These rules may not always result in successful venting in actual buildings. Only testing can provide an indication of the operation. We have found many units where the appliance is not able to operate correctly and that just happened to keep the unit from being a major liability to health and safety. Changing any portion of the operation may affect safety. The client's operation of the unit can also affect safety.
Luck is the final card.
It is the random combination of the first four factors and other things that affect the building. Simple things like unclogging a dryer vent, fixing a bath fan, repairing ducts, or insulating walls, can change the operating patterns of the combustion devices. Very few HVAC installers have the equipment necessary to ensure a safe installation of a combustion appliance is completed. Many units create CO because of improper setup and testing. Problems with gas pressure, orifice size, and improper venting are the most common.