An indoor fan is typically found in which type of system?

An indoor fan plays a crucial role in both heat pumps and air conditioners, which is why the correct choice is that it can be found in both types of systems. In these systems, the indoor fan is responsible for circulating air within the indoor space, helping to distribute either cooled or heated air throughout the environment.

In air conditioning systems, the indoor fan assists in pulling in warm indoor air, passing it over the evaporator coil where it is cooled, and then recirculating the chilled air back into the living areas. Similarly, in heat pumps, the fan helps move air over the heat exchanger, allowing for effective heating or cooling as the system reversibly extracts heat from the air outside and transfers it indoors, or vice versa.

Considering that the other options suggest limitations to either one of these systems or to filtration systems, they do not adequately capture the functionality and application of the indoor fan. This demonstrates the versatile and essential role of the indoor fan in enhancing comfort and efficiency in heating and cooling systems.