The measurement and interpretation of indoor carbon dioxide CO₂ concentration can provide information on building indoor air quality and ventilation. On the other hand, concentration of carbon monoxide CO can show as how combustion process run and if the boiler is safe. When there is not sufficient air available to complete the combustion process, some of the fuel is left unburned, resulting in inefficiency and undesirable emissions. An examination of the CO₂ and CO concentration in boiler and interpretation results help to improve indoor air quality. The paper presents characteristics of concentration CO₂ and CO depend on used fuel in tested boiler rooms. The concentration curves show how each fuel combustion affect the amount of CO₂ and CO that is produced.