Venting rules

The café design had one venting problem that Maya did not spot at first. The two washrooms sat against an exterior wall, far from any existing vent stack, and the engineer had shown their fixtures vented through a single loop that ran up into the ceiling and across to the nearest vent stack. The arrangement looked fine on the drawing. When Maya checked it against the code, she found that one lavatory's trap arm was longer than the code allowed for its size and slope, so its trap was not protected. Luis was pleased: "That's exactly the kind of thing that passes on paper and fails in real life, with a gurgling sink and a sewer smell nobody can find."

This lesson works through the venting part of the code. PLB-120 explained why venting matters and described the main kinds of vents. Here the focus is on finding and applying the rules: which fixtures need vents, how far a trap may be from its vent, how vents are sized, where they may connect and terminate, and when alternatives such as air admittance valves are permitted.

What the venting rules cover

In the IPC, Chapter 9, Vents, covers when vents are required, vent terminals, outdoor vent extensions, vent connections and grades, fixture vents and trap-to-vent distances, individual, common, wet, waste stack, circuit and combination vents, island fixture venting, vent pipe sizing, relief vents, and air admittance valves.

Most venting questions on a job are about one of five things:

• Does this trap need a vent, and which kind of vent is allowed here?
• Is the trap close enough to its vent, measured as the code says?
• Is the vent big enough for the load and its length?
• Does the vent connect and rise correctly, so it cannot fill with water or trap condensate?
• Does the vent end in the right place outdoors, or is an air admittance valve permitted instead?

Which traps need vents

The basic rule is that every trap must be protected by a vent, so that air pressure on both sides of the trap stays close to atmospheric and the seal is not siphoned or blown out. The code then describes many ways of providing that protection: an individual vent for one trap, a common vent for two traps at the same level, a wet vent where a drain also serves as a vent for another fixture, a circuit vent for a row of fixtures on a branch, a stack vent where fixtures connect close to the top of a stack, and others.

Each method comes with conditions. A wet vent, for example, is allowed only for certain fixtures, only within certain loads and only in certain arrangements. Using a method outside its conditions is the same as having no vent at all. When a drawing shows a venting method, the first check is whether the code permits that method for those fixtures in that arrangement.

Distance from trap to vent

The most common venting failure in the field is a trap that is too far from its vent. If the drain falls too far below the trap weir before the vent connection, flowing water can fill the pipe and siphon the trap. The code limits the length of the trap arm, depending on the trap size and sometimes the slope, and also says that the vent connection must not be below the trap weir in a way that turns the trap arm into a siphon.

The IPC sets the maximum distance from a fixture trap to its vent in a table by trap size and slope. For example, a 1-1/4 in trap arm at a quarter inch per foot is limited to 5 feet, a 1-1/2 in trap arm to 6 feet, and a 2 in trap arm to 8 feet. The total fall of the trap arm must not exceed one pipe diameter, so that the vent opening is not below the trap weir. The Ohio Plumbing Code keeps this table.

The program below checks a list of trap arms against a table of limits. The table uses the IPC values for a quarter inch per foot slope; check the Ohio amendments and the edition in force before relying on them.

# Check trap arm lengths against the IPC table at 1/4 in per foot (feet)
limits = {1.25: 5, 1.5: 6, 2: 8, 3: 12, 4: 16}   # trap size (in) -> max distance to vent (ft)
arms = [("cafe lav 1", 1.25, 4), ("cafe lav 2", 1.25, 7), ("mop sink", 3, 10), ("3-comp sink", 2, 8.5)]
for name, size, length in arms:
    lim = limits[size]
    verdict = "ok" if length <= lim else f"too long by {length - lim:g} ft: add a vent or move the fixture"
    print(f"{name:12s} {size:g} in trap, arm {length:g} ft (limit {lim} ft): {verdict}")

cafe lav 1   1.25 in trap, arm 4 ft (limit 5 ft): ok
cafe lav 2   1.25 in trap, arm 7 ft (limit 5 ft): too long by 2 ft: add a vent or move the fixture
mop sink     3 in trap, arm 10 ft (limit 12 ft): ok
3-comp sink  2 in trap, arm 8.5 ft (limit 8 ft): too long by 0.5 ft: add a vent or move the fixture

Vent sizes and lengths

Vents must be large enough to move the air needed to protect the traps they serve. The code sizes vents by the drainage load they serve and by their developed length: a long vent needs to be larger than a short one serving the same load, because air loses pressure as it travels. The code usually also sets minimum sizes, such as a minimum fraction of the drain it serves or a minimum absolute size, and requires the main vent or stack vent of each building to be at least a certain size.

• Individual vents: often at least half the diameter of the drain served, with a minimum size.
• Vent stacks and stack vents: sized by the load on the stack and the developed length of the vent, from a table.
• Branch vents and vent headers: sized for the total load they serve and their length.
• Relief vents: required at intervals on tall stacks and at offsets, sized by the code.

The IPC includes a table that gives the maximum developed length of vents for combinations of soil or waste stack size, drainage fixture units and vent size, and rules for relief vents on stacks with more than ten branch intervals. Kingsway Tower's stacks have vent stacks connected at the base and at intervals, as the code requires for buildings of its height.

How vents connect and rise

Vents carry air, but they can also carry water if they are installed wrongly. A vent that connects below the flood level of a fixture can fill with waste when a drain blocks, and then it is no longer a vent. The code therefore sets rules for how vents connect to drains and how they rise before turning horizontal.

• A vent must connect to the drain above the centerline of the horizontal drain, so that waste does not run into it.
• A dry vent must rise vertically to a point above the flood level rim of the fixture it serves before turning horizontal, with exceptions.
• Vent pipes must slope back toward the drain so that condensation and rain drain away; they must not form traps or pockets.
• Vents may join together above the flood level rim of the highest fixture served, and then connect to a vent stack or stack vent.
• Vents must not be used as drains, and drains must not be used as vents except where the code allows wet venting.

On the café drawing, the engineer's vent loop ran up and across the ceiling, high above the fixtures, which was correct. Maya's only problem was the trap arm length. If the vent had been shown running horizontally below the lavatory rims, that would have been a second problem.

Vent terminals

Vents must end outdoors, usually through the roof, in a place where sewer gas will not enter the building or bother people. The code sets the minimum height of the vent above the roof, the distance from windows, doors, air intakes and property lines, and the minimum size of the vent terminal in cold climates so that frost does not close it.

The IPC requires vents to extend a minimum height above the roof, more where the roof is used for occupancy, sets minimum distances from doors, windows and air intakes, and allows the local jurisdiction to require vents in cold climates to be enlarged before passing through the roof to prevent frost closure. Ohio sets its own frost-closure size rule.

Air admittance valves and other alternatives

An air admittance valve is a one-way valve that lets air into the drainage system when pressure drops, then closes to keep sewer gas in. It can protect a trap where running a vent to the outdoors is difficult, such as at an island sink. Codes treat air admittance valves cautiously, because they only admit air: they cannot relieve positive pressure, and the system still needs at least one vent to the outdoors.

The IPC permits air admittance valves under conditions: they must be listed to the referenced standard, installed a minimum distance above the horizontal branch or fixture drain they serve and above insulation in attics, accessible, in a space with air, and the drainage system must have at least one vent to the outdoors. The Ohio Plumbing Code keeps these rules.

Other venting alternatives include single-stack systems, which use special fittings to control pressure in tall stacks without separate vent stacks, and sovent-type aerator systems. These are usually engineered systems approved for specific buildings, as Lesson 4 described.

Venting checklist

Maya added a venting list to her drawing review routine.

• Every trap has a vent of a kind the code permits for that fixture and arrangement.
• Every trap arm is within the length limit for its size, measured from the weir to the vent fitting along the pipe, and within the slope limits.
• Vent sizes suit their loads and developed lengths; relief vents and vent stacks are shown where required.
• Dry vents connect above the drain's centerline and rise above the flood level rim before turning horizontal.
• Vents slope to drain, have no pockets and terminate correctly outdoors.
• Air admittance valves, if used, are permitted for that location and installed to their conditions.