Filters must be installed such that all return air is filtered upstream from which of the following components?

Filters must be installed such that all return air is filtered upstream from the heat exchanger because this component is critical in ensuring that only clean air passes through to the conditioned space. The heat exchanger operates by transferring heat between two fluids—typically by maintaining a barrier between them. If contaminated air enters the heat exchanger without being filtered, it can lead to build-up of dirt and debris on the heat exchange surfaces, which can hinder efficiency and increase wear on the system. By filtering the air before it reaches the heat exchanger, the system not only improves indoor air quality but also enhances the lifespan and performance of the HVAC equipment.

Filtering the air upstream of the heat exchanger also prevents microbial growth and reduces the risk of contamination of the conditioned air, which is vital for maintaining a healthy environment. It is essential for maintaining operational efficiency in HVAC systems, making this the most appropriate choice among the options provided.