Dirt streaks around supply diffusers that are not caused by a dirty ventilation system are most likely caused by dirty room air at the diffuser. When the room air contains dust and other particles, these can accumulate around the diffuser as the air is discharged, leading to visible streaks.

A package unit typically contains a compressor, an evaporator coil, a condenser, a supply fan, a thermostat, and a filter. These components are essential for the functioning of a packaged HVAC system. Other options mentioning components such as a VAV box or an exhaust fan are not typical for standard package units.

The most common types of fuel consumed in modern residential HVAC systems are natural gas, propane, and oil. These fuels are widely used for their efficiency and availability in residential heating and cooling systems.

A forward-curved fan is typically used for transferring large volumes of air with a minimal wheel diameter. These fans are capable of moving a significant amount of air at lower speeds and are commonly used in HVAC systems due to their efficiency in providing high airflow with low noise levels.