Forced draft systems and portions of induced draft systems must be designed to be __________ to prevent leakage of combustion products.

In the context of forced draft systems and certain sections of induced draft systems, the primary goal is to prevent combustion products from leaking into the environment, which can pose health hazards and contribute to pollution. Therefore, these systems must be designed to be gas tight.

Being gas tight ensures that any potentially harmful gases, such as carbon monoxide and unburned hydrocarbons, are contained within the system and do not escape into the atmosphere. This requirement is crucial for maintaining safety and compliance with environmental regulations. A gas-tight design typically involves sealing joints, using appropriate materials that can withstand high temperatures and corrosive substances, and ensuring that any potential pathways for gas escape are eliminated.

While other options might appear relevant, they do not specifically address the need for managing potentially toxic gases produced during combustion processes. For instance, being air-tight could suggest limiting air leakage, which is important but not the primary concern here. Similarly, water tightness is irrelevant in this context since we are focusing on gases, and pressure resistance relates more to the structural integrity under operating conditions than to the containment of exhaust gases. Thus, the requirement for these systems is specifically about achieving a gas-tight configuration to ensure safe operation.