A New Old Way to Vent a Kitchen Island

The shared island loop (pictured here) may become less popular now, as the combination of litter and venting is legal again.

Synopsis: The code expert Glenn Mathewson describes three ways to ventilate a kitchen island: the island vent, the air intake valve and the combined waste and ventilation system. Combined venting is not a popular solution for island venting as it was not allowed to include a food waste disposer on the island in the past. However, this rule was not based on research and was corrected in the 2018 edition of IRC. Since many states and municipalities are using older versions of the code, combo venting can take a while to catch on, but Mathewson explains that this could be a welcome alternative to the other types of island venting.

Most builders and installers are familiar with two ways of venting island sinks: the tried and tested island vent and the newer air intake valve (AAV). Neither method is exactly foolproof – they are legal under certain circumstances, and their acceptance and design are not universal for municipalities, states, or even code-writing units. There is arguably a better way to vent island faucets, however – we’ll get to that in a moment.

As nifty as modern homes are, a simple cup of water is essentially the only thing preventing sewage gases from entering our homes. This cup of water is in what is known as a P-trap. Contrary to some assumptions, his name has nothing to do with the type of liquid it contains, but rather with the shape of the piping, which resembles a letter P that has fallen on his face. Every time a device is used, the water in the trap is extinguished and replaced. If the drain is not adequately ventilated, the jet of water from a sink or bathtub can suck the last bit of water out of the trap. Venting eliminates this suction process and ensures that the last cup of water that gets into the trap will remain there until the next time the device is used. This is a science tested. You would think everything in code is determined that way, but it’s not always true.

Vent the island fasteningVent the island attachment

The “old school” method of venting island sinks is appropriately referred to as the “island mount vent” in IRC section P3112. This island loop, which was first included in the Uniform Plumbing Code in 1958, was later recognized in most other model installation codes and has remained largely unchanged over the years. This relaxes some exceptions to the standard sanitary rules for kitchen design. A vent is connected to the trap arm within standard restrictions, but instead of going up through the roof, this vent is allowed to rotate horizontally and fall back under the ground. It can rotate before it reaches the long-standing minimum height of 6 inches above the sink’s flood edge, provided it rises above the drain at the bottom of the sink.

If you rise at least above the sink outlet, the sink will be able to drain through the vent, but not completely. If it is clogged, water will remain in the sink to alert the user to the problem. This type of “alarm” is an integral part of plumbing design. An island loop vent is only allowed to rotate under the top of the sink as cleaning work is required to clear any clogs in the vent. Access for a drain coil replaces the perfect plumbing design. The distance to this entrance is specified in IRC section P3005.2.9 and requires 18 inches measured horizontally in front of the cleaning opening. This generally means orienting the cleaning so that it is facing the cabinet doors.

One point of criticism of the island loop is that it takes up a lot of space in the sink cabinet. It also requires a lot of fittings and a certain geometry, and it costs a lot of time and money to install. These shortcomings led to the search for an alternative solution.

Prime with an AAVPrime with an AAV

This solution – the AAV – was invented in Europe by Sture and Doris Ericson (Stu-Dor), whose “Studor Vent” became the current technical language for the device. AAVs arrived in the U.S. in 1986 but weren’t listed in model installation codes until 1995. These one-way valves let air down the drain to protect the trap from sucking water, but do not let sewage air into your home. Most appealingly, there is no need for a vent to go vertically through the roof or below the floor, making it ideal for remodeling applications where adding a new vent through the walls and roof of an existing building can be prohibitive. And they take up very little space. AAVs can be placed up to 4 inches above the unit drain to be vented or the branch horizontal drain. Not only are they the go-to place for island vents or basement sinks that are far removed from existing vents, but also for locations where the cost of piping and labor would exceed the cost of the valve.

They can only be installed after the plumbing system has been tested for gas tightness and must be accessible like ventilation slots for island loops. Most importantly, they are not a universal alternative as some local codes do not allow this. It’s not clear why they aren’t generally approved – the IRC and various installation codes allow AAVs to be used in various applications, and there is an ASTM testing standard that legitimizes their manufacture and performance. In places where they are not allowed, it can be an all too common case that “we always did it that way”.

But another way to vent island fixtures has been around for ages, even though it’s not widely used – the combined waste and vent system. This system uses an oversized drain pipe that has a dual function as both a drain and a vent. The larger cross-section of the drain pipe and its maximum slope of 1/2-in-12 in the horizontal sections ensure that the sewage draining through the pipe never fills it, leaving an air space above the sewage to vent the system. There is only one vertical pipe – the connection between the device’s drain and the horizontal portion of the combined waste and ventilation system – which is also oversized and can drop as much as 8 feet before connecting to the horizontal portion. Unlike some of the other vent options in the code, the horizontal length of a combined waste and vent system is unlimited.

Of the three methods mentioned here for venting an island, the combined waste and venting system is arguably the simplest. Why didn’t it catch on? Probably because you weren’t able to hook up a food waste handler to a combined waste and ventilation system until the International Residential Code changed in 2018. Even if you don’t have a dedicated disposer at the sink, many dishwashers contain similar devices. For convenience, dishwashers are usually installed alongside sinks. If you can’t connect a dishwasher to a combined waste and ventilation system to grind food, you will have to resort to other options that will allow it. Hence the common use of the island loop and the AAV.

The reason for the ban on connecting disposers to combined waste and ventilation systems is set out in the commentary in previous versions of the code, such as this notice on IRC 2015: “Because of the low flow rates and volumes that occur in these systems collected solids, which would lead to clogging and a reduction in the internal cross-sectional area of ​​the pipelines. Therefore, sinks equipped with disposers and food waste devices that dispose of solid waste must not be operated through a combination of drain and vent. “

It turns out that this rationale is not based on science. It was an assumption. And it’s wrong.

When the code is updated, the International Code Council will publish a book entitled “Major Changes to the International Residential Code”. In the 2018 edition, the ban on attaching disposal devices to combined waste and ventilation systems is clarified. “When this restriction was incorporated into the code, there was no research and no field issues were identified to support the restriction.” The American Society of Plumbing Engineer Research Foundation conducted the investigation, and “Their technical findings found no technical justification for this to limit the discharge of food waste disposers into combined waste and ventilation systems. “

combined waste and ventilation system

So the IRC was changed. Now, with the latest version, you can hook up your dishwasher and food macerating waste disposer to a combined waste and ventilation system. But for an office worker, the International Plumbing Code would probably have dropped the ban as well – and it will be in the 2021 update. This is good news for anyone planning an otherwise difficult-to-vent kitchen island, especially in countries where AAVs are not allowed.

While it’s a relatively minor change, it can have a significant impact on the way builders explore kitchen islands, making the job easier and cheaper. However, most states and municipalities are using older versions of the code, so it may take a while for them to catch on. And just as certain parts of the code continue to assume that “we’ve always done it this way,” some traders and states are likely to continue to use other methods for the same reasons.

This doesn’t mean the combo waste and ventilation system is the answer to every kitchen island venting puzzle. However, it may be preferable to island-mount ventilation and is a welcome alternative in areas where AAVs are not allowed.

From Fine Homebuilding # 286

More information about rough pipelines:

Drain ventilation systems – Although hidden behind walls as well as in floors and ceilings, the pipes that make up the drain-exhaust system of a house form the equilibrium of the plumbing equation.

14 tips for bathroom installation – From utility lines to waste lines, these simple rules will make your job better.

Framing with the plumber in mind – A few tips to help you get your sticks and nails out of the way of the plumbers.

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