A bathroom sink that hasn't been used for two months develops a faint sewer smell. No other fixtures in the house are affected. What is the most likely cause?
AA crack in the drain pipe beneath the sink
BA blocked vent stack preventing air from entering the drain
CThe P-trap's standing water has evaporated, leaving an open path for sewer gas
DThe main shutoff valve is partially closed, reducing pressure to that fixture
A P-trap holds a small amount of standing water that physically blocks sewer gas from entering the home. If a fixture is unused for weeks, this water evaporates — removing the seal. The fix is simply running the tap for 30 seconds to refill the trap. Because only one unused fixture is affected, this rules out a main-line or vent issue. A cracked pipe or blocked vent would typically affect more than one fixture or manifest differently.
Question 2 Multiple Choice
A kitchen drain that is used daily begins making a gurgling sound when water drains. The drain is not overflowing and has no obvious blockage near the top. What does the gurgling most likely indicate?
AThe P-trap water has evaporated, allowing air through
BWater pressure in the supply lines is too high
CA venting problem — insufficient air is entering the drain system, creating suction through the trap
DA partial clog directly in the P-trap requiring snaking
Gurgling is the diagnostic sound of air being pulled backward through the P-trap because the vent system isn't admitting enough air above the draining water. Without venting, draining water creates a partial vacuum that sucks air through the trap seal — producing the gurgle and eventually pulling the trap dry. A simple clog would slow drainage but not necessarily gurgle; an evaporated trap causes sewer smell, not gurgling. Supply pressure has no effect on drain behavior.
Question 3 True / False
The residential water supply system operates under continuous pressure (typically 40–80 PSI), which is what allows water to flow upstairs and to multiple fixtures simultaneously.
TTrue
FFalse
Answer: True
Unlike the gravity-dependent DWV system, the supply system is pressurized — either by the municipal main or a well pump and pressure tank. This pressure is what pushes water upward to second-floor bathrooms, maintains flow when multiple fixtures run at once, and delivers spray force from showerheads. The main shutoff valve controls all of this pressure for the whole house.
Question 4 True / False
Drain pipes in a residential home carry wastewater under pressure, similar to how supply pipes carry fresh water.
TTrue
FFalse
Answer: False
The drain-waste-vent (DWV) system operates entirely by gravity, not pressure. This is why all drain pipes must maintain a continuous downward slope (typically ⅛ to ¼ inch per foot) toward the sewer or septic system. DWV pipes are also vented through the roof to admit air that prevents siphoning and allows free flow. Confusing the two systems leads to misdiagnosis: supply problems involve pressure; drain problems involve slope, traps, and ventilation.
Question 5 Short Answer
What is the purpose of a P-trap, and what condition allows it to fail at that function?
Think about your answer, then reveal below.
Model answer: A P-trap is the curved pipe section under every sink that retains a small amount of standing water. This water seal physically blocks sewer gas — including methane and hydrogen sulfide — from traveling up through the drain into the living space. It fails when the trap runs dry: if a fixture goes unused long enough, the trapped water evaporates, removing the seal. Refilling takes only 30 seconds of running water.
The P-trap's water seal is simple but essential — it is the only barrier between the living space and the sewer system at each fixture. The dry-trap failure mode explains why rarely-used fixtures (guest bathrooms, floor drains) can develop sewer smell without any damage or blockage in the pipes themselves.