Home Inspector Plumbing DWV Systems Questions and Answers — Questions and Answers
Question 1: During a home inspection, you observe a kitchen sink drain that makes a 'gurgling' sound after water has been drained and a faint sewer gas odor is present. What is the MOST likely cause of these issues?
- A blocked or improperly installed vent stack. (Correct answer)
- The drain line has an excessive slope.
- The P-trap is installed backward.
- The supply line water pressure is too high.
Correct answer: A blocked or improperly installed vent stack.
The vent stack's primary functions are to allow sewer gases to escape to the exterior and to introduce air into the DWV system to prevent a vacuum. Gurgling sounds and sewer odors are classic signs that the vent is blocked or improperly installed, causing negative pressure that siphons water from the P-trap, breaking the water seal that normally blocks sewer gas.
Question 2: What is the generally accepted minimum slope for a horizontal 3-inch drainpipe in a residential DWV system according to most plumbing codes?
- 1/2 inch per foot
- 1/4 inch per foot
- 1/8 inch per foot (Correct answer)
- 1/16 inch per foot
Correct answer: 1/8 inch per foot
According to the International Plumbing Code, the minimum slope for a horizontal drainage pipe with a diameter of 3 to 6 inches is 1/8 inch per foot. This slope is crucial for ensuring that solid waste is carried along with liquids, preventing clogs. A slope of 1/4 inch per foot is required for pipes 2 1/2 inches or smaller.
Question 3: A home inspector identifies a sink drain with an 'S-trap' configuration. Which of the following statements BEST describes the issue with this type of trap?
- It is more prone to clogging than a P-trap.
- It is susceptible to self-siphoning, which can remove the water seal. (Correct answer)
- It does not allow for the connection of a mechanical vent.
- It is constructed from materials that are no longer approved for plumbing use.
Correct answer: It is susceptible to self-siphoning, which can remove the water seal.
S-traps are prohibited by modern plumbing codes because their design, which drains straight down, can create a siphon effect that pulls the water out of the trap. This removes the water seal that is necessary to block sewer gases from entering the home. P-traps, which connect to a horizontal drain line in the wall, are designed to work with a vent system to prevent this siphoning.
Question 4: What is the primary purpose of a P-trap in a plumbing drain waste and vent (DWV) system?
- To increase the velocity of the wastewater.
- To provide an easy access point for drain cleaning.
- To filter out large debris before it enters the main sewer line.
- To hold a small amount of water to block sewer gases. (Correct answer)
Correct answer: To hold a small amount of water to block sewer gases.
A P-trap is a U-shaped pipe designed to constantly hold a small amount of water. This water acts as a seal, effectively blocking unpleasant and potentially harmful sewer gases from backing up through the drain and into the living space.
Question 5: While inspecting a newly installed bathroom sink, you notice the trap arm, which is a 1 1/2-inch pipe, connects to the vent stack only 2 inches from the trap weir. This installation is a defect because:
- The trap arm must be at least 6 inches long.
- The vent connection is too close, creating a prohibited crown vent. (Correct answer)
- The trap arm must slope upwards towards the vent.
- A 1 1/2-inch pipe is too small for a bathroom sink.
Correct answer: The vent connection is too close, creating a prohibited crown vent.
Plumbing codes require a minimum distance between the trap weir and the vent to prevent siphoning. This minimum distance is two times the pipe's diameter. For a 1 1/2-inch pipe, the minimum trap arm length is 3 inches (1.5 x 2). An installation with only 2 inches creates a 'crown vent,' which is prohibited because it can easily become clogged and lead to the trap siphoning dry.
Question 6: Which of the following would NOT be considered a component of a residential Drain-Waste-Vent (DWV) system?
- Vent stack terminating through the roof.
- P-trap under a kitchen sink.
- Horizontal cast iron soil pipe in the crawlspace.
- Pressure relief valve on a water heater. (Correct answer)
Correct answer: Pressure relief valve on a water heater.
A DWV system is comprised of all the piping that removes sewage and greywater from a home. This includes drain lines, waste stacks, P-traps, and vent stacks. A pressure relief valve is a safety device for a pressurized vessel, like a water heater, and is part of the potable water supply system, not the gravity-fed DWV system.
Home Inspector Plumbing DWV Systems Questions and Answers
During a home inspection, you observe a kitchen sink drain that makes a 'gurgling' sound after water has been drained and a faint sewer gas odor is present.
What is the MOST likely cause of these issues?