I was conducting a home inspection on a duplex last week. The chimneys did not have covered screens installed. As a result of that the flue liner was cracked and deteriorated from rain getting in there and the freezing and thawing cycles.
From the image you can see a nest that is blocking the flue. This flue was also used to vent the exhaust from the gas appliances. This is safety hazard not only from carbon monoxide but also from the nesting materials catching fire.
So always have a covered screen for safety, damage prevention and fire prevention.
- Ohio home inspection, Ohio home inspector, Images of home inspection finds. Finds of 2012 (ohiohomeinspectionvideos.com)