Vent termination rules. Episode fifteen of the Gas Service Animated series.

A vent can do everything right inside the house and still fail at the last inch. If flue gas exits too close to a window or an air inlet, the building's own pressure field can carry that exhaust right back indoors. That is called re-entrainment, and it means carbon monoxide and moisture end up in the living space. Termination clearances are not about looks. They exist to keep the flue gas outside, where it belongs.

Up on the roof, a gas vent finishes with a listed cap installed to its listing. How high the vent must rise depends on the roof pitch. The code's termination table lets a small vent on a flat or gently sloped roof terminate lower, while a steep roof demands more height, so read the table for the exact pitch. And that table only applies to vents twelve inches or smaller that sit at least eight feet from a vertical wall. Closer than that, or larger than that, the vent must rise clear like a chimney, at least two feet above the nearby construction.

Masonry chimneys have their own numbers. A chimney serving residential type appliances must extend at least three feet above the highest point where it passes through the roof, and at least two feet higher than any part of the building within ten feet. It must also stand at least five feet above the highest connected draft hood outlet or flue collar. And decorative shrouds are allowed only when they are listed for that specific chimney system.

On a sidewall, the code draws a clearance map around the terminal. A direct vent terminal sits at least twelve inches above grade and above any deck or porch, and at least seven feet above a paved public walkway. Clearance to an operable window, door, or gravity air inlet scales with appliance input: six inches for the smallest units, nine inches up to fifty thousand Btu per hour, and twelve inches up to one hundred fifty thousand. Terminals that are not direct vent need far more room: four feet below or beside an opening, and one foot above it. Corners, soffits, and fixed windows follow the table and the appliance listing.

One number on that wall towers over the rest: the mechanical air intake. A powered intake fan does not wait for exhaust to drift in, it pulls it in. So a vent terminal must stay at least ten feet horizontally from a mechanical air supply inlet, or sit at least three feet above it. On the roof, the same idea applies: a gas vent terminates at least three feet above a forced air inlet located within ten feet. Find every intake before you call a termination good.

The code also protects the path itself. Nothing may obstruct a venting system except specific listed devices installed to their instructions. In the field the obstruction is usually simpler: a bird nest under the cap, a crushed or corroded cap, a collapsed liner. Block the top, and the failure shows up at the bottom, as flue gas spilling from the draft hood far below. Screens belong in place, intact, and completely clear.

Terminations are approved on the day they are installed, but buildings keep changing. Snow drifts bury a low sidewall terminal. A new deck gets framed around one. An addition turns an outside wall into an inside corner, and suddenly the old clearances no longer apply. After any exterior remodel, the terminations must be checked again.

So on a service call, the termination gets measured, not eyeballed. Confirm the cap is listed, intact, and clear. Pull the tape on heights and clearances to windows, inlets, grade, and walkways. Look for new construction crowding the terminal, and investigate any condensate stains on the wall. When in doubt, defer to the code tables, the appliance listing, and your company's procedures.

Where the flue lets go decides whether it ever comes back. Roof and chimney heights, the sidewall clearance map, the big mechanical intake number, and a clear unblocked cap. Measure them all. Next episode: masonry chimneys and liners.