Which factor is indicated to negatively affect draft in a venting system?

The factor that is indicated to negatively affect draft in a venting system is long horizontal runs. In a venting system for gas appliances, draft refers to the flow of combustion gases and air through the venting system. Longer horizontal runs can create resistance to airflow due to gravity and increased surface area, which may lead to the accumulation of condensation and limit the ability of the exhaust gases to rise effectively.

When there are long horizontal runs in the venting system, the gases can cool and become denser, which can hinder their upward movement. This is particularly important in maintaining a proper draft, as an ineffective draft can lead to poor appliance performance and potential safety hazards, such as the backflow of combustion gases into the living space.

In contrast, short vertical runs facilitate better draft as they allow hot gases to rise more easily, and increased appliance efficiency generally leads to better combustion and exhaust performance. The use of flexible venting material does not inherently lead to negative draft; rather, it depends on how the system is installed and the overall configuration.