Pre-fabricated fireplaces and chimneys will wear out over time, not intended to withstand repeated high-temperature fires. Often, repairs will involve replacement, and chase covers, a common repair, are no exception. In all likelihood, if your pre-fabricated chase cover needs repair, your chimney professional will rightly recommend its replacement.
Surrounding the flue and extending above the roof, chimney chase covers are exposed to rain and melting snow and may permit puddles of water to form. These rust the chase covers, usually made of galvanized steel if original, ultimately eroding them. This allows water to enter the chimney itself, so worn out chase covers have to be replaced.
A warped appliance is another common repair for fireplaces, caused by repeated high-temperature fires or over-feeding fires in pre-fabricated fireplaces. Typically made from lightweight metal, pre-fabricated fireboxes may warp or crack under intense heat. Unusable in this condition, a damaged firebox or chimney must be replaced before the fireplace can be used.
Perhaps the most common repair is the replacement of a missing chimney cap, which is essential. Needed to prevent small animals and birds from nesting in your chimney appliance, these toppers also help to keep out rain and snow. If the chimney is obstructed, draw is reduced and the performance of the fireplace as a whole is negatively impacted.
Another problem with pre-fabricated fireplaces that may call for attention is the increased infiltration of air. Generally double-walled, pre-fabricated chimneys are intentionally designed to intake outside air while heated inside air is drawn up the inner shaft. The solution to this is not obvious, since the insulation that seems like a good idea fills up air space required by city codes, so a chimney professional needs to be consulted. They will be able to give you the best advice for fireplace and chimney maintenance.