When is CST typically used regarding HVACR systems?

CST, or Composite Structural Thermal, typically refers to a method or consideration in the context of HVACR systems, particularly in how heating and cooling is managed in buildings. The use of CST is most relevant below grade when performing retrofits due to the unique challenges and requirements that these conditions present.

When working below grade, like in basements or subterranean environments, insulation and thermal management are critical because of the potential for moisture infiltration and heat loss. By using CST in these scenarios, practitioners can effectively assess and implement solutions that optimize energy efficiency and prevent issues related to dampness or temperature fluctuations. This practice ensures that retrofitting existing systems can be done in a way that addresses the specific environmental conditions found below grade, which is vital for the comfort and efficiency of the HVAC system.

In contrast, the other choices pertain to situations where CST is either less relevant or not applicable. For example, while new installations may require similar considerations, they often follow different design protocols that do not necessarily invoke the use of CST in the same manner as retrofitting below grade spaces. Thus, focusing on the unique requirements of below-grade retrofitting highlights the importance and correct application of CST in those specific contexts.