The Vent That Passed the Code and Failed the Instructions

Why this matters

A tech can measure every clearance in the code book's vent termination table, get every one right, and still fail the inspection, because the appliance's own installation instructions are also enforceable and they usually ask for more. This case was reconstructed afterward from the shop's own job file, and the most useful finding was not in the measurements. It was in what the file showed the tech had and had not looked at.

The correction notice

A shop installed a 60,000 Btu/h input condensing furnace, two-pipe direct vent (intake and exhaust both run to the outdoors, so the burner draws no combustion air from the house), terminating through a side wall on the north face of a house. The final inspection failed on one line:

Vent termination not installed per manufacturer's instructions: clearance to inside corner and height above grade.

The lead tech was certain it was right. "I measured all of it. It meets the code." Both statements were true.

Reading the job file backwards

The service manager started from the last record and worked toward the first, asking at each point what the tech knew at that moment.

The final inspection photo set. Termination from 10 feet back, termination close up with a tape measure showing 13 inches from grade to the bottom of the exhaust, and a tape showing 14 inches from the exhaust to the bottom of an openable window above it. No photo showed the distance to the inside corner where the house's wall met a bump-out, about 10 inches from the intake pipe.

The commissioning sheet. Temperature rise, gas pressure, a combustion analysis with acceptable CO. All fine. Commissioning measures how the appliance burns, not where it vents.

The tech's notes from the rough-in. "Term meets code: 12 in min above grade, 12 in min from window for direct vent over 50k." That is a correct paraphrase of the direct-vent row in the fuel gas chapter's termination table (G2427.8 in the 2021 IRC, as the jurisdiction adopted it): for a direct-vent appliance over 50,000 Btu/h input, the terminal sits at least 12 inches from any door, operable window or gravity air inlet, and the bottom of the terminal at least 12 inches above grade. The tech had measured against the table and the table passed.

The instruction sheet photo. The file held one photo of the sheet: its cover and revision block. None of the venting pages. The tech had captured the sheet's identity, which the shop's procedure asked for, and never opened it to the termination section.

The quote. "Sidewall termination, north wall, by bump-out." The estimator had picked the location from the inside, where the furnace sat close to that wall and the run was shortest.

That sequence is the finding. From quote to final, every decision about the termination's location was checked against the code table and never against the sheet, and the record shows the exact point where the sheet was in the tech's hand and unread.

What the sheet asked for

The service manager opened the venting section of the sheet that had shipped with the furnace. Its termination diagram, which ran to a full page, carried requirements the code table does not address at all, plus some that were stricter than the table:

• a minimum distance from the terminal to an inside corner of the building, larger than the 10 inches as installed, because exhaust trapped in a corner can be drawn back into the intake;
• a minimum height of the terminal above the expected snow level, stated as so many inches above the highest anticipated snow line, not just above grade;
• minimum separations between the intake and exhaust, and from soffits, the gas meter and regulator, and adjacent terminations.

The numbers below are the shape of what this sheet asked for, not a figure to carry to another appliance; every manufacturer's diagram is different, which is why the diagram is the source.

 NORTH WALL ELEVATION (as installed)

        +--------+   operable window
        |        |
        +--------+
            |  14 in (table: 12 min)  OK
         [EXH][INT]-- 10 in --| inside corner
            |                 |  (sheet: more) FAIL
            |  13 in above grade
  - - - - - - - - - - - - - - - snow line (sheet)
  ========== grade ==========
   sheet: terminal 12 in above snow line
   county design snow depth puts that near 24 in

The inside-corner distance failed outright. The height failed once the snow line was applied: the sheet called for the terminal to sit 12 inches above the expected snow level, and with the depth the county's building department uses for design, that meant roughly 24 inches above grade, not 13.

The inspector's reading, and why it was right

The tech's instinct was that the code sets the clearances and the sheet is advice. The inspector's reading was the opposite, and it follows the text. The fuel gas chapter of the IRC, as adopted, requires appliances to be installed in accordance with their listing and the manufacturer's installation instructions, and the termination table is written as a set of minimums that does not displace those instructions. A listed appliance's instructions are part of what was tested; the lab evaluated the terminal at positions the manufacturer specified. When the sheet and the code table differ, the installer meets both, which in practice means meeting the stricter of the two at every clearance. See related: The Installation Instructions Are Part of the Code, and When the Instructions and the Code Disagree, which owns the case where they genuinely conflict rather than one being stricter.

The table was never wrong. It is a floor written for every appliance; the sheet is the specification for this one.

Interim safety while the fix was scheduled

Before deciding the fix, the service manager dealt with the house as it stood. A terminal close to an inside corner and close to the snow line can recirculate exhaust into the intake or get buried, and in a direct-vent furnace that usually shows up as pressure switch lockouts, poor combustion or, in the worst case, exhaust around the house's openings. The tech returned the next morning, confirmed the house had a working CO alarm on the level with the furnace and outside the sleeping areas (tested with its test button, not by exposing it to gas), re-ran a combustion analysis with the burner at steady state, and told the customer in plain terms: keep the pipes outside clear of snow and drifts, and if the CO alarm sounds, everyone leaves the house and calls 911 from outside. The fix was scheduled within the week.

The move

Three options were priced in crew hours.

Relocate the termination along the wall, away from the bump-out. Moving 4 feet along the wall cleared the corner but did nothing for the snow-line height, and it added two elbows inside. About 3 crew hours plus siding patch. Solves one of two failures.

Go through the roof instead. Clears both, but the run up through a finished closet and the roof penetration and flashing took it to roughly 7 crew hours and added a roofing detail the shop does not like to create in winter.

Raise the terminal on the outside with the manufacturer's riser configuration, at a new hole 4 feet along. The sheet showed a vertical riser arrangement on the exterior of the wall to lift a sidewall terminal for snow, with its own support and clearance requirements. Combined with the 4-foot move, it cleared both the corner and the snow line. The service manager checked the new total equivalent vent length, counting each elbow at the sheet's stated equivalent length, against the sheet's maximum for that pipe diameter before approving it; the new run stayed inside the limit. About 4 crew hours plus siding patch at the old hole.

The shop chose the riser. The re-inspection passed, with the sheet's venting pages photographed and in the file this time, and a photo of the tape at the corner and from the snow-line mark.

What the shop changed

• Termination location is picked against the sheet at the estimate, not the rough-in. For any new vent termination, the estimator downloads the current instructions by model and reads the termination diagram before choosing a wall.
• The photo checklist now asks for the venting pages, not just the cover. The shop's listing and instructions check already required the revision block. It now also requires a photo of the termination diagram next to a photo of the measured location for each clearance in it. See related: The Listing and Instructions Check SOP.
• "Meets code" is not written on a vent note by itself. The note reads "meets table and sheet," with both minimums written down.

Where the same check turns out differently

On a south wall in a snow-free county, the same installation might have passed: the snow line requirement collapses to the grade requirement, and if the corner had been farther away the code table and the sheet would have agreed. That is the trap. A shop that installs in mild climates learns that the table is enough, then installs the same way in a jurisdiction with real design snow, and the habit fails there first. A different manufacturer's sheet can also be looser than this one on the corner and stricter on the soffit, so a habit built from one brand's diagram is not portable either.

References

• International Residential Code, Chapter 24 (Fuel Gas), G2427.8 in the 2021 edition, as adopted by your jurisdiction: through-the-wall vent terminal clearances, including the direct-vent row.
• The appliance's own installation instructions, venting and termination section, at the revision shipped with the unit.
• See related: Venting Systems Listed With the Appliance; How to Read a Code Section, Exceptions and All.