Kai Mallari's Notes'

CO-Is-the-Load-Bearing-Hazard-of-Any-Gas-Heating-System (Home Systems)

4 min read

CO Is the Load-Bearing Hazard of Any Gas Heating System

idea

Claim: In a gas furnace or boiler, the heat exchanger is the only barrier between combustion gases (including carbon monoxide) and your breathing air. When that barrier cracks, CO leaks silently into the home. This is not a gradual, visible failure — it is invisible, odourless, and lethal. Annual service by a licensed gas fitter is the only reliable detection mechanism; a CO detector on every sleeping floor is the only survival fallback.

Mechanism

A gas furnace burns natural gas inside a sealed metal heat exchanger. Return air from the home circulates around the outside of the exchanger, picks up the heat, and is blown through ducts. Combustion gases — including carbon monoxide — are supposed to stay inside the exchanger and exhaust through the flue.

As the furnace ages (typically 15–20 years of heating seasons), the metal undergoes repeated thermal expansion and contraction cycles. This eventually causes hairline cracks in the exchanger. When cracks form:

  • Combustion gases mix with the circulating air stream
  • CO — colourless, odourless, tasteless — enters the duct system
  • Every heating cycle pumps CO into every room

Technical Safety BC’s annual gas-appliance service bulletin explicitly lists CO measurement as a required step in annual furnace service, and identifies this as the primary safety concern for gas appliances in BC.1

The compounding risk: CO poisoning symptoms (headache, nausea, dizziness, fatigue) mimic the flu. Occupants often don’t recognize the source. Prolonged low-level CO exposure causes cumulative harm before high-level acute exposure triggers alarm responses.

Why Annual Service Catches This (and Nothing Else Does)

A cracked heat exchanger is not visible from outside the furnace. It does not produce a smell (CO is odourless). It does not cause a gas smell (gas odorant is in the supply line, not the flue). The burner may flame normally. The furnace may heat normally.

The only reliable detections:

  • Annual licensed gas fitter service: includes heat exchanger visual inspection (and camera inspection on aging units) plus live CO measurement during operation
  • CO detector activation: the fallback when the exchanger has already cracked

Neither filter checks nor thermostat adjustments catch this. Owner-performed maintenance cannot detect it.

Scope

  • Applies to gas furnaces and gas boilers — any appliance that burns natural gas and has a heat exchanger
  • Applies to gas water heaters — same heat exchanger mechanism, same CO risk (see water-heater (Home Systems))
  • Does NOT apply to heat pumps, electric baseboards, or electric furnaces — no combustion, no CO risk
  • Does NOT apply to fireplace gas inserts or log sets — those have separate venting considerations (see gas-appliance-venting (Home Systems))

Idea Compass

North: Where this comes from

  • heating-system (Home Systems) — the furnace component this idea lives inside
  • Technical Safety BC’s annual gas-appliance service bulletin — the regulatory source1

East: Tensions / failure

South: Where this leads

West: What’s similar

Sources

Footnotes

  1. Technical Safety BC, the BC gas-safety regulator — Information Bulletin: Annual Servicing for Gas Appliances; CO measurement is a required annual service item; “all gas fired appliances require regular maintenance to operate efficiently and safely” — https://www.technicalsafetybc.ca/regulatory-resources/regulatory-notices/information-bulletin-annual-servicing-gas-appliances ↩ ↩2

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