National Home Inspector Exam (NHIE) Plumbing Systems Summary

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.

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.

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.

The P-trap is the U-shaped bend under the sink. After water drains, some stays in the bottom of the U. This standing water is the trap seal. It blocks sewer gases from coming up through the drain into the room. In the drawing, the shaded water sits in the U-bend marked B.

Wrong choices

• C: C is the trap arm, the sideways pipe between the trap and the vent connection. It holds no standing water.
• A: A is the tailpiece, the straight pipe from the sink to the trap.
• D: D is the vent pipe going up. It lets air into the system.

The drain system connects to the sewer or septic system, which is full of gases. The water seal in each fixture trap blocks these gases from entering the living space. Traps that dry out, such as in unused floor drains, can let odors in.

Wrong choices

• To keep the drain from freezing: Traps are not freeze protection; the water in them can freeze.
• To measure water use: Water use is measured by the meter.
• To make water drain faster: The trap does not speed up drainage.

Polybutylene (PB) supply pipe was widely installed from about the late 1970s to the mid-1990s. It is often gray. It and its fittings have a history of leaks and failures. Inspectors report it and recommend evaluation by a qualified plumber.

Wrong choices

• PEX, a current approved material: PEX is commonly red, blue, or white and is widely used today.
• CPVC: CPVC is usually cream or tan and rigid.
• Copper: Copper is metal, not gray plastic.

Galvanized steel pipe has a zinc coating that wears away over decades. The steel then rusts from the inside, narrowing the pipe and reducing flow, and can eventually leak. Low flow at fixtures in an older home is a common clue.

Wrong choices

• They become too flexible: Steel pipe is rigid.
• They melt in hot water: Steel does not melt in hot water.
• They attract termites: Termites do not eat steel.

Hot water can burn skin quickly. At about 120 °F, it takes several minutes to cause a serious burn, while at 140 °F and above it can happen in seconds. About 120 °F is a commonly recommended setting, especially for homes with children or older adults.

Wrong choices

• 160 °F: This temperature can scald skin in about a second.
• 180 °F: This is very dangerous for household use.
• 90 °F: This is too cool for normal hot water needs.

A garden hose may lie in a puddle or be used to spray chemicals. If water pressure drops, that dirty water could be sucked back into the house pipes. A vacuum breaker at the faucet lets air in and blocks this backflow.

Wrong choices

• To keep the faucet from freezing: Frost-free faucet design protects against freezing, not the vacuum breaker.
• To increase hose pressure: A vacuum breaker does not raise pressure.
• To measure water use: The meter does that.

When water rushes down a drain, it can pull a vacuum that sucks the water out of the trap. The vent pipe lets air into the system to stop this. In the drawing, the vent is the part of the wall pipe that goes up from the connection, marked D. The part going down, E, is the drain.

Wrong choices

• E: E is the drain line going down, which carries waste away.
• C: C is the trap arm, which carries water from the trap to the wall.
• B: B is the trap, which holds the water seal.