2021 IPC Chapters 8 and 10
Some drains must never connect straight to the sewer. A walk-in cooler, a steam sterilizer or a commercial dishwasher can pull sewer air, or worse, dirty water back toward food or clean equipment if the drain is solidly connected. Chapter 8 lists what must drain indirectly, through a visible gap, into a trapped receptor. Chapter 10 covers the traps themselves and the interceptors that keep grease, oil and sand out of the public sewer. Houston enforces the Uniform Plumbing Code (Lesson 1), so treat every number here as a 2021 IPC number and check the adopted code before you correct a permit.
Section 802.1 sends food-handling equipment (outside dwelling units), clear-water waste, humidifiers, dishwashing machines, pots, pans and dishwashing sinks to an indirect waste pipe. Food storage, preparation and handling equipment needs an air gap (802.1.1), and each well of a multiple-compartment sink drains independently to a receptor. Floor drains inside walk-in coolers and freezers drain by air gap too (802.1.2), though an air break is allowed where a backwater valve protects the drain; if the space can freeze, the waste line is not trapped and discharges to a receptor outside the frozen area. Potable clear water from sterilizers and relief valves needs an air gap (802.1.3), and so does pool water, filter backwash and pool deck drains (802.1.4). Nonpotable clear water from process tanks, filters, drips and boilers may use an air break or an air gap (802.1.5). A commercial dishwasher and the sinks used to wash, rinse or sanitize dishes, pots and utensils outside dwellings may also use either (802.1.6 and 802.1.7).
An air gap is open air between the end of the waste pipe and the flood level rim of the receptor. It must be at least twice the effective opening of the pipe (802.3.1). For a round waste pipe, think of the effective opening as its diameter: a 2 inch pipe needs at least 4 inches of clear air. An air break is different: the pipe ends below the flood level rim but above the trap seal of the receptor (802.3.2), so the drain does not touch the water. An air break is easier to build, but it gives no protection if the receptor floods, which is why food equipment must use the gap.
The receptor is the part connected to the drainage system, and it must be trapped and vented. Indirect waste piping that runs more than 30 inches horizontally, or 54 inches in total developed length, must itself be trapped (802.3). A receptor that receives only clear water and does not connect directly to a sanitary drain needs no trap (802.3 exception).
Floor sinks, standpipes, hub drains and floor drains can serve as receptors. Most need a removable strainer or basket over the outlet. A receptor may not sit in a concealed space, plenum, crawl space, attic, or the space above a ceiling or below a floor, and it needs ready access. It is sized for the maximum discharge of every pipe that drains to it, and installed so it does not splash or flood (802.4.1). A hub drain is a hub or pipe rising at least 1 inch above a water-impervious floor. A standpipe is individually trapped and extends 18 to 42 inches above the trap weir, with access for rodding (802.4.3). A laundry tray can drain to an automatic washer standpipe if that standpipe rises at least 30 inches above its weir and above the tray rim, and the tray outlet is within 30 inches, measured horizontally, of the standpipe (802.4.3.1).
Corrosive liquids, spent acids and other harmful chemicals may not enter the plumbing system until they are diluted, neutralized or treated in an approved device that automatically supplies its own dilution water or neutralizing medium, and the method must be approved before installation (803.1). A chemical drainage and vent system is kept completely separate from the sanitary system, and its vent goes to the roof separately from the sanitary vent (803.2, 901.3). Think school science labs, film or lab processing rooms, and battery rooms.
Every fixture is separately trapped and never double trapped. The fixture outlet may be no more than 24 inches above the trap weir and no more than 30 inches horizontally from the trap inlet (1002.1). A combination fixture, such as a two-compartment sink, can share one trap if the compartments differ in depth by 6 inches or less and the outlets are within 30 inches of each other. Traps must be self-scouring and level. Prohibited types (1002.3): traps that depend on moving parts, bell traps, crown-vented traps, most partition-type traps, S traps and drum traps (a drum trap used as a solids interceptor is allowed). Building or house traps are prohibited (1002.6). The trap seal is at least 2 inches and no more than 4 inches (1002.4), and a trap is never larger than the drain it discharges into (1002.5).
Emergency floor drains and any trap whose seal can evaporate, such as a seldom-used mechanical room floor drain, need seal protection. The listed methods are a potable water-supplied primer valve (ASSE 1018), a graywater or reclaimed-water primer, a wastewater-supplied primer device (ASSE 1044), a barrier-type device (ASSE 1072), or a fixture drain connection. A lavatory or hand sink drain can prime a floor drain, trench drain or floor sink in the same room, and a drinking fountain drain can do the same in the same or an adjacent room. The priming drain may not run on or above the floor and must enter below the flood level rim and above the trap inlet (1002.4.1.5). A primer valve line enters the trap above the seal on the inlet side (1002.4.1.1 to 1002.4.1.3).
A grease interceptor or automatic grease removal device is required for grease-laden waste in restaurants, hotel and school kitchens, hospitals, bars and similar food preparation areas, from pot sinks, prerinse sinks, soup kettles, wok stations, floor drains that receive kettle drainage, hood wash units and dishwashers without a prerinse sink (1003.3.1). A dwelling unit does not need one (1003.3.4), and a food waste disposer may not discharge to one (1003.3.2). Emulsifiers, chemicals and enzymes are prohibited additives (1003.3.3). The interceptor is vented so it cannot become air bound (1003.9), needs access for service (1003.10), and its discharge connects directly to the sanitary drain (1003.3.8).
Hydromechanical units are rated by flow-through in gpm, and Table 1003.3.5.1 gives the grease they must hold (selected rows are shown below). The pattern is 2 pounds of retention for each gpm, and above 100 gpm you double the gpm for the pounds. The unit needs a vented flow-control device so the flow cannot exceed the rating (1003.3.5.2). Gravity interceptors hold at least 500 gallons and a retention time of at least 30 minutes, so the required volume is the peak drain flow in gpm times 30 (500 gallons and 30 minutes come from the Chapter 2 definition; the calculation is in 1003.3.7).
| Flow-through rating (gpm) | 4 | 10 | 20 | 25 | 50 | 100 |
|---|---|---|---|---|---|---|
| Grease retention (pounds) | 8 | 20 | 40 | 50 | 100 | 200 |
Worked example. A new church fellowship hall kitchen has a three-compartment pot sink, a prerinse sink and a floor drain that takes kettle drainage. The designer calculates a peak flow of 22 gpm using the manufacturer's method. No table row is 22, so choose a unit rated at the next listed value, 25 gpm, which must hold 50 pounds (25 x 2). If the owner instead picks a gravity interceptor for a peak of 40 gpm, the volume is 40 x 30 = 1,200 gallons, well above the 500 gallon floor.
Oil separators are required at repair garages with floor or trench drains, car washes, factories with oily or flammable wastes, and hydraulic elevator pits (an elevator pit may skip it with an approved alarm that does not stop the pumps firefighters rely on). A separator is at least 2 feet deep below the invert of its discharge drain and its outlet has at least an 18 inch water seal. Garage capacity is 6 cubic feet for the first 100 square feet drained plus 1 cubic foot for each additional 100 square feet (1003.4.2). A parking garage where no servicing, repair, washing or fueling happens needs none. Sand and similar interceptors need ready access and at least a 6 inch water seal (1003.5). Commercial clothes washers drain through an interceptor with a removable basket that stops solids 1/2 inch and larger; dwelling-unit washers do not need one (1003.6).
Worked example. A repair bay with floor drains covers 500 square feet. The first 100 square feet need 6 cubic feet, and the other 400 add 4 cubic feet, so the separator holds at least 10 cubic feet.
In the field
A rehab crew converts a former storefront into a small community cafe with a hand sink, a three-compartment sink, a mop sink and a floor drain by the ice machine. Ask early where each one drains. The ice machine drain needs an air gap to a floor sink, the three-compartment sink drains through an indirect pipe (each well independently), and the pot sink and floor drain, if they take kettle water, go through a grease interceptor. Find out where the interceptor will fit and how it will be reached for pumping. One buried under finished counters or tile is a common rework item.
1. A 1 1/2 inch indirect waste pipe from a walk-in cooler in a church kitchen ends above a floor sink. Taking the effective opening as the pipe diameter, what is the smallest air gap to the flood level rim under Section 802.3.1?
2. A produce prep sink in a school kitchen (Group E) is used to store and prepare food. Under Section 802.1.1, how must it discharge to the drainage system?
3. A commercial dishwashing machine in a hospital kitchen (Group I-2) is being connected to drainage. Under Section 802.1.6, its discharge goes to a waste receptor through:
4. A steam sterilizer in a dental clinic (Group B) discharges potable clear water to the drain. An inspector checks the connection against Section 802.1.3. Which arrangement passes?
5. An indirect waste pipe from a coffee station in a Group B break room runs 20 inches horizontally and has a total developed length of 60 inches. Under Section 802.3, the pipe:
6. A plans reviewer sees a floor sink for ice machine drains located in the ceiling space above a Group B break room, reached by a lift-out ceiling tile. Under Section 802.4, the receptor:
7. A standpipe for an automatic clothes washer is being installed in a community center laundry room. Under Section 802.4.3, the standpipe extends above the trap weir by:
8. An inspector measures the water seal of a lavatory trap in a Group B restroom at 1 1/2 inches. Under Section 1002.4, the trap:
9. At a rough-in inspection of a small clinic (Group B), which trap type is prohibited under Section 1002.3?
10. Under Section 1002.1, what is the greatest vertical distance allowed from a fixture outlet to the trap weir?
11. A seldom-used floor drain in the mechanical room of a Group B building keeps losing its seal to evaporation. Which fix is NOT a listed trap seal protection method in Section 1002.4.1?
12. A restaurant kitchen (Group A-2) will discharge 20 gpm to a hydromechanical grease interceptor rated at 20 gpm. What grease retention capacity does Table 1003.3.5.1 require?
13. A school kitchen (Group E) has a peak drain flow of 35 gpm into a gravity grease interceptor. What is the required capacity under Section 1003.3.7?
14. A repair garage (Group S-1) has floor drains serving 500 square feet that must pass through an oil separator. What is the minimum separator capacity under Section 1003.4.2.2?