What should no portion of a venting system extend through?

The correct choice is based on safety and efficiency standards for venting systems. Venting systems are designed to safely discharge combustion gases from equipment to the outside atmosphere without allowing harmful gases to accumulate within the structure.

When venting systems extend through fabricated air ducts, there is a risk that these ducts can interfere with airflow and possibly lead to the accumulation of harmful gases or combustion byproducts inside the duct system. This can create hazardous conditions, including potential exposure to toxic gases within living spaces. Therefore, it is critical that venting systems do not penetrate fabricated air ducts to maintain proper ventilation practices.

The other options are scenarios where venting systems may be appropriate or acceptable under correct installation practices provided they are designed accordingly and adhere to safety codes. Building walls can be penetrated for venting systems if proper installation methods are applied, as can open ceilings if done correctly. However, extending through fabricated air ducts contradicts the safety protocols needed to ensure that venting works effectively without compromising air quality and safety within the building.