CO Rises and Draft Falls After an Economizer Bypass Is Installed
A 300 hp gas-fired firetube runs at 150 psig with a new flue economizer and bypass damper installed last month. Before the work: O2 3.0%, CO 25 ppm, stack temperature 470 F, breeching draft -0.09 in. w.c. Now, at high fire only: O2 5.8%, CO 750 ppm, stack temperature 390 F, draft -0.02 in. w.c., a low rumble from the burner, a hazy orange flame at the sight glass, and the flame scanner signal swinging plus or minus 15%. Low fire behaves normally. A portable CO meter reads 30 ppm near the breeching and the room smells faintly sooty.
1. Higher O2 should mean more air and less CO. Explain why both went up, and give the most likely physical cause plus how you would confirm it before touching burner air settings.
Read the worked answer
The contradiction is the clue. If the O2 sample were truly measuring air entering the burner, an O2 rise would drive CO down. When O2 and CO rise together, the analyzer is almost certainly seeing air that entered the flue downstream of the burner - tramp air dilution. The burner itself is running air-starved, producing CO; the dilute sample hides it.
Most likely cause: the economizer bypass damper, its linkage or its actuator is not in the position the control thinks it is, and a joint or damper-shaft packing is leaking air into the flue. That explains:
- O2 reading high (dilution);
- CO high (actual combustion is rich);
- draft at the breeching degraded (the leak is downstream of the draft tap, and the gas path is disturbed);
- lazy orange flame, rumble, and an unstable scanner signal;
- and the stack temperature 'improvement' from 470 F to 390 F may itself be an illusion - diluting hot flue gas with cool room air lowers the measured temperature without improving efficiency.
Confirm before adjusting anything:
- Shut the burner down. Inspect the damper blade, linkage, actuator and end switches; verify actual position against commanded position.
- Inspect the economizer and breeching joints, gaskets, and expansion joints for failures.
- With the burner at low fire, leak-check the breeching (smoke pencil or velometer at joints) and read draft at the furnace/burner and at the breeching to see where it is being lost.
- Re-zero and check the analyzer; sample at a second point if available.
2. State your immediate actions on discovering the 30 ppm room reading, and explain what not to do.
Read the worked answer
Immediate actions:
- Treat the room reading as an exposure event, not a tuning nuisance. Shut the burner down, start ventilation, and move non-essential people out of the boiler room.
- Confirm with a second instrument. A single portable meter, or a residential-style CO alarm, is not adequate evidence in a boiler room - and it is also not adequate evidence that the room is safe.
- Notify the chief engineer and plant safety. If anyone reports headache, dizziness or nausea, that person gets fresh air and medical attention.
- Do not re-enter to 'watch the flame' until the space is ventilated and the source is found.
What not to do:
- Do not open the air shutter or crank up excess air to chase the CO number. If the problem is draft and dilution, adding air to a lazy flame worsens the situation, and you will be tuning the burner against a false O2 reading.
- Do not raise firing to 'blow it out.' More fuel into an air-starved, poorly drafted combustion chamber is how you get a puffback or a furnace explosion.
- Do not accept the stack temperature drop as an efficiency gain until you have proven the gas path is intact.
3. After the repair, what would you require before the boiler is returned to normal automatic operation?
Read the worked answer
- Repair the damper and any failed joint, then verify actual-versus-commanded position through the full stroke, including the end switches and the bypass interlock.
- Verify draft at the furnace and breeching across the firing range, and confirm the values are back in the range recorded at commissioning.
- Retune the burner with a calibrated analyzer, using O2 and CO together rather than O2 alone, at low, mid and high fire. Hold at each point until readings stabilize.
- Confirm the flame signal is steady and the scanner does not drop out at any firing rate, and that the flame-failure response time is unchanged.
- Re-check the boiler room atmosphere with a calibrated instrument at high fire before anyone works in the space routinely.
- Document the pre-failure and post-repair readings side by side, the parts replaced, and who did the work - this is the record that proves the economizer is delivering real efficiency rather than diluted numbers.