Analyzer Reads Low O2 and High CO After a "Tune-Up"

The burner is starved of air, and it is not mixing well enough to finish the job.

• O2 at 1.0% is far below the 2.5–3.5% you would normally target on a packaged gas burner. At that level there is essentially no margin - any upset in draft, gas Btu content, or linkage position pushes the unit into incomplete combustion.
• CO at 1,800 ppm is the proof it already has. CO is unburned carbon monoxide; it means fuel is being broken down but not oxidized to CO2. This is both an efficiency loss and a serious health hazard if it migrates into the boiler room.
• CO2 at 11.6% looks "good" and is meaningless. CO2 rises whenever you cut excess air, whether or not combustion is complete. Chasing a high CO2 number is exactly how the contractor got here. CO2 alone cannot distinguish good combustion from a rich, smoking one.
• The draft readings explain the rest. Firebox at -0.02 in. w.c. against a -0.10 design, and breeching at -0.04 against -0.18, means the unit is not pulling its design air volume. No amount of damper fiddling will fix a draft problem; the damper adjustment is what broke it.

Treat the 1,800 ppm CO as an immediate hazard, not a tuning parameter.

Restore draft before touching the fuel/air settings. Tuning a burner against a blocked or restricted flue is wasted effort.

1. Verify the breeching damper, barometric damper, and stack for obstruction or a damper stuck shut. Check the FD fan inlet, screens, belt condition, and fan rotation. Confirm the chimney is not blocked (bird screen, collapsed liner, debris).
2. Re-establish design draft - firebox about -0.10 in. w.c., breeching about -0.18 - at high fire.
3. With draft restored, open the air until you have roughly 3% O2, and then confirm that CO falls below about 100 ppm (ideally to single digits). CO2 for natural gas at 3% O2 lands near 9.5–10%. If O2 is at 3% and CO is still high, the problem is mixing, not total air - check the burner register, swirler, and gas manifold pressure.
4. Repeat at low fire and mid fire. A modulating burner is only safe if it is tuned at every point on the curve; the linkage and cam positions must follow the manufacturer's fuel/air curve, not just the high-fire point.
5. Record O2, CO, CO2, stack temperature, and draft at each firing rate, and pin or lock the manual damper so it cannot be relocated by the next person who wants a prettier CO2 number.

Note on the stack temperature: 415 F at 70% load is not a reliable efficiency indicator while the draft is wrong - the unit is not operating on its design curve. Re-baseline it after the tune.

During the work:

• Put a portable CO monitor in the boiler room and in any adjacent occupied space; combustion at 1,800 ppm CO can spill through an open breeching or a failed gasket and the gas is invisible and odorless.
• Do not work alone, and do not prop open the boiler room door to "air it out" - that changes the draft you are trying to measure.
• Expect delayed ignition after any burner adjustment. Verify the pilot and main flame establish properly at each restart; watch for a rumble or a hard light-off.
• Watch the flame signal - a flame that is being starved of air can be unstable enough to nuisance-trip or, worse, to lose the scanner while still burning.

Before leaving it in service:

• Confirm stable low-fire operation (the most common place for a modulating burner to go unstable).
• Re-check CO at each firing rate, not just at the rate you tuned.
• Verify the flame safeguard, the high-pressure limit, and the low-water cutoff trip tests are current.
• Inspect the fireside for sooting or discoloration from yesterday's rich operation.
• Write everything into the log, including the contractor's original settings, so the next operator knows what was changed and why.