IPC Traps, Interceptors, and Separators 2 — Questions and Answers
Question 1: According to the IPC, what is the required water seal depth for a standard fixture trap?
- 1 inch
- 2 inches (Correct answer)
- 3 inches
- 4 inches
Correct answer: 2 inches
The IPC requires a minimum water seal depth of 2 inches and a maximum of 4 inches for standard fixture traps.
The IPC specifies that standard fixture traps must maintain a water seal of not less than 2 inches and not more than 4 inches. The minimum 2-inch seal provides adequate protection against sewer gas passage into the occupied space under normal operating conditions. The maximum 4-inch limit prevents excessive seal depth, which could impede drainage flow and create self-siphonage conditions. The 2-to-4-inch range balances gas protection with drainage performance. Some special-use traps, such as those for floor drains in specific occupancies, may have different requirements. Maintaining the trap seal is one of the most fundamental safety functions of a plumbing system.
Question 2: What does the IPC specify about the use of double trapping on a plumbing fixture?
- Double trapping is encouraged for extra protection
- Double trapping is prohibited (Correct answer)
- Double trapping is only prohibited for water closets
- Double trapping is required for commercial fixtures
Correct answer: Double trapping is prohibited
The IPC prohibits double trapping — installing two traps in series on the same fixture drain — because it creates a trapped air space that impedes drainage.
The IPC explicitly prohibits double trapping, which occurs when two traps are installed in series on the same fixture drain line. While it might seem that two traps would provide better sewer gas protection, the air space trapped between the two water seals creates significant drainage problems. This trapped air cannot be vented and acts as a cushion that resists drainage flow, causing sluggish drainage, gurgling, and potential trap seal loss through siphonage of one or both traps. The prohibition applies to all fixtures — including situations where a fixture with an integral trap (like a water closet) is connected to an external P-trap.
Question 3: Under the IPC, what type of trap is specifically prohibited for use in drainage systems?
- P-trap
- Drum trap larger than 4 inches
- S-trap (Correct answer)
- Running trap with cleanout
Correct answer: S-trap
The IPC prohibits S-traps because their configuration makes them highly susceptible to self-siphonage, which can empty the trap seal.
The IPC prohibits S-traps in drainage systems because their design inherently creates self-siphonage conditions. An S-trap drops vertically from the fixture into a trap, then drops vertically again into the drainage piping. When water flows through this configuration, the column of water in the second vertical leg creates a siphon that can pull the water seal out of the trap, leaving it empty and allowing sewer gases to enter the building. P-traps, which exit horizontally after the trap weir, do not create this siphonage condition and are the standard approved trap configuration. S-traps are one of the most commonly identified plumbing deficiencies in older buildings.
Question 4: According to the IPC, what is the minimum size grease interceptor required for a commercial kitchen with three compartment sinks?
- The size is determined by flow rate calculation, not a fixed size (Correct answer)
- 20 gallons
- 50 gallons
- 100 gallons
Correct answer: The size is determined by flow rate calculation, not a fixed size
The IPC requires grease interceptor sizing based on flow rate calculations considering the number and type of fixtures connected, not a single fixed size.
The IPC requires grease interceptors to be sized based on the flow rate from the connected fixtures, using either the DFU method or the manufacturer's rated capacity. The sizing calculation considers the number and type of fixtures connected (sinks, dishwashers, floor drains), their individual flow rates, and the interceptor's retention time needed for effective grease separation. A three-compartment sink alone might require a certain capacity, but the total interceptor size must account for all connected fixtures. The IPC references sizing standards such as PDI-G101 (hydromechanical grease interceptors) or provides criteria for gravity grease interceptors, which are larger underground units with longer retention times.
Question 5: What does the IPC require regarding maintenance access to interceptors and separators?
- Access is only needed for initial installation
- They must have readily accessible covers for cleaning and maintenance (Correct answer)
- Only annual access is required
- Access hatches are optional if the unit is self-cleaning
Correct answer: They must have readily accessible covers for cleaning and maintenance
The IPC requires that all interceptors and separators have readily accessible covers or access points for regular cleaning and maintenance.
The IPC mandates that interceptors and separators be installed with readily accessible covers or access openings for cleaning, inspection, and maintenance. Grease interceptors, oil separators, sand interceptors, and other such devices require regular servicing to function effectively. Without proper access, these devices cannot be cleaned and will eventually fail, allowing prohibited substances to enter the drainage system. The code specifies that access must not require the removal of permanent building components. For underground interceptors, the access cover must be at grade level and properly sized for equipment access. Maintenance records may be required by the local jurisdiction.
Question 6: Under the IPC, what is the purpose of a trap primer?
- To prevent freezing of trap water
- To maintain the water seal in traps subject to evaporation (Correct answer)
- To increase the trap seal depth beyond 4 inches
- To prevent backflow through the trap
Correct answer: To maintain the water seal in traps subject to evaporation
A trap primer maintains the water seal in floor drain traps and other infrequently used traps by periodically adding water to replace water lost to evaporation.
The IPC requires trap primers for floor drains and other traps that are subject to losing their water seal through evaporation. When a trap is connected to a fixture that is rarely used — such as a floor drain in a mechanical room — the water in the trap seal gradually evaporates. Once the seal is lost, sewer gases can enter the occupied space, creating health hazards and odor problems. Trap primers solve this by periodically adding a small amount of water to the trap to maintain the seal. The IPC accepts several types of trap primers including distribution-type primers connected to frequently used supply lines, electronic primers with timer controls, and trap seal primer valves. The primer must be accessible for maintenance and connected to a potable water supply.
IPC Traps, Interceptors, and Separators 2
According to the IPC, what is the required water seal depth for a standard fixture trap?