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What is a bow vent, and when does Massachusetts allow one?

Updated 2026-09-27 · Checked against 248 CMR 10.16, 248 CMR 10.15, 248 CMR 10.08

248 CMR does not define "bow vent" in words; 10.16(9) permits it and Example 16 draws it. In practice a bow vent is a vent that loops up under the counter and back down, instead of rising straight up, because there is no room for it to rise above the fixture. 248 CMR 10.16(9) allows one only when the vent for an individual sink or lavatory cannot rise 6 inches above the flood level rim of the fixture before turning horizontal, and it must be sized from 10.16(11) Table 2.

The textbook case is a kitchen island. The sink sits in the middle of the room, there is no wall behind it to carry a vent up, and the sink's flood-level rim (the top edge of the sink itself) sits at about counter height, or a little below it for an undermount bowl. The normal venting rule cannot be met, so the code gives you the bow vent.

What 248 CMR 10.16(9) actually says

The whole section is two sentences. From 248 CMR 10.16 on Cornell LII:

• 10.16(9)(a): "Bow vents are permitted when a vent for an individual sink or lavatory cannot rise six inches above the flood level rim of the fixture before turning horizontal."
• 10.16(9)(b): "The bow vent shall be sized in accordance with 248 CMR 10.16(11): Table 2: Size and Lengths of Vents."

The code follows those sentences with Example 16, Typical Bow Vent Installations, a drawing. Cornell's web copy does not reproduce the image, so get the figure from a printed or licensed copy of 248 CMR. It is the reference an inspector will have in mind, and it is exactly the kind of picture Part II isometric questions are built from.

Every word of 10.16(9)(a) is a condition you can be tested on:

• "individual": one fixture, not a group.
• "sink or lavatory": not a water closet, urinal, shower or floor drain.
• "cannot rise six inches above the flood level rim": the bow vent is an exception for when the normal rule is impossible, not a choice you make because it is easier.

Why the normal vent rule fails at an island

The rule the bow vent is an exception to is in 10.16(4)(b)1.b: for other than floor-mounted fixtures, the vent pipe shall rise vertically, or at a 45-degree angle from the vertical, to a point at least 6 inches above the flood-level rim of the fixture it is venting, before it may offset horizontally. 10.16(4)(c)2 adds that horizontal vent pipes forming branch vents, relief vents or loop vents shall be installed at least 6 inches above the flood-level rim of the highest fixture served.

The reason is simple. A vent that turns horizontal below the flood rim can fill with water if the fixture drain backs up, and then it is a drain, not a vent. Keeping horizontal vent runs above the rim means they stay dry.

At a wall-mounted sink you just run the vent up inside the wall. At an island there is nothing above the counter to run it in. You cannot get 6 inches above the rim, so 10.16(4)(b) cannot be met, and 10.16(9) opens up.

The other options, and why they are closed or Board-gated in Massachusetts

This is where Massachusetts parts company with the model codes, and it is why so much of the national advice about island sinks does not apply here.

Air admittance valve. Many national articles say "just put an AAV under the island sink." In Massachusetts, 10.16(1)(e) says "Automatic vents and air admittance valves are not permitted without Special-Permission from the Board." An AAV is not the default answer here.

Wet venting. 10.16(5)(b) bars kitchen-sink, garbage disposal, dishwasher, washing machine and culinary-fixture piping from serving as a wet vent, and 10.16(5)(a) allows wet venting only in the bathroom arrangement. Put those two together and an island kitchen sink cannot be wet vented. Note which rule does which job: (5)(b) stops the kitchen sink's piping from venting anything else, and (5)(a) is why nothing else can vent the kitchen sink.

Combination waste and vent. 10.16(1)(a) prohibits a combination waste and vent system without Special-permission by the Board, and even then every drainage pipe must be at least two pipe sizes larger than 10.15 requires, and it is limited to floor drains and sinks (10.16(1)(a)1). So for a sink it is a real option, but only one the Board has to approve; it is not something you can simply choose on a permit.

Island fixture venting under the ICC codes. The IPC and IRC describe a similar looped arrangement under the name "island fixture venting." The concept is close, but do not bring their wording or numbers into a Massachusetts answer. On the exam and on an MA job, the rule is 10.16(9) and the sizing is 248 CMR Table 2.

That leaves the bow vent as the ordinary, code-compliant way to vent an island sink in Massachusetts without asking the Board for anything.

Anatomy of a bow vent

The code text does not spell out the geometry; Example 16 in the printed 248 CMR is the controlling picture, and it is in the official regulation on mass.gov, not on Cornell. The shape varies with the cabinet and the framing. A typical bow vent, as commonly installed (confirm against Example 16 in the printed code):

1. Trap and trap arm. The sink trap, then a trap arm to the vent takeoff. The trap arm still has to meet 10.16(10) Table 1 (5 ft for 1-1/2 in., 6 ft for 2 in.), and the vent cannot be within two pipe diameters of the weir (10.16(1)(b)). Our trap arm length post covers both.
2. The bow. The vent rises from the drain as high as the cabinet allows under the counter, then bends over and runs back down. The rise gives air a path above the waste flow even though it cannot get 6 inches above the rim.
3. The return. The looped vent comes back down below the floor and connects to the drainage piping. Because 10.16(4)(a) requires every vent to be graded to drain back to a soil or waste pipe by gravity, any water that gets into the loop runs out of it instead of sitting in it.
4. The vent run out to a wall. From the lower part of the loop, vent piping runs to a place where it can rise and connect into the building's venting system, following the normal rules once it gets there: connections to a vent stack at least 6 inches above the flood-level rim of the highest fixture served (10.16(4)(c)1).

Notice what this arrangement does that ordinary venting may not: its horizontal run sits below the flood-level rim, which 10.16(4)(c)2 would otherwise forbid. 10.16(9) is what permits that here, and only for an individual sink or lavatory.

Two code points bite at this stage. A vent taken off a horizontal drain must be taken off above the center line of the drain (10.16(4)(b)1.a). And no part of a vent installed underground or below a basement floor may be less than 2 inches in diameter (10.16(11)(e)). Island sinks on a slab put part of the bow vent underground, so that underground run is 2-inch minimum even if the vent above the floor could be smaller.

For a residential kitchen sink, 10.15(8)(c) applies as well: the branch waste is not less than 1-1/2 in. and connects independently to the drainage system, and a full-size cleanout is required under the kitchen sink, unless a two-piece trap that can be taken apart is used in its place.

Sizing a bow vent from Table 2

10.16(9)(b) sends you to 10.16(11) Table 2, Size and Lengths of Vents. The same table sizes most vents in 248 CMR, and the code gives the procedure right under it:

1. Compute the total fixture units connected, using 10.15 Table 1 (a residential kitchen sink is 2 DFU, with or without disposer) and 10.15(2)(b) for any continuous flow.
2. Determine the size of the drain (for a stack, from 10.15 Table 3 or 4; for a branch, the branch size).
3. With that size and the fixture-unit load, read Table 2 for the vent diameter whose allowed developed length covers your vent run.

Read the Table 2 lengths from the official printed table. Cornell's web rendering of Table 2 flattens it into a list that is easy to misread, so we do not quote its lengths here.

The developed length is the length along the centerline of the vent pipe and fittings (10.03), so the up, over and back down of the bow counts, along with the run to the wall. A long island run can push you into a larger vent size.

Two sanity checks help on a test. The individual vent floor in 10.16(11)(a) is at least 1-1/4 inch and not less than half the diameter of the drain it connects to. And the underground portion is 2-inch minimum no matter what Table 2 allows. When a multiple-choice answer offers a vent smaller than either, it is wrong.

Bow vent inspection checklist

Use this to check your own rough-in before the inspector does. Every line is tied to a code section:

• The fixture is a single sink or lavatory, and a normal vent really cannot reach 6 inches above its rim (10.16(9)(a)).
• The trap arm meets Table 1 and the vent is not within two pipe diameters of the weir (10.16(10), 10.16(1)(b)).
• The vent leaves the drain above the drain's centerline (10.16(4)(b)1.a).
• All vent piping is graded to drain back by gravity; no dead-level sags that hold water (10.16(4)(a)).
• The vent is sized from Table 2, and the underground part is 2 inches or larger (10.16(9)(b), 10.16(11)(e)).
• No AAV or automatic vent without Special-Permission (10.16(1)(e)).
• The kitchen sink is not wet vented and not used as a wet vent (10.16(5)(a), 10.16(5)(b)).
• Kitchen sink branch is at least 1-1/2 in., connected independently, with a full-size cleanout or a two-piece trap (10.15(8)(c)).

Bow vents on the exam

Bow vent questions tend to test the conditions, not the plumbing. Expect a question that names four fixtures and asks which one may use a bow vent (the sink or lavatory), one that asks what condition triggers it (the vent cannot rise 6 inches above the flood rim before turning horizontal), and isometric drawings where a looped vent has to be read correctly. Because 248 CMR uses the term "bow vent" where the ICC codes say "island fixture venting," the wording itself tells you which code a question is written to.

The Massachusetts journeyman exam guide explains where venting sits in the Part I and Part II blueprint, the code book has 10.16 in study form, and the practice app has drawn isometric problems. If you are also sizing the drains the bow vent serves, see drain pipe sizing by fixture units.

Want more like the questions above? Start 7 days free to work the full venting set under exam timing.

Independent study aid — always confirm requirements in the current 248 CMR and the PSI Candidate Information Bulletin.