First, make sure it isn’t gas
If you’re not sure the smell is sewer and not natural gas, especially near the gas meter, water heater or furnace, treat it as gas. PGW says to leave the area immediately and call its 24-hour line at (215) 235-1212 from a safe place. Outside PGW’s territory, call your own gas utility. Sewer gas isn’t harmless either. EPA notes that hydrogen sulfide, one of its components, can irritate the eyes, nose and throat and make breathing difficult at high enough levels. If the smell is strong or someone feels sick, get out and call for help from outside. If sewage is backing up, stay out of it.
Refill the trap and watch what happens
If the odor is mild and there’s no backup, slowly pour plain water through the floor drain grate. Stop if it pools instead of draining. A typical trap holds a water seal about 2 inches deep, according to the CDC’s Healthy Housing Reference Manual. Skip drain cleaners and fragrances; they hide the clue you’re looking for.
Then write down the date and what happened. A smell that goes away and returns weeks later points strongly to evaporation. EPA says the drains that dry out are rarely sinks or toilets, which get used every day. It’s usually a floor drain in an out-of-the-way spot, like a mechanical room. A basement utility area next to a furnace or air handler fits that pattern, and EPA warns that heating and cooling equipment can spread the odor through the rest of the building.
If the smell doesn’t go away
A full trap can’t block gas escaping from a different opening. Clues worth telling a plumber:
The smell lingers even right after you refill the trap. In a NIOSH investigation of an older building with sewer odors, investigators found dry drain traps but also original cast-iron waste and vent piping, which can become brittle and crack with age. After the visit, the owner added trap devices to the floor drains and repaired cracks in the sewer lines, and odor complaints dropped.
Toilets or sinks gurgle when something else drains. The CDC manual explains that venting protects trap seals, and that pressure changes in a poorly vented system can siphon or push water out of a trap. Our guide to why a toilet gurgles when another fixture drains covers that problem.
Water drains slowly or backs up. The CDC manual notes that solids building up in drain lines will block them. That’s a blockage, not a dry trap, and refilling won’t fix it.
What’s yours to fix in Philadelphia
The Philadelphia Water Department’s customer guide puts the sewer lateral that carries wastewater from your house to the street on the property owner, along with the main house drain, curb trap, vent pipe and vent cover. If you think the lateral has a problem, the guide says to call PWD at (215) 685-6300 to schedule an inspection. If PWD finds a defect, it can issue a Notice of Defect that gives up to 10 days for repairs, and a licensed plumber must make them. Our guide to which plumbing jobs need an L&I permit covers the permit side.
If your floor drain backs up during heavy rain, that’s different from a smell. PWD asks you to call the same hotline while the water is still there, and its Basement Backup Protection Program installs backwater valves for homes that qualify. In Bucks County, find out first who runs your sewer; our guide on finding your Bucks County water or sewer provider shows how.
Keeping the trap from drying out again
For a drain that dries out because nobody uses it, EPA’s multifamily guidelines recommend a simple maintenance plan: add water on a schedule. Set a phone reminder and adjust the interval to how fast your smell returns. If that’s impractical, EPA points to two fixes a plumber can install: an automatic trap primer, which feeds a little water into the trap, or an inline drain-trap seal. Ask which one suits your drain and how you’ll check that it’s working.
Before you approve any repair, ask the plumber what they found, how they confirmed it, and whether the fix addresses the source. A trap seal device won’t stop gas leaking from a cracked pipe.