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Drainage system

Q: Section 603.4.8 of the code requires that sizing of drain lines serving backflow devices must be in accordance with manufacturer?s flow charts. How do we size a drain for a 4-inch backflow device based on the manufacturer?s discharge rate such as a reduced pressure principle backflow assembly (RPZ)?

A: A discharge rate for a 4-inch RPZ device is approximately 550 to 600 GPM at 55 psi from a manufacturer?s flow chart table. It is reasonable to calculate the device discharge rate as intermittent flow under Table 702.2(b). Our calculation is based on the manufacturer?s rate of approximately 600 GPM divided by 30 GPM and multiply by 6 DFUs totally 120 DFUs which is a 4-inch size drain line. The minimum drainage line size for 600 GPM discharge rate from a 4-inch backflow device is 4-inch trap and drain. Please consult with your plumbing designer and manufacturer for additional information.


Q:Do all installations of domestic dishwashing machines require an air gap fitting under the 2015 Minnesota Plumbing Code? Are there any exceptions?

A: Yes. The code requires installation of domestic dishwashing machines to discharge indirectly through a listed air gap fitting into a waste receptor, a wye branch fitting on the tailpiece of a kitchen sink, or dishwasher connection of a food waste grinder (see Minnesota Rules, Chapter 4714, Sections 414.3 and 807.4). This air gap fitting must be listed to a standard intended for the application. An example is a drain airgap fitting listed to IAPMO PS 23.

Replacements of domestic dishwashers where there are no alterations, no changes to the existing plumbing system, and no modification to the design, and where the system was installed to code at the time it was installed, would not require the additional airgap fitting for this work. 

It is reasonable to consider this as repair and replacement work without altering the design of the existing plumbing system. The reconnection of the discharge drain line must be fastened as high as possible under the countertop, and using existing rough-ins of the existing plumbing system.

Remodeling of the kitchen including sink replacements, altering the water and/or drainage piping are subject to the requirements of the current code.

Consult with your local authority for any permit requirements and necessary approvals for your project prior to installation.

Q:Can stainless steel sinks or urinal fixtures be fabricated on-site by licensed individuals without being listed to a standard?

A: No. Stainless steel fixtures including the urinals, commercial kitchen sinks, lavatories, or any other plumbing fixtures must meet the requirements of the commercial stainless steel standard ASME A112.19.3 listed in Chapter 14 of the 2015 Minnesota Plumbing Code (see Sections 401.1 ? and 420.1). Compliance with the standard ensures that it consistently constructed of durable, smooth, bowl depth as it relates to minimum thickness of steel, and free from concealed fouling surfaces.

Materials and support

Q: Are dishwashers drainage systems still required to be supported since it is not specific in the 2015 Minnesota Plumbing Code?

A: Yes. All piping and fixtures must be adequately supported. See Chapter 4714, Sections 312.2, 313.5, and the manufacturer?s installation instructions.

Q: Do galvanized steel pipe need be cut groove method or can the steel pipe be rolled groove under the 2015 code?

A: All grooving of galvanized pipe must be the cut groove method. Rolled groove method causes obstruction of flow of water and waste, which subjects the piping to undue wear and erosion and must not be utilized (see Chapter 4714, Section 310.5).

Water conditioning

Q: The footnote of Table 611.4 says the sizing for residential water softeners for more than four bathroom groups requires the softener be engineered for the specific installation. Does this mean, we have to hire a Minnesota-licensed professional engineer to design the water softeners?

A:  Our agency  will continue to accept plumbing construction plans and specifications prepared by the master plumber that designed and installed their systems as permitted within the limitation prescribed in MS 326.02, or plumbing plans designed by a Minnesota-licensed professional engineer.


Q: I am designing a cabin with one bathroom group and a kitchenette sink. I have a 4-inch building sewer. Can I install just one 3-inch full-size vent through the roof to meet code?

A: Yes, the 3-inch full-size vent is acceptable for this design. Although the design of the building sewer is 4-inch, the required building sewer for one bathroom group and a kitchenette sink is actually 3-inch in size as determined in Table 703.2. Therefore, your cross sectional area of the 3-inch vent is equal to the required 3-inch building drain, and meets the requirement of this section.

Q: I am designing for a three-compartment sink and a commercial dishwasher installation for a restaurant. I see Section 704.3 of the 2015 Plumbing Code requires a tell-tale floor on each of the fixture to protect from sewer backing up. Since the floor drains are only needed to protect the fixtures and do not receive discharges from any indirect waste piping, do I still need to vent each one of the floor drain?

A: Yes, each floor drain, including tell-tale floor drain,s must be individually vented within the trap to vent distance of Table 1002.2. There are no venting exceptions specified in the code for floor drains. Additionally, Section 704.3 specifically requires the tell-tale floor drain to be trapped and vented in accordance with the code. For more information, see Minnesota Rules, Chapter 4714, Sections 418.2, 704.3 and 1002.1.