10.2 NC Residential Code DWV, Venting, Traps & Fuel Gas Provisions for Dwellings

10.2 NC Residential Code DWV, Venting, Traps & Fuel Gas Provisions for Dwellings

Quick Answer: In a dwelling, the building drain carrying water closets is a minimum of 3 inches, and no more than three water closets may discharge to a 3-inch horizontal branch. Residential wet venting allows a bathroom group to be vented through the dry vent of the most downstream fixture, with the wet vent sized at 2 inches where it serves more than one fixture drain. Air admittance valves are permitted in dwellings, but at least one vent must extend to the outdoors, and every AAV must be accessible and installed above the trap arm at the required height. Vents terminate at least 6 inches above the roof and 6 inches above the anticipated snow depth, and 10 feet from, or 3 feet above, any door, window, or air intake. Residential fuel gas is in NCRC Chapter 24: an appliance shutoff within 6 feet, a sediment trap at appliances other than illuminating appliances, ranges, and clothes dryers, and CSST bonded to the electrical service.

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Where the Residential Code Diverges from the Plumbing Code

The two books agree on physics and on most numbers. They diverge in three predictable places, and those are exactly where exam questions live.

graph TD
    A["Same in both books:<br/>trap seal 2-4 in, slope table,<br/>test pressures, prohibited fittings,<br/>backflow principles"] --> B["Diverge:<br/>1. DWV sizing simplifications<br/>2. Venting allowances (wet vent, AAV)<br/>3. Cleanout requirements"]
    B --> C["Answer from the book the<br/>occupancy in the stem points to"]

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Residential Drainage Sizing

• Building drain and building sewer. A dwelling drain that receives one or more water closets is a minimum of 3 inches. A 4-inch drain is common and is required once the fixture unit load exceeds what a 3-inch drain carries at the installed slope.
• The three-water-closet rule. No more than three water closets may be connected to a 3-inch horizontal branch or a 3-inch building drain in a dwelling. The fourth water closet forces a 4-inch line. This is one of the most frequently asked residential drainage questions.
• Fixture drain minimums. Water closet 3 inches; bathtub, shower, and lavatory 1-1/2 inches; kitchen sink 1-1/2 inches (2 inches where a food waste disposer and dishwasher share it in some designs); clothes washer standpipe 2 inches; floor drain 2 inches.
• Standpipe rules. A clothes washer standpipe is a minimum of 18 inches and a maximum of 42 inches above the trap weir, with the trap set between 6 and 18 inches above the floor.
• Slope. Identical to the Plumbing Code: 1/4 inch per foot for 2-1/2 inches and smaller, 1/8 inch per foot for 3 through 6 inches.
• Fittings. The horizontal-to-horizontal prohibition on sanitary tees applies without change; use a wye and eighth bend or a long-turn fitting. A double sanitary tee (cross) may not be used where the barrel is horizontal, and back-to-back water closets on a double sanitary tee require the fitting and spacing the code specifically allows.
• Cleanouts. A dwelling requires a cleanout near the junction of the building drain and building sewer, at each change of direction exceeding 45 degrees in the building drain where required, and at intervals not exceeding 100 feet. Cleanouts must be accessible, and a two-way cleanout at the property line is a common local requirement.

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Residential Venting

Individual, Common & Wet Vents

• Individual vent: the default — each trap vented by its own dry vent.
• Common vent: two fixtures on the same floor level connecting at the same point may share a vent, with the vertical drain sized for the combined load.
• Wet vent — the residential workhorse. A bathroom group (water closet, lavatory, bathtub or shower, and in some layouts a bidet or floor drain) may be wet vented from the dry vent connection of the most downstream fixture drain in the group. Rules to memorise:
  • Only fixtures within the bathroom group may be wet vented.
  • The dry vent connects to the wet vent at the fixture drain of the most downstream fixture, and that dry vent is sized from the total load it carries.
  • The wet vent is a minimum of 2 inches where it serves more than one fixture drain, and 1-1/2 inches where it serves a single fixture drain within the limits the code sets.
  • The water closet drain connects downstream of the wet-vented fixtures.
  • No more than two bathroom groups may be served by a single wet vent under the residential provisions.
• Vent sizing generally. A vent is not smaller than half the diameter of the drain it serves and never less than 1-1/4 inches. The vent stack or stack vent is sized from the total drainage load and the developed length.

Air Admittance Valves in Dwellings

AAVs solve island sinks and remodels where a dry vent cannot be run, and the residential code permits them — with conditions:

• At least one vent must extend to the outdoors. An entire dwelling cannot be vented on AAVs alone; the system needs a path to relieve positive pressure, and an AAV only admits air.
• Height. An individual or branch-type AAV is installed at least 4 inches above the horizontal branch drain or fixture drain it serves; a stack-type AAV is installed at least 6 inches above the flood level rim of the highest fixture served.
• Accessibility and ventilation. The valve must be accessible for replacement and located in a space with air circulation — not sealed inside a wall.
• Prohibited locations. Not in a space where it could be submerged, not where it would be subject to freezing, and not for venting a sump vent or a system requiring a relief vent for positive pressure.
• Listing. Must comply with ASSE 1051.

Vent Terminations

• Above the roof: vents through a roof terminate not less than 6 inches above the roof and not less than 6 inches above the anticipated snow depth.
• Near openings: a vent terminal must be at least 10 feet from, or at least 3 feet above, any door, openable window, or air intake of the building or an adjacent building.
• Roof used for a purpose other than weather protection: the terminal extends not less than 7 feet above the roof.
• Frost closure: in cold regions the code requires an increased terminal size; North Carolina's climate rarely triggers it, but mountain jurisdictions may.
• Sidewall terminations are permitted only where the code and the local jurisdiction specifically allow them, with the same clearance rules to openings.

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Traps in Dwellings

The trap rules are essentially identical to Chapter 6 of this guide, and the residential book restates them:

• Trap seal 2 inches minimum, 4 inches maximum, except for deep-seal traps approved by the code official.
• One trap per fixture, with the multi-compartment sink exception where the compartments are adjacent, the outlets are within 30 inches centre to centre, and the depth difference does not exceed 6 inches.
• Trap arm developed length is governed by the same table used in Section 6.1 and by the one-pipe-diameter fall rule.
• Prohibited traps: full S-traps, bell traps, crown-vented traps, drum traps, and traps with internal moving or partition parts.
• Trap primers or trap seal protection devices are required for floor drains and emergency drains subject to evaporation.

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Residential Fuel Gas (NCRC Chapter 24)

Chapter 7 of this guide covers the NC Fuel Gas Code in depth. The residential chapter mirrors it, and these provisions are the ones asked about most in a dwelling context:

Exam Trap — Where a Drip Leg Is Not Required. The sediment trap is required at most appliances, but the code lists exceptions, and the ones candidates forget are ranges, clothes dryers, and illuminating appliances. A question that puts a drip leg on a gas range is testing whether you know the exception list, not whether you know what a drip leg does.