If the problem is in the plumbing itself, like a broken vent pipe or a bad drain connection past the AC’s line, we’ll tell you straight that it’s a plumber’s job. On the AC side, Gold Eagle’s technicians clear drain lines, clean coils and pans, and seal leaky return ducts across the Austin area; see our AC services.

What should you do next?

Run water in every drain once a month, especially guest baths and floor drains.
If the smell comes back fast or seems to come from a wall or the attic, call a plumber.
If the smell comes out of the vents, or the AC smells musty when it starts, book a Gold Eagle visit to check the AC drain line, pans, coil and return ducts.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our AC services, or call 512-489-4653 to schedule a visit.

Related Gold Eagle service: AC repair in Austin

Why Does a Room Smell Like Sewer? Dry Drain Traps and Your AC Drain