Which of the following is a common issue that can arise from improperly sized duct systems?

Improperly sized duct systems can lead to a range of issues that negatively affect both the performance of the HVAC system and the comfort of the occupants.

Inconsistent temperatures occur when the ductwork cannot adequately distribute conditioned air throughout the building. If the ducts are too small, air can't flow freely, leading to hot and cold spots; if they are too large, the air may move too quickly, causing inadequate heating or cooling in different areas.

Higher energy costs are often a direct consequence of improperly sized ducts. When the HVAC system has to work harder to push air through undersized ducts, it consumes more energy, leading to increased utility bills. Conversely, oversized ducts can also cause inefficiencies, as the system may not operate at peak efficiency.

Poor air quality can result from improper sizing as well. Restriction of airflow can lead to reduced ventilation, which can cause accumulation of indoor pollutants, allergens, or stagnant air. Inadequate air exchanges can mean that harmful airborne particles do not get filtered out effectively.

Given these interconnected issues, all of the mentioned problems can arise from an inadequately sized duct system, making "all of the above" the most comprehensive and correct choice.