Carbon monoxide is a poisonous gas that forms as a result of incomplete combustion. The combustion process requires the correct mixture of fuel and air. Examples of fuel are natural gas, wood, kerosene, and gasoline. Air consists primarily of oxygen and nitrogen. If there is too little air, fuel goes unburned and carbon monoxide (CO) is created. CO is measured in parts per million (PPM).


People get CO poisoning by inhaling it. From the lungs, it is transported throughout the body by way of the bloodstream. Once in the bloodstream, CO attaches to the hemoglobin and depletes the body of oxygen. At a high enough concentration, CO can cause an individual to become unconscious, suffocate, and die. Conditions related to CO poisoning include:

  • Brain damage;

  • Nausea;

  • Vomiting;

  • Dizziness;

  • Body weakness;

  • Headaches; and

  • Flu-like symptoms.

    Nearly as dangerous as exposure to increased levels of CO is exposure to lower levels of CO over a long time period.


    Equipment that uses the combustion process produces carbon monoxide. CO is also produced from campfires and charcoal grills, but for our discussion we will concentrate on HVAC-related equipment. If the equipment is properly tuned, the integrity of the flue gas passageway is whole, there is sufficient fresh air for combustion, and the venting pathway is clear and sound, CO is vented to the atmosphere. Alter this equation in any way, and CO has an increased chance of remaining in the building and threatening the safety of the occupants. Common factors that contribute to CO production relative to HVAC equipment include:

  • Excessive gas pressure;

  • Inadequate gas pressure;

  • Poor venting and drafting;

  • Burner misalignment;

  • Rust, soot, and dirt;

  • Mechanical deficiencies;

  • Air and fuel impingement; and

  • System ignition and shutdown.


    It is suggested that CO alarms meet the UL 2034 listing. Factors to consider when choosing an alarm include whether it reads in real time, its sensor and battery life, calibration, and sensitivity. Detectors must alarm within a certain time period based upon concentration levels of CO. The alarm setpoints under UL 2034 are designed for healthy people. This type of alarm can be found at home improvement and hardware stores. Infants and people with chronic illnesses are susceptible to lower levels of CO, which cannot be measured by store-bought alarms. A more sensitive alarm is recommended for these individuals. This type of alarm is available to contractors through wholesale suppliers.


    This question is asked in nearly all of the leading reference guides and by better educational institutions regarding testing for CO: Where is the first and most important place to test for CO inside a building? After obtaining a baseline reading outside of the building, it should be an established priority to first check for CO in the area where you will be working. In order to perform thorough diagnostic procedures inside a structure, the atmosphere must be safe to test within. If CO levels are excessive upon entering the structure, one is advised to leave immediately. Personnel with protective breathing apparatus can re-enter the building, turn off the equipment, and air out the building.


    Today the most common piece of testing equipment used is the electronic digital carbon monoxide meter. The digital CO meter is very sensitive, with some having the ability to read under five parts per million. It also has the ability to sample CO directly from the flue or other high-temperature areas. The sample is cooled by way of a hose and probe assembly attachment.


    Although CO testing procedures and locations are beyond the scope of this overview, a methodical approach to CO diagnostics should be adopted. The technician should have a thorough knowledge of how the HVAC equipment, ductwork, and venting systems interact with the building. Furthermore, the technician should incorporate testing equipment that is routinely calibrated, inspection checklists, and safety guidelines in his or her diagnostic approach. The Carbon Monoxide Safety Aware-ness Advisory Council has put together a Standards and Best Practices protocol for CO diagnostics.

    Rothacker is a director of For questions or comments on the Tech Page, contact Rothacker at (e-mail).

    Publication date: 11/18/2002