What it is

Combustion is just fuel plus oxygen plus an ignition source, releasing heat. On a gas furnace we want complete combustion: methane (or propane) plus enough air burns to carbon dioxide and water vapor, and that's it. When combustion goes incomplete — not enough air, poor mixing, flame impingement, a starved or sooting burner — you get carbon monoxide (CO), an odorless, colorless, deadly gas. CO is the whole reason combustion work is safety work.

The heat exchanger is the steel barrier that keeps those combustion products on one side and the house air on the other. The blower moves house air over the outside of the exchanger; combustion happens inside it; the flue carries the byproducts out. If that steel cracks, flue gas can mix into the air you're heating the home with — which is why a cracked exchanger is a red-tag, not a "watch it."

How it works

Complete combustion of natural gas burns clean and blue, producing mostly CO₂ and water vapor, with a small amount of excess air left over (you always run some extra air for safety margin). A clean, sharp blue flame is the visual signature.

Incomplete combustion happens when the fuel can't find enough oxygen to fully oxidize, or the flame gets chilled/quenched before it finishes burning (flame touching cold metal = "impingement"). Carbon that should have become CO₂ stops at CO instead. The usual causes: insufficient combustion air (tight mechanical room, blocked louvers, exhaust-fan depressurization); dirty/sooted or misaligned burners and lifting/floating flame; overfiring; flame impingement from a cracked or rusted exchanger; and a blocked or restricted flue that starves the burner of draft.

A combustion analyzer reads the flue gas and tells you what's actually happening: oxygen (O₂), carbon monoxide (CO), and stack/flue temperature, and it calculates combustion efficiency from those.

In the field

Run a combustion analysis (the right way to judge a furnace):

  1. Let the furnace run to steady state (give it several minutes; readings drift until the exchanger is hot).
  2. Sample the flue gas in the correct spot per the analyzer/appliance (typically in the flue, in the products stream). On a Category I (natural draft / 80%) unit you sample the flue; on a Category IV (condensing) unit you sample the proper port.
  3. Read O₂, CO (air-free is the meaningful number), and flue temp.

Finding a cracked heat exchanger — there's no single magic test, so stack evidence:

  • Visual / camera inspection. Pull the blower or use an inspection camera to look at the exchanger faces and corners. Cracks love the bends and the weld seams. This is the most direct evidence.
  • Flame disturbance test. Watch the burner flames with the burners lit, then have the indoor blower cycle on. If the flames suddenly roll, flutter, change shape, or pull/blow when the blower starts, air is getting from the blower side into the combustion side — that's a breach. (Sooted/dirty burners can mimic this, so corroborate.)
  • CO behavior. A spike in CO when the blower kicks on can indicate a breach disturbing combustion.
  • Visible flame rollout, soot tracks, rust scaling, or heat discoloration at the burner box.

Don't condemn on one ambiguous sign alone — but if you have solid visual confirmation of a crack or hole, that's a shutdown.

Normal values & targets

These are general field reference ranges for a residential gas furnace at steady state. Always defer to the manufacturer's stated targets.

  • O₂ in the flue: roughly 6–9% on many natural-draft/80% furnaces (excess air for safety). Condensing units often run a bit different — check the spec.
  • CO (air-free): as low as possible. Clean, well-tuned furnaces commonly read well under ~100 ppm air-free. Readings climbing into the few-hundreds ppm and up signal a combustion problem that needs correcting. High and climbing CO is your warning sign — find the cause.
  • Flue/stack temperature: varies widely by furnace type and firing — condensing units run cool (often well under ~140°F net so they can condense), 80% units run much hotter. Use it for efficiency and to spot over/underfiring, not as a pass/fail by itself.
  • Ambient CO in the living space: should be ~0 ppm. Any meaningful CO in occupied space is an immediate problem.
  • Temperature rise across the furnace: must fall within the rating-plate range, commonly something like 35–65°F (the plate gives the exact window). Outside that range points to an airflow or firing problem that also affects combustion and exchanger stress.

Common faults & what they mean

  • High CO, normal-ish O₂ — flame impingement or poor burner condition rather than pure air starvation. Look at burner alignment, soot, exchanger condition.
  • High CO and low O₂ — not enough combustion air, or overfiring. Check combustion-air openings, room depressurization (kitchen/bath fans, dryer), and manifold pressure.
  • CO spikes when the blower starts — strong indicator of a heat-exchanger breach; confirm visually.
  • Temperature rise too high — weak airflow (dirty filter, closed registers, slow blower). High rise overheats and stresses the exchanger — a leading cause of cracks over time.
  • Temperature rise too low / cold flue / condensation on an 80% unit — overfiring or excessive airflow; sustained condensation rots out exchangers and flues.
  • Lazy yellow flames, soot — incomplete combustion; expect elevated CO. Find the air or fuel problem.

Tech tips & gotchas

  • CO is air-free CO that matters. "As-measured" CO gets diluted by excess air; analyzers calculate air-free CO so two furnaces are comparable. Quote air-free.
  • A furnace can make dangerous CO and still "work." The homeowner feels heat. Only the analyzer tells you it's poisoning the air. Put the probe in the flue on every combustion service — that's the standard of care.
  • Cracked exchangers usually come from a history of overheating — chronic low airflow (filters left in too long, undersized returns) cooks the steel through thermal cycling. When you find a crack, find why, or the replacement cracks too.
  • Don't run the flame-disturbance test as your only proof. Dirty burners flutter too. Pair it with a camera look and CO behavior.
  • A blocked flue is an emergency. It starves combustion (CO up) and can spill products into the house. Verify draft and termination aren't obstructed (nests, ice, collapsed liner, disconnected pipe).
  • Worst-case depressurization test: turn on every exhaust appliance (range hood, baths, dryer) and re-check draft and spillage. A house that pulls the furnace into spillage under exhaust load is a real hazard even if the furnace tests clean in isolation.

Safety / code notes

  • Confirmed cracked or breached heat exchanger = take it out of service. Shut it down, red-tag it, and inform the customer in writing. Do not "monitor" a breach that can put CO into living space.
  • Always verify CO with a calibrated combustion analyzer before and after combustion work — disclaimers don't substitute for measurement. Keep your analyzer calibrated.
  • Combustion-air openings are sized per the fuel-gas/mechanical code combustion-air provisions (e.g., the two-opening method commonly figured around 1 in² of free area per 1,000 BTU/h, with the all-indoor and outdoor-air methods detailed in the code). Confirm the appliance has adequate combustion air before chasing CO.
  • Venting must match the appliance category (I–IV) and follow the fuel-gas code venting sections for sizing, slope, clearances, and termination.
  • CO alarms in the dwelling are required by many jurisdictions — recommend/verify working CO alarms regardless. If you measure CO in occupied space, get occupants to fresh air and treat it as an emergency.