Doing a combustion analysis on a 90+ condensing furnace.

Key Readings for 90+ Condensing Furnace Combustion Analysis:

1. Oxygen (O₂): Target 2-4% (condensing furnaces operate with excess air).
2. Carbon Monoxide (CO): Must be <50 ppm (ideally <10 ppm) in undiluted stack gas.
3. Carbon Dioxide (CO₂): Typically 8-10% for natural gas.
4. Stack Temperature: Should be <125°F (ideally 90-110°F) for high efficiency.
5. Efficiency: Should be ≥90% (as rated).
6. Draft/Pressure: Negative draft in combustion chamber; vent should be neutral to slightly negative.

Stop-the-Job Numbers (Unsafe Conditions):

• CO > 50 ppm in stack gas (immediate hazard).
• O₂ > 15% or <1% (indicates combustion air issues).
• Stack temp > 200°F (possible heat exchanger breach or improper venting).
• Visible soot or rollout at heat exchanger (safety hazard).

Code Reference: Per NFPA 54 (2020), Section 7.11, CO in occupied spaces must not exceed 35 ppm (8-hour exposure). For stack CO, ANSI Z21.45 requires <50 ppm for gas appliances.

Action: If any stop-the-job number is hit, shut down the furnace, ventilate, and investigate (e.g., cracked heat exchanger, blocked vent, or improper gas pressure). Retest after corrections.

Target Readings (High Fire, after 5–10 min run time):

| Parameter | Target Range | Code/Standard Ref | | :--- | :--- | :--- | | O₂ | 4.5% – 6.5% (≈ 8.5%–9.5% CO₂) | NFPA 54 / ANSI Z223.1; Mfr specs | | CO (air-free) | < 100 ppm (Ideally < 50 ppm) | AHRI Guideline X; NFPA 54 §10.1.1 | | CO/CO₂ Ratio | < 0.008 (0.8%) | AHRI Guideline X | | Stack Temp | 80°F – 120°F (must be < 140°F to condense) | Mfr specs; Condensing physics | | Draft/Pressure | Positive (per mfr, typically +0.05” to +0.20” w.c.) | Mfr install manual | | ΔT (Supply-Return) | 35°F – 65°F (check nameplate rise range) | Mfr specs; ACCA Manual S |

🛑 STOP-THE-JOB NUMBERS (Shut down immediately):

1. CO (air-free) > 400 ppm — Immediate hazard. (AHRI Guideline X / NFPA 54 §10.1.1).
2. CO (as-measured) > 100 ppm in flue with O₂ < 3% — Indicates severe rich burn/flame impingement.
3. Flue Gas Spillage / Negative Draft on a Category I/II/IV appliance not designed for it — CO entering CAZ.
4. Stack Temp > 140°F on a 90%+ unit — Not condensing; heat exchanger fouled, over-fired, or airflow critically low.
5. CO in Supply Air > 9 ppm (ambient limit) or any CO detected in living space — Cracked HX or backdrafting.

Critical Workflow:

1. Zero analyzer in fresh air.
2. Clock gas meter — verify input = nameplate ±2%. Wrong input invalidates all other readings.
3. Test High Fire first (worst case for CO).
4. Test Low Fire — CO often spikes here on modulating units.
5. Measure combustion air at burner inlet (O₂ ~20.9%). If < 20.5%, solve air starvation first.
6. Adjust gas valve only if O₂/CO₂ off spec and input clocked correct. Adjust in 1/8-turn increments. Re-clock after adjustment.

If CO > 100 ppm air-free: Check gas pressure, primary air, burner alignment, HX blockage, venting. Do not leave running.