If a house has 50 CFM going out through the bath exhaust, how many CFM must be coming back in?

To determine the correct amount of air that must come back into the house when 50 cubic feet per minute (CFM) of air is being expelled through exhaust, one must consider the principle of air balance. When an exhaust fan is operating, it removes air from the interior, and for the house to maintain proper air pressure, an equivalent amount of air needs to be introduced back into the space.

In this case, since 50 CFM is being exhausted, another 50 CFM must be allowed to enter the house to keep the pressure balanced and ensure adequate air circulation. This balance is vital for preventing issues such as negative pressure, which can lead to backdrafting of combustion appliances or moisture issues due to improper airflow.

Thus, the correct answer reflects the need for a balance of air entering and exiting the space; hence, 50 CFM coming back in matches the amount being expelled.