When might a combination of air and flue gas be problematic in a venting system?

The combination of air and flue gas can become problematic in a venting system primarily when condensation occurs. This is because flue gases often contain water vapor as a byproduct of combustion. When these gases cool, especially when mixed with cooler air, the water vapor can condense into liquid water.

This condensation poses several issues: it can lead to the corrosion of venting materials, compromise the effectiveness of the venting system, and potentially create hazardous conditions by allowing exhaust gases to escape back into the living space instead of being properly expelled outside. Maintaining a proper temperature in the venting system is crucial to prevent this condensation, ensuring that flue gases remain above their dew point.

In contrast, during installation, the presence of air and flue gases may not be an immediate concern as the system is being set up for proper functioning. With type "B" vents used outdoors, while there are specific installation and operational guidelines to follow, the risk associated with air and flue gas is more critical once operational processes lead to cooling and potential condensation. Additionally, while operational characteristics and efficiencies are important, the condensation issue is specifically tied to the interaction of temperature and the presence of moisture in the exhaust gases.