The simplest way to keep a P-trap from drying out is to run water through the drain regularly — and for drains that don’t get regular use, to pour water down them manually about once a month. A P-trap’s water seal evaporates over time whenever the drain is idle, and in Colorado’s very dry climate that evaporation happens faster than most homeowners expect. A dry P-trap is the most common cause of sewer gas odors inside a home, and it’s one of the easiest problems to prevent.

Why P-Traps Dry Out in Colorado

Denver averages around 14 inches of annual precipitation — one of the driest major metros in the country — and indoor relative humidity often drops below 20% during winter months when heating systems are running. This low humidity dramatically accelerates evaporation from P-traps. A floor drain in a basement that’s only used when the water heater pressure relief valve discharges can dry out in as little as two to three weeks during a dry winter stretch.

This is particularly relevant for floor drains, guest bathroom drains, spare room sinks, bar sinks, laundry room drains, and any fixture that doesn’t see daily use. Seasonal homes and vacation properties are especially vulnerable — a P-trap left to evaporate over a winter can allow sewer gases to accumulate in a closed house for months.

The Monthly Water Method

For any drain that doesn’t see regular use, pour approximately one quart of water down it once a month. This is enough to refill the trap and restore the water seal. Add a small amount of vegetable oil (a tablespoon or two) on top of the water — the oil floats on the water surface inside the trap and slows evaporation significantly, extending the time before the trap dries out again. This is an old plumber’s trick that works reliably in dry climates.

Trap Primers: The Permanent Solution for Floor Drains

For floor drains in particular, a mechanical trap primer is the most reliable long-term solution. A trap primer is a small device that connects to a nearby supply line (often under a sink or at the water meter) and automatically sends a small amount of water to the floor drain trap whenever a pressure differential occurs — typically when another fixture is used. Most modern building codes require trap primers on new floor drain installations, but many older homes in Denver and Aurora don’t have them.

Installing a trap primer is a straightforward plumbing job that eliminates the need to remember monthly manual priming. Done’s plumbers can add one to an existing floor drain installation if your home doesn’t have one.

Other Causes of Sewer Odors That Mimic a Dry Trap

  • A broken or missing P-trap — some older drains were installed without one, or the trap was removed and not replaced
  • A cracked or leaking P-trap that can’t hold water
  • A damaged vent pipe that allows sewer gases to bypass the trap (a clogged or broken vent can create a siphon that pulls water out of P-traps)
  • A cracked sewer line under the slab, which allows gases to seep up through the floor independently of the trap

If you’ve been priming a floor drain consistently and still notice sewer odors, the trap may not be the issue — it’s worth having a plumber investigate. A camera inspection of the drain line and vent system can identify whether the odor source is something more serious than a dry trap.

When to Call Done

If monthly priming isn’t solving the odor problem, or if you’d like to have a trap primer installed to permanently protect a floor drain, Done’s plumbing team can help. Persistent sewer odors inside a home should always be investigated — the cause is often simple (a dry trap) but can occasionally signal a more serious issue like a cracked sewer line.

Visit our plumbing services page to schedule an inspection, or our drain cleaning page if you suspect a clog is contributing to the problem. For sewer odor causes that go deeper than the trap, our sewer services team can run a camera inspection.