What Frost Closure Does to a Vent Terminal

Why this matters

A vent terminal in a cold climate does not plug with debris. It plugs with the building's own breath. Warm, water-saturated air rises out of the drainage system, meets pipe wall at outdoor temperature in the last few feet, and gives up its moisture there as rime, which grows inward from the wall rather than filling from the bottom. That geometry is the whole story: open area falls with the square of the remaining bore, so the first half inch of ice costs more than half the vent and the reader who thinks in diameters underestimates it every time. It also explains why the control is terminal diameter rather than terminal height, and why the buildings that close first are the quiet, humid, lightly-used ones rather than the busy ones. This card follows one January call all the way through and then does the geometry that made the answer obvious in hindsight.

The call

Three-storey walk-up, six units, mid-January, the coldest stretch of the winter. Sewer smell in both top-floor units, reported worst between about six and nine in the morning. Nothing from the four units below. The building has a single drainage stack continued through the roof as a stack vent, no air admittance valves anywhere, and no recent work.

What got eliminated, and on what evidence

A dry trap in an unoccupied space. The obvious first candidate in any smell complaint and it dies fast here: both top units are occupied and in daily use, and every trap in both was checked full, including the two that get used least, a floor drain in one unit's utility closet and a rarely-run second lavatory. A dry trap is also a steady leak, and this complaint has a clock on it.

A failed air admittance valve. These fail closed and produce exactly this symptom, so it is worth ten minutes of looking. There are none in the building. The sibling card comparing air admittance valves and vent stacks covers what that failure looks like; it is not what this was.

A defect in the piping itself. A crack or an open joint in a concealed vent line leaks continuously, and again, this complaint is not continuous. The six-to-nine window is the building's morning peak discharge, which means the smell is being driven by flow, not seeping in when nothing is happening. That single observation moves the diagnosis from a leak to a pressure problem, and pressure problems in a drainage stack are air problems.

A blockage low in the system. Ruled out by the direction of the symptom. In a stack carrying water, the falling film drags air down with it, so the air pressure in the upper portion of the stack sits below atmospheric and the pressure at the base, where the flow turns into the horizontal drain and decelerates, sits above it. Negative pressure siphons trap seals; positive pressure blows them out as a burp during a surge. The complaint is at the top and it is a steady smell, not a burp, so the fault is on the negative side, at the top of the air circuit. A restriction low in the system would have moved the complaint to the ground floor and changed its character.

What the roof showed

Winter roof access first, and it is the most dangerous part of this whole call. Fall protection is rigged before the ladder goes up, under 29 CFR 1910.28 in general industry or 29 CFR 1926 Subpart M on construction work, and those two Parts do not share a trigger height, so decide which your job falls under before rigging. On ice, the ladder gets a secured top and a spotter, and the roof gets walked with the assumption that the membrane under the snow is a skating rink.

The 3-inch terminal had a visible rime lip. Measured with a rule down the bore, the ice ring extended about 5 inches down from the top and was roughly 0.6 to 0.8 inch thick at the tightest point, thickest on the windward side.

Stand upwind and to the side while probing it. When a closure lets go, the stack vents everything it has been holding, and the control for an inhalation route is placement and airflow, not a glove.

The geometry that makes a small number look big

Take the tightest section as an annulus for the arithmetic. Real closure is a lipped collar in the top few inches, thicker on the cold side, so this uniform model is a convenience for computing area and not a description of the shape.

Use an illustrative inside diameter of 3.0 inches. Actual DWV inside diameter is not the nominal size and varies with material and wall thickness, and that correction runs against you: at an actual 2.9 inches the clean area is pi x 1.45 squared = 6.61 sq in rather than 7.07, about 6.5 percent less, so every remaining-area figure below is slightly optimistic.

Clean area at 3.0 in: pi x 1.5 squared = 7.07 sq in.

The first half inch takes 56 percent of the opening. At the measured 0.75 inch, this terminal was down to a quarter of its area, which is why a building that had vented itself perfectly well for decades started siphoning its top-floor traps in one cold week.

Now the same arithmetic on a 4-inch terminal, illustrative inside diameter 4.0 in, clean area pi x 2.0 squared = 12.57 sq in:

• 0.50 in of ice leaves a 3.0 in bore and 7.07 sq in, which is the entire clean area of the 3-inch terminal.
• 1.00 in of ice leaves a 2.0 in bore and 3.14 sq in, which is what the 3-inch terminal had at half that ice thickness.

Going up one size buys about half an inch of additional ice thickness at any given absolute open area. That is the derivation behind cold-climate amendments that increase terminal size, and it is why height is not the lever. A taller terminal is more exposed cold pipe, which is more surface for deposition, so adding height makes closure marginally worse rather than better. Where the siting rules force a tall terminal, as they do on a used roof, increasing the diameter is the only control left.

Why the quiet buildings close first

Deposition happens whenever moist air meets cold wall. Removal happens when a slug of warm water goes down the stack, warming the pipe and surging air past the terminal. Closure is the net of the two, so it is worst where removal is weakest.

• A lightly used stack in a heated, humid building supplies plenty of moisture and almost no flushing. It closes.
• A busy commercial stack in the same climate and the same weather rarely closes, because it gets warmed and purged several times an hour.

That is the direction to remember and it is the opposite of the intuition that heavy use plugs things. If a building on your route has one stack serving a handful of quiet fixtures and another serving a laundry, the quiet one is the one to check in January.

Clearing it, and what not to reach for

No flame, no torch, no heat gun run to softening, and this is not a preference. Plastic DWV pipe overheated releases hydrogen chloride and other decomposition products; that is an inhalation hazard and the control is not applying the heat in the first place, because there is no practical respiratory control for someone kneeling over a roof penetration in the wind. Heat also softens the pipe at exactly the point that has to stay round.

Warm water poured down the terminal is the ordinary field method and it works, with two hazards your own instruction creates. Carry it up in a closed container with a lid, never an open bucket on a ladder, because a scald on a roof in January is a two-casualty event. And it lands on the roof and on the ladder footing as ice, so salt or sand the access before you pour, not after.

The permanent fix is the terminal size increase specified by the local cold-climate amendment, plus reducing exposed length to the minimum the siting rules permit, which is often not much of a reduction once the receptor rules covered in the sibling siting card have had their say.

Confirming it rather than assuming it

• Re-verify during cold weather, not in April. A terminal that reads clear at 40 degrees F tells you nothing about the same terminal at 5 degrees F with the same building behind it.
• Record the bore you measured and the depth you measured it at. "Iced" on a ticket is not a finding. "Ring approximately 0.75 in thick extending about 5 in down from the top of a 3 in terminal" is one, and it is what justifies the size increase to the customer and to the inspector.
• Check the symptom's clock, not just its presence. A smell tied to the building's discharge peak is a pressure problem. A smell that is there at three in the morning with nobody awake is a leak or a dry seal, and it is a different card.
• Confirm the direction before you commit. Traps siphoned at the top and a burp at the bottom are the two ends of one pressure gradient. If the complaint is at the bottom and it burps, do not go looking on the roof first.
• Run the non-invasive vent check before roof access where the weather makes the roof the hazard it is in January. The sibling HowTo on telling whether a vent is blocked without opening anything covers that test; it is worth doing from inside before anybody climbs.

References

• 29 CFR 1910.28, walking-working surfaces and fall protection in general industry; 29 CFR 1926 Subpart M, the construction counterpart, with a different trigger height
• The model plumbing code as adopted and amended by your local jurisdiction, which is the only version that binds, for terminal size increases in cold climates and for minimum height above the roof
• Manufacturer documentation for the temperature limits of the DWV material in use
• See related: Why a Vent Terminal Location Is a Code Decision, Not a Convenience; What a Drainage System Is Actually Doing With Air; How to Tell Whether a Vent Is Blocked Without Opening Anything