When was the last time your chimney or flue was cleaned?
The National Fire Protection Association Standard 211 says,

Our chimney experts perform full cleanings while leaving no-mess.
While cleaning your chimney, we perform a full chimney inspection to ensure functionality and safety. We thoroughly inspect the chimney cap, chimney crown, flashing, brickwork, and for loose roof shingles. Furnaces and stoves will also have an inspection of the smoke chamber, damper, firebox, lintel, hearth, and the ash dump door. Some of the areas in Brooklyn that we service chimney cleaning are Bushwick, Bedford Stuyvesant, Sheepshead Bay, Sunset Park

The chimney liner is one of the most important components of your fireplace and heating system. When our technicians perform a chimney inspection we look closely at the condition of the chimney liner. The liners exhaust the flue gases and smoke out of your fireplace or furnace flue and allow the hot and toxic gases to exit your home safely. Any breaks or openings in the chimney liner can lead to serious problems, potentially allowing a spark or floating ember to escape into the area between the lining and the chimney or the intrusion of carbon monoxide into the living space.

Cracks, breeches or deterioration of your flue can affect the way your system performs. If flue gases or particulate matter from a wood burning appliance escape into attics, walls or ceiling spaces there is potential for fire or carbon monoxide poisoning. These cracks or breeches in the system also affect the way your system draws. The flue system acts like a straw, causing the flue gases to be pulled up and out of the system from the top. If the flue has cracks or missing mortar joints, it will not perform as intended, similar to drinking out of a straw with holes in it.

Relining your fireplace, woodstove, furnace or water heater flue with stainless steel is the most common method used to repair a damaged or deteriorated chimney flue.

When a new furnace, boiler or water heater is installed, a new liner may be needed to properly size the more efficient appliance. The sizing is important because as the flue gases rise, they cool and condense. If the flue/liner is too large the gases may not vent properly.

A chimney cap keeps out rain, snow, birds and other animals as well as leaves, twigs and other debris which could lead to a blockage or chimney fire. A chimney cap will keep sparks from leaving the chimney and igniting nearby combustibles. Additionally a chimney cap will help eliminate wind induced downdrafts.
The moisture from rain and snow does not dry fast due to the lack of sunlight in the chimney. The moisture erodes the mortar joints and weakens the brick.