Plumbing Systems
Supply, drain, and vent
Plumbing splits into potable supply (pressurized, ideally 40-80 psi and regulated above that) and drain-waste-vent, which relies on gravity and slope, roughly 1/4 inch per foot for pipe 2 inches and smaller and 1/8 inch per foot for larger lines. Every fixture needs a trap, and the vent protects the trap seal. Gurgling drains or sewer odor often indicate a vent or trap-seal problem, not a clog. Know the pipe families: copper (soldered or press), PEX (crimp, clamp, or push fittings), CPVC (solvent-cemented, hot water rated), PVC and ABS (drain use only, with different cements that are never swapped), and aging galvanized steel that corrodes from the inside.
Fixtures and water heaters
Toilet anatomy is heavily tested: continuous running is usually a failed flapper; a weak flush is a clogged rim jet or low tank level; a rocking toilet or leak at the base means a failed wax ring. Set the tank water line about 1 inch below the top of the overflow tube. Faucet drips come from a cartridge, stem washer, or O-ring depending on valve type; always close the angle stops first and verify. Water heaters: set near 120 degrees F, test the T&P valve, run the discharge pipe to within 6 inches of the floor, flush sediment annually, and check the anode rod.
Traps
Never plunge or snake a drain full of chemical cleaner, do not over-tighten plastic fittings, and know when a licensed plumber is required (gas piping, main sewer, slab leaks). Freeze protection and backflow prevention are common question topics.
Worked example
A toilet runs constantly: lift the tank lid and check the water level. If water is at the overflow, adjust the fill valve. If water trickles into the bowl, replace the flapper and clean the seat. Fast, cheap, and correct most of the time.