Final answer:
Venting spillage can be caused by an improper combustion air mixture, excessive elbows in venting systems, or a blocked condensate furnace drain, which all disrupt the safe and effective expulsion of combustion gases.
Step-by-step explanation:
Venting spillage can be caused by multiple factors, including: A. Improper combustion air mixture, which affects the appliance's ability to effectively burn fuel; B. Excessive elbows in venting system, which can restrict the flow of exhaust gases and cause backdraft; and D. Blocked condensate furnace drain, which can affect the pressure and the safe operation of the furnace. A cracked heat exchanger (C) is also a serious issue but typically results in mixing combustion gases with indoor air rather than venting spillage.
Combustion pollutants, such as carbon monoxide, are produced when appliances burn fuel - and venting is crucial to safely expel these pollutants from indoor spaces. Excessive elbows can impede the smooth flow of gases, and a blocked drain can change the internal pressure dynamics of the appliance, both contributing to venting spillage.