In building codes, why is venting contractors advised against common venting?

The recommendation against common venting in building codes is primarily due to the risk of cross-contamination of fumes. When multiple appliances are connected to a common venting system, the exhaust gases from one appliance can potentially flow back into the air supply of another appliance. This can lead to hazardous situations, particularly if harmful gases, such as carbon monoxide, are present. Proper venting design ensures that each appliance can operate safely and efficiently without impacting the performance or safety of others within the same building.

The other answer choices, while they may touch on relevant factors to consider in venting design, do not directly address the critical safety concern associated with common venting systems. Being aware of the potential for cross-contamination helps ensure compliance with safety regulations and protects the health of building occupants.