3.3 Water Closets, Urinals & Flushing Devices

Water Closets, Urinals & Flushing Devices

Water closets and urinals represent the primary sanitary soil fixtures in both commercial and residential construction. Because they discharge human metabolic waste and pathogens into the drainage network, their hydraulic design, structural mounting, water conservation standards, and backflow protection systems are rigorously regulated by MPC Chapter 4. A licensed Michigan journeyman plumber must understand the structural mechanics of carriers, flange connection rules, compartment spacing clearances, and flushometer pressure dynamics to pass the state examination and install durable, code-compliant systems.

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1. Water Efficiency Ratings & Conservation Mandates

Water conservation standards in the Michigan Plumbing Code stem from the federal Energy Policy Act (EPAct) and ASME A112.19.2 / CSA B45.1 standards for vitreous china plumbing fixtures. Maximum water consumption ratings include:

• Standard Gravity & Pressure-Assist Toilets: Maximum 1.6 gallons per flush (GPF) (6.0 liters per flush).
• High-Efficiency Toilets (HET): Maximum 1.28 GPF (4.8 liters per flush), representing a 20% water reduction over baseline EPAct standards (standard WaterSense rating).
• Dual-Flush Water Closets: Engineered with a split-actuator mechanism delivering a full 1.6 GPF flush for solid waste and a reduced 1.1 GPF flush for liquid waste, achieving an effective composite flush volume not exceeding 1.28 GPF.
• Commercial Urinals: Standard urinals are limited to a maximum of 1.0 GPF (3.8 LPF). High-Efficiency Urinals (HEU) operate at 0.5 GPF down to 0.125 GPF (1 pint).
• Prohibited Fixtures: Fixed-tank commercial water closets exceeding 1.6 GPF, continuous-flowing trough urinals, and non-water-conserving blowout bowls are illegal in new construction or renovation.

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2. Water Closet Design & Bowl Flushing Dynamics

Water closets are classified by their hydraulic flushing action:

1. Siphon Jet: Features a submerged water jet in the trapway throat that rapidly fills the trap leg, initiating an immediate, powerful siphonic pull. It maintains a large water surface area (water spot) inside the bowl, making it quiet, sanitary, and the standard in residential and light-commercial gravity-tank installations.
2. Reverse Trap: Similar to siphon jet, but with the trapway discharging toward the rear. It possesses a slightly smaller water spot and lower hydraulic capacity than a standard siphon jet.
3. Blowout Bowls: Engineered without a siphonic trapway. Instead, a high-velocity jet of water from a commercial flushometer blasts water and solids directly over a large-diameter trapway (minimum 3-inch internal diameter) into the soil branch. Blowout bowls are extremely resistant to clogging from foreign debris, making them ideal for correctional institutions, sports stadiums, and busy transit hubs. However, they are loud, require high operating water volume and dynamic pressure (minimum 25 to 35 psi), and cannot operate on residential gravity supply.

Floor-Mounted vs. Wall-Hung Installations

Wall-Hung Carrier Fittings (In-Wall Support)

Under MPC Section 405.4.3, wall-hung water closets must not transmit structural cantilever loads to the finished wall surface or the drainage piping itself. Wall-hung bowls must be supported by an engineered in-wall cast iron carrier fitting (such as Zurn, Wade, or JR Smith) complying with ASME A112.6.1M. The carrier's structural foot supports must be anchored directly into the structural concrete floor slab using heavy-duty expansion bolts. Neoprene or graphite-impregnated felt horn gaskets must be used between the fixture horn and the carrier faceplate; wax rings are strictly prohibited on wall-hung water closets because cantilever deflection and vertical shear will squeeze out the wax, causing catastrophic sewer gas and wastewater leaks.

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3. Closet Flanges: Thickness, Fastening & Seals

Under MPC Section 405.4 (Floor and wall drainage connections) and 405.4.2 (Securing floor outlet fixtures), the connection between a floor-mounted water closet and the drainage system must be made with an approved closet flange or an approved listed connection:

• Flange Materials and Standards: Closet flanges are manufactured of copper alloy, cast iron, galvanized steel, PVC or ABS and must comply with the referenced standards for the material (for example ASME A112.4.3 / CSA B45.9 for plastic flanges and ASME A112.6.4 for cast-iron soil pipe flanges). The 7/16-inch minimum thickness commonly quoted in the trade for a cast-brass or cast-iron flange is a manufacturing standard, not a numbered section of the plumbing code - do not cite it as an MPC section on the exam.
• Mechanical Fastening (MPC 405.4): Floor-outlet fixtures must be rigidly secured to the drainage connection and to the building structure with corrosion-resistant fasteners. Fastening the flange only to finish flooring, or leaving it unsupported on loose tile, does not satisfy that requirement.
• Vertical Setting Elevation: The bottom surface of the closet flange must rest flat on top of the finished floor. Setting the flange below the finished floor level (e.g., when tile is installed over an existing subfloor flange) creates an excessive gap that a single wax ring cannot reliably seal, risking concealed floor rot.
• Closet Bolts: Water closets are secured to the flange with two or four 5/16-inch or 1/4-inch brass or stainless steel closet bolts seated in the flange slots (standard bolt spread is about 5-1/2 inches). Plated steel bolts corrode rapidly in urine exposure and should not be used.
• Gaskets: Watertight seals must be achieved using an approved petroleum wax ring (with or without an integral polyethylene sleeve/horn) or an elastomeric compression gasket.

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4. Minimum Clearances & Compartment Layouts

Under MPC Section 405.3.1, water closets, urinals, and bidets must maintain strict spatial clearances to allow safe physical access and sanitary maintenance:

• Side Clearance (Centerline): A water closet, urinal, lavatory or bidet must not be set closer than 15 inches (381 mm) from its centerline to any finished side wall, vanity cabinet, partition, or other obstruction.
• Adjacent Fixture Spacing: A minimum of 30 inches (762 mm) center-to-center must be provided between adjacent water closets, urinals, or bidets.
• Front Clearance: A minimum of 21 inches (533 mm) of clear, unobstructed space must be provided in front of the water closet or bidet bowl to any wall, door swing, or opposing fixture.
• Water Closet Compartment (Stall) Dimensions: Compartment sizes are not fixed by MPC Chapter 4; wheelchair-accessible compartment dimensions (60 inches wide by 56 inches deep for wall-hung bowls, 59 inches deep for floor-mounted bowls) come from ICC A117.1 and the ADA Standards and are enforced through the Michigan Building Code.

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5. Urinals: Types, Clearances & Waterless Mandates

Under MPC Section 419, urinals installed in public or commercial buildings must have smooth, non-absorbent vitreous china or stainless steel surfaces with integral flushing rims. Wall-hung urinals are classified as blowout, siphon jet, or washdown.

Urinal Partitions & Privacy Screens

Under MPC Section 405.3.5, in public restrooms where two or more urinals are installed, privacy partitions must be installed between adjacent urinals. The partition screen must extend out from the wall at least 18 inches (457 mm), reach a height of at least 60 inches AFF, and terminate not more than 12 inches above the finished floor.

Waterless (Non-Water) Urinals (MPC Section 419)

Waterless urinals conserve municipal water by utilizing a replaceable trap cartridge filled with a proprietary liquid sealant. The sealant has a specific gravity lower than urine ($<1.0$), allowing liquid urine to flow down through the sealant layer while the sealant remains floating on top, effectively sealing out foul sewer gases. When installing waterless urinals, Michigan plumbing regulations enforce two mandatory requirements:

1. Rough-In Potable Water Supply: MPC Section 419 requires every non-water urinal installation to have a water distribution line roughed in to the urinal location so that an approved backflow prevention device and water supply can be installed if a water-supplied urinal is fitted later. This ensures that if the facility owner abandons the waterless system in the future, a conventional flushometer water supply is immediately available without opening finished wall surfaces.
2. Drainage Piping Material Restrictions: Undiluted human urine is highly acidic and corrosive. When not diluted with water, urine produces concentrated ammonia and struvite (magnesium ammonium phosphate) crystals. As a result, copper DWV piping is prohibited for direct waterless urinal drainage branches; non-metallic piping (PVC or ABS Schedule 40) or acid-resistant cast iron must be utilized.

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6. Flushometer Valves & Backflow Protection

Commercial water closets and urinals rely on flushometer valves (complying with ASSE 1037 / ASME A112.19.2) rather than gravity storage tanks. Flushometers discharge a high-volume, short-duration slug of pressurized water directly from the building supply main.

Mechanics: Diaphragm vs. Piston Valves

• Diaphragm Type: Operates using a flexible rubber diaphragm dividing the valve into an upper and lower chamber, separated by a microscopic bypass orifice. Water pressure in the upper chamber holds the diaphragm seated against the valve outlet. When the operating handle tilts the relief valve, pressure in the upper chamber drops instantly, allowing supply pressure below to thrust the diaphragm upward, initiating the flush cycle. As water slowly bleeds back through the bypass orifice into the upper chamber, pressure equalizes, closing the valve.
• Piston Type: Uses a sliding brass or polymer piston with a cup seal. Piston valves are less sensitive to severe water pressure spikes and mineral accumulation than thin elastomeric diaphragms.

Operating Pressures & Supply Sizing

Flushometer valves will not close or flush properly if dynamic water pressure falls below design thresholds. Siphon jet bowls require a minimum dynamic flow pressure of 15 to 25 psi, while blowout bowls demand 25 to 35 psi. Commercial water closet flushometers require a minimum 1-inch (25 mm) dedicated water supply branch, whereas urinals require a minimum 3/4-inch (19 mm) supply branch.

Atmospheric Vacuum Breaker (AVB) Elevation

Because a flushometer valve connects directly between the potable water distribution system and a fixture containing raw sewage, backsiphonage presents a severe biological hazard. Under MPC Section 425.2, flushometer valves must comply with ASSE 1037 and be installed with the outlet not less than 1 inch (25 mm) above the flood level rim of the fixture. The vacuum breaker serving a flushometer valve must be installed with its critical level not less than 6 inches (152 mm) above the flood level rim of the water closet or urinal bowl. Installing a flushometer with its vacuum breaker submerged or closer than 6 inches to the rim allows back-siphonage of raw sewage into the drinking water supply during negative street pressure events.