To limit the probability that:
- a lack of shielding will lead to the entry of water
from rain or melting snow into the system, or
- a lack of screens, or screen mesh that is too large,
will lead to the entry of birds or animals into the building, which
could lead to damage to equipment.
This is to limit the probability of
negative effects on the air quality of indoor spaces, which could
lead to harm to persons.