A trap seal is the small plug of standing water held in the curved bend of a plumbing trap, such as a P-trap under a sink. That water blocks sewer gas from rising through the drain into the room. Plumbing code requires the seal to be between 2 and 4 inches deep.
A trap seal is the plug of water that sits in the bend of every plumbing trap. Look under a sink and you will see a U-shaped or P-shaped piece of pipe, the P-trap. That curve is not just a shape. It holds a small amount of standing water at all times, and that water is the trap seal. It is the single thing keeping sewer gas out of the room.
The drain pipes below a fixture connect to the building's sewer or septic system, which is full of foul, sometimes dangerous gas. Without a barrier, that gas would drift straight up the drain and into the house. The trap seal blocks it. Water fills the low point of the bend and acts like a cork. Air and gas from the sewer side cannot push past the water, so they stay in the pipe where they belong. Every time the fixture is used, fresh water flows through and refreshes the seal.
Plumbing code sets an exact depth for that plug of water. The seal has to be at least 2 inches deep and no more than 4 inches deep. That range is not arbitrary. Too shallow, and normal drainage or evaporation can empty it too easily, letting gas through. Too deep, and the trap tends to clog and drains sluggishly. The 2-to-4-inch window is the tested sweet spot that keeps the seal reliable without choking the drain. For example, a bathroom sink used every day almost never loses its seal, because each use tops it back up.
A trap seal fails when that water disappears, and there are a few common ways it happens. A fixture that goes unused for weeks, such as a guest-bathroom shower or a floor drain, can lose its seal to plain evaporation. A venting problem can siphon the water out through suction when a nearby fixture drains. And a partial clog can slowly pull the level down. However it happens, the symptom is the same: a faint but persistent sewer-gas smell near a drain that has no visible leak. The fix is usually as simple as running water to refill the trap, unless a venting or clog issue keeps emptying it. For a floor drain that dries out on its own, a trap primer keeps the seal full automatically.