Carbon monoxide (CO) from heating systems is produced primarily through the incomplete combustion of fossil fuels in gas-powered appliances. This odorless, colorless gas poses serious health risks when it accumulates indoors due to leaks, poor ventilation, or malfunctioning equipment. The most common sources in residential heating systems include gas furnaces with cracked heat exchangers, improperly installed or maintained appliances, and blocked venting systems. While air conditioners themselves don't produce CO, they can circulate existing leaks from heating systems through ductwork, exacerbating exposure risks.

Key findings from the sources reveal:

• Gas furnaces and boilers are the primary CO sources when combustion processes malfunction ^[2][4]
• Cracked heat exchangers in furnaces account for many residential CO leaks ^[5][7]
• Older heating systems with natural draft venting are particularly vulnerable to CO buildup ^[4]
• Leaky or unsealed HVAC ducts can distribute CO throughout homes ^[1][6]

Causes of Carbon Monoxide in Heating Systems

Combustion Malfunctions in Gas Furnaces

Gas furnaces produce carbon monoxide as a natural byproduct of combustion, but proper functioning systems vent this gas safely outside. The danger arises when combustion becomes incomplete or venting fails. Cracked heat exchangers represent the most critical failure point, as they separate combustion gases from breathable air. When these metal components develop fissures from age or stress, CO can leak directly into home air circulation ^[5][7]. Older furnaces (typically over 15 years) show higher failure rates due to material degradation ^[9].

Key malfunction causes include:

• Cracked heat exchangers: Result from thermal stress over years of heating/cooling cycles, allowing CO to bypass venting systems ^[5]
• Pilot light issues: Improperly burning pilot lights produce excessive CO and indicate combustion problems ^[9]
• Blocked vents: Obstructed chimneys or flues prevent proper CO exhaust, causing backdrafting into living spaces ^[4]
• Fuel line damage: Leaking gas lines create incomplete combustion conditions that generate higher CO levels ^[9]

Technicians emphasize that annual professional inspections can identify these issues before they become dangerous. The U.S. Department of Energy specifically recommends checking for soot accumulation near appliances and rusting on vent pipes as visual indicators of potential CO problems ^[8].

Ventilation and System Design Failures

Even properly functioning heating systems can become CO hazards when ventilation fails. Natural draft systems in older homes rely on passive airflow to remove combustion gases, but these designs often prove inadequate for modern sealed construction ^[4]. Leaky ductwork compounds the problem by drawing CO from furnace areas into living spaces through negative pressure ^[1][6].

Critical ventilation issues include:

• Unsealed ductwork: Gaps in HVAC ducts pull CO-contaminated air from furnace rooms into general circulation ^[6]
• Improper detector placement: CO detectors near supply vents may give false readings or fail to alert due to air movement patterns ^[3]
• Backdrafting: Leaky vent pipes create reverse airflow that pushes CO back into homes instead of outside ^[5]
• Inadequate fresh air intake: Tightly sealed modern homes without proper makeup air systems starve combustion appliances of oxygen, leading to incomplete burning ^[4]

HVAC professionals stress that proper system design must account for both active venting of combustion gases and passive air exchange. The installation of multiple CO detectors - particularly near sleeping areas and on each home level - provides critical early warning when ventilation fails ^[3][7].