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National Home Inspector Exam (NHIE) · Home Inspection (All Domains)

National Home Inspector Exam (NHIE): Plumbing Systems Concepts

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Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.

Overview

This chapter covers the water that comes into the house and the water that leaves it. It includes supply piping, the drain-waste-vent system, water heaters, and fixtures. The inspector runs water, checks for leaks and poor drainage, and looks for missing safety parts.

Every fixture drain has a trap, a bend that stays filled with water so sewer gas cannot come up into the room. Under common model codes the water seal is typically 2 to 4 inches deep. The trap arm runs from the trap to the vent and slopes slightly downhill, typically 1/4 inch per foot for small pipes. The vent lets air into the system so draining water does not siphon the trap dry. S-traps are not allowed because they tend to siphon.

The water heater must have a temperature and pressure relief valve. Its discharge pipe should be full size, run downhill, have no valve or threads at the end, and typically end no more than 6 inches above the floor or at an approved location. Supply pipe material matters too: polybutylene and old galvanized steel are known for failures.

Key concepts

  • Trap and trap seal

    A trap is a curved section under a fixture that holds water. That water, called the trap seal, blocks sewer gas. Under common model codes the seal is typically at least 2 inches and no more than 4 inches deep.

    Memory tipSeal depth: 2 to 4 inches.

    Source: International Residential Code (ICC) · as of 2021

  • Trap weir

    The highest point inside the trap outlet, where water spills over into the trap arm. The standing water in the trap stays level with the weir, so the seal depth is measured from the weir down to the top of the dip.

    Memory tipWater can only stand as high as the weir.

  • Trap arm

    The pipe from the trap outlet to the vent connection. Under common model codes it typically slopes 1/4 inch per foot for pipes 2 1/2 inches and smaller, and its length is limited by pipe size.

    Memory tipToo long or too steep, and the trap can siphon dry.

    Source: International Residential Code (ICC) · as of 2021

  • Vent

    A pipe that lets air into the drain system and lets sewer gas escape above the roof. It keeps pressure even so water rushing down the drain does not suck the trap empty.

    Memory tipGurgling drains and slow draining often point to venting problems.

  • S-trap

    A trap whose outlet turns straight down instead of running level to a vent. It tends to siphon itself dry. Under common model codes S-traps are not allowed.

    Memory tipOutlet turns down = S-trap = report it.

    Source: International Residential Code (ICC) · as of 2021

  • TPR valve discharge

    Under common model codes, the water heater relief valve pipe is full size, drains by gravity, has no valve or threaded end, and typically ends no more than 6 inches above the floor or at another approved place.

    Memory tipFull size, downhill, no valve, no threads, 6 inches from the floor.

    Source: International Residential Code (ICC) · as of 2021

  • Supply pipe materials

    Common supply pipes are copper, PEX, and CPVC. Gray polybutylene and old galvanized steel are known for leaks and clogging and are often flagged for further review.

    Memory tipGray plastic supply pipe? Note possible polybutylene.

See it in a picture

A sink tailpiece drops into a U-shaped P-trap holding water. The water surface lines up with the trap weir, the high point inside the trap outlet. The trap arm runs from the trap to a vertical stack, sloping slightly down; the stack goes up as the vent and down as the drain.
Parts of a P-trap: the seal water stands level with the weir.

Often tested

  • The trap seal water stands level with the trap weir.
  • The trap arm slopes slightly downward toward the vent and stack.
  • S-traps and missing traps let sewer gas into the house.
  • A capped, valved, or upward-running TPR discharge pipe is a safety item.
  • Run water at fixtures to check flow, drainage, and leaks below.

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedHome Inspection (All Domains) › Plumbing Systems★★★★★

Question 1. A water heater's temperature and pressure relief (TPR) valve discharge pipe should typically end:

  1. ① Pointing up toward the ceiling
  2. ② Threaded and capped at the end to prevent drips
  3. ③ Reduced to a smaller pipe size
  4. ④ Within about 6 inches of the floor or at another approved location, with no threads at the end
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ Within about 6 inches of the floor or at another approved location, with no threads at the end

Key point: TPR discharge: full size, gravity, ends near floor, no threads

The TPR discharge pipe carries scalding water safely away if the valve opens. Under common codes, it is typically the same size as the valve outlet, drains by gravity, has no valves, ends within about 6 inches of the floor or at another approved place, and has no threads at the end (so it cannot be capped).

Wrong choices

  • Threaded and capped at the end to prevent drips: A capped line would block the valve from working.
  • Pointing up toward the ceiling: The pipe must drain by gravity; it should not run uphill.
  • Reduced to a smaller pipe size: The pipe must not be smaller than the valve outlet.
Practice questionReviewedHome Inspection (All Domains) › Plumbing Systems★★★★★

Question 2. What does a water heater's temperature and pressure relief (TPR) valve do?

  1. ① Controls the water temperature setting
  2. ② Opens to release water if the temperature or pressure gets too high
  3. ③ Reduces the incoming water pressure
  4. ④ Prevents backflow into the city water main
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ② Opens to release water if the temperature or pressure gets too high

Key point: TPR valve prevents water heater explosion

If a water heater's controls fail, the water can overheat and pressure can build until the tank bursts. The TPR valve opens at about 150 psi or 210 °F to release water and prevent this. Its discharge line must be installed so the water goes somewhere safe.

Wrong choices

  • Controls the water temperature setting: The thermostat controls water temperature.
  • Prevents backflow into the city water main: That is the job of backflow preventers or check valves.
  • Reduces the incoming water pressure: A pressure-reducing valve does that.
Practice questionReviewedHome Inspection (All Domains) › Plumbing Systems★★★★★

Question 3. Why is an S-trap typically considered a defect under modern plumbing codes?

  1. ① It can siphon its own water seal and let sewer gas in
  2. ② It slows down the water supply
  3. ③ It is the same as a P-trap, just older
  4. ④ It is too hard to clean
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ① It can siphon its own water seal and let sewer gas in

Key point: S-traps can self-siphon the trap seal

An S-trap turns down into the drain right after the trap, so draining water can create a siphon that pulls the seal out. Sewer gas can then enter the room. Modern codes call for a P-trap with a trap arm connected to a vented drain.

Wrong choices

  • It is the same as a P-trap, just older: An S-trap drops straight down without a proper vent connection, so it can siphon.
  • It is too hard to clean: Cleaning is not the main problem.
  • It slows down the water supply: Traps are on the drain side, not the supply side.
Practice 5 questions from this unit →